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TO AVOID FINES return on or before date due.
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5/08 K'IProMAcc8Pres/CIRCIDateDue‘indd

METAL DECORATION OF EXFOLIATED GRAPHITE NANOPLATELETS (xGnP)
FOR FUEL CELL APPLICATION
By

In-Hwan Do

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY

Department of Chemical Engineering and Materials Science

2006

ABSTRACT

METAL DECORATION OF EXFOLIATED GRAPHITE N AN OPLATELETS (xGnP)
FOR FUEL CELL APPLICATION

By

In-Hwan Do

The synthesis and characterization of metal particles at nanometer length scale has
been the object of much research in modern nanotechnology due to their great impact on
new nanoscale scientific and technological applications. Nanoscale metal particles
possess unique optical, thermal, electronic, magnetic properties and chemical reactivity
since the size of the resulting materials is on the same order as the fundamental
interaction distances that give rise to physical properties and thus shows the quantum size
effect which is not observed in their bulky status. Therefore, an effective synthetic
method is required to obtain uniform small metal powders with controlled size and a
narrow size distribution and also to produce nanocomposites consisting of either metals
or metal oxides supported on carbons or metals dispersed on metal oxides for a variety of
applications in chemical industries, automobiles, energy and power generating devices,
hydrogen economy as well as for sensors.

On the other hand, although their excellent mechanical, thermal and electrical
conductivity, excellent corrosion and oxidation resistance, and low impurity levels which
are required as a breakthrough material to increase performance of next generation
energy devices, exfoliated graphite nanoplatelet (xGnP) has not been studied as deeply as
recent new nanostructured carbon materials such as single wall carbon nanotubes

(SWNT), multi-wall carbon nanotubes (MWNT), carbon nanohorn (CNH), graphite

nanofiber (GNF), and fullerenes. In addition, xGnP is much cost-effective compared to
other carbon nanosturctures. Hence, it is interesting to evaluate the applicability of xGnP
as a support material for fuel cell which is one of promising energy devices for the future.

In this research, a new simple, efficient and economic way is presented for the
synthesis of noble metal nanoparticles such as Pt, Ru, Pd, etc and their deposition on
various carbon supports and metal oxides via microwave heating in the presence of
various types of room temperature ionic liquids (RTIL). The resulting metal nanocrystals
were characterized by means of UV-vis spectroscopy, transmission electron microscopy
(TEM), powder x—ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and
cyclic voltammetry measurement. Homogeneous metal nanocrystals with sizes of
O.9~3nm were synthesized and deposited on various carbon and metal oxide support
materials through the reduction of the corresponding metal precursors. The size of metal
nanocrystals could be easily and finely tuned by simply controlling experimental factors
such as the concentration of RTIL.

xGnP-supported Pt based catalysts were prepared and tested for utilization in a direct
methanol fuel cell. They showed much higher catalytic activity compared to commercial
carbon black supported catalysts. Excellent activity of the xGnP-supported catalysts over
the commercial catalysts originates from the morphology of xGnP facilitating contact
between the reactant and catalytically active phase as well as the improved interaction
between xGnP and metallic phases. This result proves the potential of xGnP as a support
to replace carbon black. xGnP nanocomposites with metal nanocrystals may find

potential applications as catalysts for other chemical reactions.

Copyright by
ln-Hwan Do
2006

 

 

--l\|r

TO MY BEST FRIEND AND LOVELY WIFE

HEE YOUNG YOU

ACKNOWLEDGEMENT

I would like to thank my advisor Prof. Lawrence T. Drzal for his support throughout my
graduate research. Without his support, this research has not been successfully
accomplished. I also like to thank Prof. Andree Y. Lee, Prof. Krishnamurthy Jayaraman,
Prof. llsoon Lee, Prof. Miller, and Prof. Swain for accepting to be my committee
members and providing valuable comments on my research.

In addition, I would like to thank all members in Composite Materials and Structures
Center for their help and guidance in learning and operating the equipments. My special
thanks go to Dr. Per for his experimental and analytical support on X-ray Photoelectron
Spectroscopes and UV-vis spectroscopy. I would also express my thanks to Dr. Xudong
F an for his good help in operating Transmission Electron Microscopy. Many thanks go to
all other students for their friendship and help.

I thank my late parents who stay in my mind for their encouragement and support. This
happy moment wouldn’t have come true without their devotion throughout my life.
Finally, I would like to express my deepest appreciation to my lovely wife, Heeyoung.
She has devoted herself to support me with her tremendous patience and mental support
and a full of humor during my graduate research. My research may not be successful

without her presence and help in my life.

vi

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................ xii

LIST OF FIGURES ......................................................................................................... xiii

CHAPTER 1 BASICS AND RESEARCH CHALLENGES IN LOW TEMPERATURE

1.1

1.2

1.3

1.4

1.5

1.6

1.7

FUEL CELLS ............................................................................................... 1
Introduction ........................................................................................................... 2
Fundamentals of Fuel Cells .................................................................................. 3

1.2.1 Basic principles ........................................................................................ 3
1.2.2 Important Reactions in Fuel Cells ............................................................ 5
1.2.3 Types of Fuel Cells .................................................................................. 9
Proton Exchange Membrane Fuel Cell (PEMF C) .............................................. 11
1.3.1 Anode in PEMFC ................................................................................... 13
1.3.2 Cathode in PEMFC ................................................................................ 14
1.3.3 Research Challenges on Catalysts in PEMFC ........................................ 16
Direct Methanol Fuel Cell (DMF C) ................................................................... 18
1.4.1 Anode in DMFC ..................................................................................... 19
1.4.2 Cathodes in DMF C ................................................................................. 21
1.4.3 Research challenges on Catalysts in DMF C .......................................... 22
Direct Ethanol Fuel Cells (DEFC) ...................................................................... 24
1.5.1 Reaction Mechanism .............................................................................. 24
1.5.2 Electrocatalysts for DEFC ...................................................................... 26
Direct F orrnic Acid Fuel Cells (DFAFC) ........................................................... 26
Bibliography ....................................................................................................... 28

vii

TABLE OF CONTENTS

CHAPTER 2 BASICES ON MECHANISM OF NANOPARTICLE FORMATION,

2.1

2.2

2.3

2.4

2.5

2.6

IONIC LIQUIDS, MICROWAVE AND POLYOL PROCESS ................. 37
Introduction ......................................................................................................... 38
Formation Mechanisms of Nanostructured Metal] Particles .............................. 38

2.2.1 General Mechanism ................................................................................ 39
2.2.2 How to Control Particle Size? ................................................................ 42
Polyol Process ..................................................................................................... 44
2.3.1 Characteristics of Polyol Process ........................................................... 45
2.3.2 General Mechanism ................................................................................ 46
2.3.3 Control of Particle Size .......................................................................... 47
Microwave Process ............................................................................................. 48
2.4.1 Fundamental Theory of Microwave ....................................................... 48
2.4.2 Effect of Microwave Dielectric Heating ................................................ 51
2.4.3 Advantages of Microwave Process ........................................................ 5 3
2.4.4 Microwave Process for Metal/Carbon Nanocomposites ........................ 54
Ionic Liquids for the Synthesis of Metallic Particles .......................................... 55
2.5.1 Characteristics 0 Ionic Liquids ............................................................... 56
2.5.2 Application of ILs for Metal/Carbon Nanocomposites .......................... 57
Bibliography ....................................................................................................... 59

viii

TABLE OF CONTENTS

CHAPTER 3 CARBON SUPPORTED METAL CATALYSTS ..................................... 67
3.1 Introduction ......................................................................................................... 68
3.2 Basic Principles of Catalyst Synthesis ................................................................ 69

3.2.1 Surface Chemistry .................................................................................. 70
3.2.2 Surface-Precursor Interaction ................................................................. 72
3.3 Synthetic Routes of Carbon-Supported Catalysts ............................................... 75
3.3.1 Deposition-Precipitation ......................................................................... 78
3.3.2 Deposition-Reduction ............................................................................. 78
3.3.3 Impregnation Method ............................................................................. 79
3.3.4 Colloid Method ...................................................................................... 79
3.3.5 Microemulsion ....................................................................................... 80
3.4 Requirements of Carbon Support ....................................................................... 81
3.5 Catalyst Design-Metal Location ......................................................................... 83
3.6 Examples of Catalysts Supported on New Carbon Nanostructures .................... 84
3.6.1 Graphite Nanofiber (GNF)-Supported Catalysts .................................... 84
3.6.2 Carbon Nanotube (CNT)-Supported Catalysts ...................................... 86
3.6.3 Other Carbon Nanomaterials-Supported Catalysts ................................ 87
3.7 Summary ............................................................................................................. 88
3.8 Research Objectives ............................................................................................ 89
3.9 Bibliography ....................................................................................................... 92

ix

TABLE OF CONTENTS

CHAPTER 4 THE SYNTHESIS OF UNSUPPORTED AND SUPPORTED METAL

4.1

4.2

4.3

4.4

4.5

NANOPARTICLES VIA MICROWAVE PROCESS IN THE PRESENCE

OF ROOM TEMPERATURE IONIC LIQUIDS (RTIL) ........................... 99
Introduction ....................................................................................................... 1 00
Experimental Details ........................................................................................ 102

4.2.1 Materials ............................................................................................... 102
4.2.2 Preparation of Unsupported and Supported Metal Particles ................ 103
4.2.3 Characterization Methods .................................................................... 104
Results and Discussions .................................................................................... 106
4.3.1 Formation of Pt Nanoparticles ............................................................. 106
4.3.2 Unsupported Metal Nanoparticles ........................................................ 115
4.3.3 Carbon Supported Pt Catalysts ............................................................. 124
4.3.4 Metal Oxide Supported Pt Catalysts .................................................... 140
Conclusion ........................................................................................................ 144
Bibliography ..................................................................................................... 145

CHAPTER 5 THE EVALUATION OF EXFOLIATED GRAPHITE NANOPARTICLES

5.1

5.2

5.3

5.4

5.5

(xGnP) AS A SUPPORT FOR Pt CATALYSTS IN DIRECT METHANOL

FUEL CELL .............................................................................................. 152
Introduction ....................................................................................................... l 53
Experimental Details ........................................................................................ 155

5.2.1 Materials ............................................................................................... 155
5.2.2 Preparation of xGnP Supported Catalysts ............................................ 156
5.2.3 Characterization Methods .................................................................... 157
Results and Discussions .................................................................................... 159
5.3.1 Comparisons of Various Carbon Supports ........................................... 159
5.3.2 XRD Analysis of Pt/xGnP ................................................................... 163
5.3.3 TEM Analysis of Pt/xGnP ................................................................... 165
5.3.4 XPS Analysis of Pt/xGnP ..................................................................... 177
5.3.5 Electro-oxidation of Methanol ............................................................. 182
Conclusion ........................................................................................................ 1 86
Bibliography ..................................................................................................... 1 87

TABLE OF CONTENTS

CHAPTER 6 PtRu NANOPARTICLE FORMATION BY RTIL-ASSISTED

6.1

6.2

6.3

6.4

6.5

MICROWAVE PROCESS AND EVALUATION OF PtRu/xGnP
ELECTROCATALYSTS FOR DIRECT METHANOL FUEL CELL.... 192

Introduction ....................................................................................................... 1 93
Experimental Details ........................................................................................ 195
6.2.1 Materials ............................................................................................... 195
6.2.2 Preparation of Unsupported and Supported Catalysts ......................... 195
6.2.3 Characterization Methods .................................................................... 196
Results and Discussions .................................................................................... 198
6.3.1 Unsupported PtRu Catalysts ................................................................ 198
6.3.2 Supported PtRu ..................................................................................... 203
Conclusion ........................................................................................................ 21 1
Bibliography ..................................................................................................... 21 3

CHAPTER 7 DETAILS ON MECHANISM FOR NANOPARTICLE FORMATION IN

SOLVENT AND IN THE PRESENCE OF SUPPORTS ......................... 216

7.1 F ormation of Nanoparticles in Solution ........................................................... 217
7.1.1 Nucleation ............................................................................................ 217

7.1.2 Growth .................................................................................................. 219

7.1.3 Formation of Metal Nanoparticles in the Absence of RTIL ................ 220

7.1.4 Formation of Metal Nanoparticles in the Presence of RTIL ................ 221

7.2 Deposition of Nanoparticles on xGnP .............................................................. 224
7.2.1 xGnP Surface-Metal Precursor Interaction .......................................... 224

7.2.2 Metal Oxide-Metal Precursor Interaction ............................................ 227

7.2.3 Pt-Pt and Pt-Graphite Interactions ........................................................ 229

7.2.4 Particle Growth of Metal Particles on xGnP ........................................ 231

7.2.5 Metal Particles Deposition on xGnP in the Presence of RTIL ............. 232

7.3 Summary ........................................................................................................... 235
7.4 Bibliography ..................................................................................................... 236
CHAPTER 8 CONCLUSIONS AND RECOMMENDED WORK ............................. 239

xi

Table 1.1.

Table 2.1.

LIST OF TABLES
CHAPTER 1

Typical characteristics and differences of various fuel cell systems ......... 10

Physical properties for common pure solvents at room temperature [59]
50

 

Table 3.1.

Table 4.1.

Table 5.1.

Examples of chemical method for synthesis of carbon-supported metal
particles 76

 

Information of Pt particles on various carbon supports synthesized via
RTIL assisted MW heating process 139

 

Characteristics of carbon supports calculated from their XRD pattern
163

 

Table 5.2.

Table 5.3.

Table 5.4.

Table 6.1.

Table 6.2.

Comparison of morphological data of XC-72R and xGnP supported Pt
(20wt. %) catalysts prepared at different molar ratio of RTIL/Pt ......... 177

Peak location of Pt4f and content of different Pt species determined from
Pt/xGnP-N and Pt/xGnP-RTIL-Z samples 179

 

Mass activity of Pt particles for Pt/xGnP catalyst prepared via microwave
irradiation at the various molar ratios of RTIL/Pt 184

 

The data from XRD and TEM for the different PtRu and Pt particles. 203

Electrochemically active surface area of PtRu particles for PtRu/xGnP
synthesized via microwave process and a commercial PtRu/CB catalyst
209

 

 

Table 7.1 [so-electric point values of some metal oxides [16, 17] - - - 228

xii

Figure 1.1.

Figure 1.2.

Figure 1.3.

Figure 1.4.

Figure 1.5.

Figure 2.1.

Figure 2.2.

Figure 3.1.

Figure 3.2.

Figure 3.3.

Figure 3.4.

Figure 4.1.

LIST OF FIGURES

 

 

 

 

 

 

CHAPTER 1
Summary of fuel cell types ....... - - - - - - - 4
Pathways of oxygen reduction on catalyst surfaces in an acid electrolyte
[15] - - _ - - - - 7
Typical structure of single cell in PEMFC (modified from [28]) ............ 11
(a) Chemical structure of Nafion ionomer and (b) three-phase boundary
model at gas diffusion electrode 12
The bridge model of oxygen reduction on Pt. _- - - 15
CHAPTER 2
The successive process for the formation of metal particles by wet
chemical reaction (modified from [10]) - 41
Microwave dielectric heating mechanism of H20 [57]. 49
CHAPTER 3

Surface groups containing oxygen and localized 1: sites (i) on a
graphene layer -- - _ 71

Three dimensional schematic of layered graphite structure (0: carbon
atom) _ 73

Schematic representing the common interactions between the active sites
of a carbon surface and metal ions and their effect on particle size and

 

 

distribution. - - - _ 74

Simplified procedures for chemical routes to prepare supported metal

particles (modified from [39]) _ - - _ _ -_ 77
CHAPTER 4

Chemical structures of room temperature ionic liquids for the synthesis
of catalysts (a: bmimCH3C02, b: bmimCH3SO4, c: bmimSCN, d:
bmimPFg). _ 103

xiii

Figure 4.2.

Figure 4.3.

Figure 4.4.

Figure 4.5.

Figure 4.6.

Figure 4.7.

Figure 4.8.

Figure 4.9.

LIST OF FIGURES

UV-vis absorption spectral change of HthCk-6H20-ethylene glycol
solution in the absence of RTIL before and after microwave reduction.
106

 

Typical UV-vis absorption spectral change of HthC16-6H20-ethylene
glycol solution containing bmimCH3C02 before and after microwave
reduction. 109

UV-vis absorption spectral change of HthC16-6H20-ethylene glycol
solution containing different amount of bmimCH3C02 before and after
microwave reduction. _ 110

Typical UV-vis absorption spectral change of HthCl6-6HzO-EG solution
in the presence of bmimCH3SO4 in MW irradiation. 111

 

Comparison of UV—vis absorption spectra obtained in the presence of
three RTILs such as bmimCH3SO4, bmimPFg, and bmimCH3C02 after
108 MW reduction. _ 112

UV-vis absorption spectral change of HthCk-6H20-EG solution
containing of bmimSCN(20) before and after microwave reduction. .. 114

UV-vis absorption spectral change showing the ability of bmimSCN
reducing Pt ions.. - - - - - 115

TEM micrographs and particle size distributions of unsupported Pt
nanoparticle in the absence (a, lb) and the presence (c, d) of a room
temperature ionic liquid. - - -- 118

 

Figure 4.10. TEM analysis of Ru nanoparticles with the addition of bmimCHgCOZ.

 

121

Figure 4.11. TEM image of Pd nanoparticles with the addition of bmimSCN ....... 122
Figure 4.12. TEM micrograph of Pt reduced via microwave heating of lmL

bmimCH3C02 containing HthCl6-6H20 123

 

Figure 4.13. X-ray diffraction patterns of Pt nanoparticles synthesized in EC at (a)

bmimCH3C02/Pt precursor = 0, (b) 2.5, (c) 5, and (d) 20 .................... 124

Figure 4.14. TEM images of (a) Pt/MWNT-N and (b) Pt/MWNT- bmimCH3SO4, (c)

Pt/MWNT-bmimPF6, (c) Pt/MWNT- bmimCH3C02, and (e)
Pt/MWNT-bmimSCN 125

xiv

LIST OF FIGURES

Figure 4.15. Average size of Pt nanoparticles supported on MWNT synthesized in

the presence of different RTILs. 128

Figure 4.16. TEM images of Pt particles dispersed on SWNT synthesized in the

presence of bmimCH3C02 - 129

Figure 4.17. TTEM morphologies of spiral graphite nanofiber (GNF)-supported Pt

(20wt.%) nanocomposites by microwave heating process (a) in the
absence of RTIL and in the presence of (b) bmimPF6 and (c)
bmimCH3COz and (d) the mean size of Pt particles obtained from
TEM analysis for all the samples 131

Figure 4.18. TEM morphologies of (a) Pt/XC-72R-N, (b) Pt/XC-72R-bmimPF6 and

(c) Pt/XC-72R-bmimCH3C02 composite 133

Figure 4.19. The average size of Pt particles in Pt/XC-72R-N, .Pt/XC-72R-bmimPF5

and Pt/XC-72R-bmimCH3C02 composites 135

Figure 4.20. TEM images of Pt nanoparticles highly loaded on (a) Pt/xGnP-N, (b)

Pt/Vulcan XC-72R-bmimCH3C02 and (c) Pt/MWNT-bmimCH3C02 .....
136

 

Figure 4.21. TEM images of of Pt particles in (a) Pt/TiOz—N, (b) Pt/Ti02-

Figure 5.1.

Figure 5.2.

Figure 5.3.

Figure 5.4.

Figure 5.5.

bmimCH3C02, (C) PUCCOz-N, (d) Pt/CeOZ-bmimCH3C02, (e) Pt/A1203-

 

 

 

 

N, and (f) Pt/A1203-bmimCH3C02. 141
CHAPTER 5

Typical TEM pictures of xGnP 156
Thermo-oxidative stability of various carbon supports in an air
environment 160
Comparison of TGA curves for xGnP and XC-72R with and without
20wt. % Pt particle. Scan rate 25°C/min - 161
X-ray diffraction patterns of various carbon supports (a: SWNT, b:
MWNT, c: CB, (1: GNF, and e: xGnP) used in the present work ......... 162
X-ray diffraction patterns of Pt/xGnP prepared (a) without RTIL-2 and

with the addition of (b) RTIL-2/Pt = 2.5, (c) 10, and (d) 50 .................. 164

XV

LIST OF FIGURES

Figure 5.6. TEM images of Pt/xGnP (a) at low magnification and at high
magnification synthesized with the addition of (b) 0, (c) 5, and (d) 50
vol. % DI water, respectively 166

Figure 5.7. The effect of water content in BC on Pt particle size determined in TEM
168

 

Figure 5.8. TEM images of (a) Pt/xGnP-N, (b) Pt/xGnP-RTIL-l, and (c) Pt/xGnP-
RTIL-2 170

Figure 5.9. The average particle size and size distribution of Pt particles in (a)
Pt/xGnP-N, (b) Pt/xGnP-RTIL-l, and (c) Pt/xGnP-RTIL-2 ................. 171

Figure 5.10. Effect of RTIL content on the size of Pt particles supported on xGnP
172

 

Figure 5.11. TEM images of Pt/xGnP-N synthesized in (a) DEG and (b) TEG,
respectively and Pt/xGnP-RTIL-Z prepared in (c) DEG and (d) TEG,
respectively 173

Figure 5.12. XPS core level spectra for the Pt 4f regions of a Pt catalyst of 3.7nm
mean particle size supported on xGnP 180

Figure 5.13. XPS core level spectra for the Pt 4f regions of a Pt catalyst of 3.7nm
mean particle size supported on xGnP 181

Figure 5.14. Comparison of catalytic activity for methanol oxidation on Pt/XC-72R,
Pt/xGnP-N, and Pt/xGnP- RTIL-1, Pt/xGnP- RTIL-2 catalysts ......... 183

Figure 5.15. Chronoamperometry curves for methanol oxidation at 0.45V on Pt/XC-
72R, Pt/xGnP-N, and Pt/xGnP- RTIL-1, Pt/xGnP- RTIL-2 catalysts ......
185

 

CHAPTER 6
Figure 6.1. TEM microscophs of (a) PtRu-N and (b) PtRu-RTIL-2 (400) .............. 198

Figure 6.2. XRD patterns of (a) PtRu-N, (b) PtRu-RTIL2(800), (c) PtRu-RTIL2(400)
and (d) Pt-N 200

Figure 6.3. The change of (220) peaks of (a) PtRu-N, (b) PtRu-RTIL2(800), (c)
PtRu-RTIL2(400) and (d) Pt-N -_ 201

xvi

Figure 6.4.

Figure 6.5.

Figure 6.6.

Figure 6.7.

Figure 6.8.

Figure 6.9.

Figure 7.1.

Figure 7.2.

Figure 7.3.

Figure 7.4.

Figure 7.5.

Figure 7.6.

LIST OF FIGURES

TEM analyses of PtRu/xGnP-N, PtRu/xGnP-RTIL-l, and PtRu/xGnP-
RTIL-2 catalysts with 40wt. % metal loading (from top to bottom).... 204

TEM analyses of (a) PtRu/xGnP-N, (b) PtRu/xGnP-RTIL2(800), (c)
PtRu-RTIL2(400), and a commercial PtRu/CB catalyst 205

 

A High resolution TEM image of PtRu/xGnP-N catalyst (scale bar: 2nm)
206

 

Cyclic voltammograms recorded at 50mV/s of PtRu/xGnP catalysts via
microwave process and a commercial PtRu/CB-ETEK 208

 

Cyclic voltammograms of the methanol oxidation in 1M H2804 + 2M
CH3OH at 50mV/S for PtRu/xGnP catalysts and a commercial PtRu/CB-
ETEK 210

Chronoamperometry curves for methanol oxidation at 0.40V on
PtRu/CB-ETEK and three PtRu/xGnP catalysts 211

 

CHAPTER 7

The simplified process for uniform particle formation synthesized via
microwave process in the absence of RTIL 221

 

The simplified process for uniform particle formation synthesized via
microwave process (a) in the presence of bmimPFg, bmimCH3SO4, and
bmimCH3C02 and (b) bmimSCN 223

 

Schematic representation of oxygen-containing groups and localized 1:
sites (i) on a graphene layer 225

 

Three dimensional schematic of layered graphite structure (0: carbon

atom) and schematic interaction between the delocalized 1r sites and Pt
precursor as an example 225

 

Schematic representing the common interactions between the metal ions
and the active sites of xGnP and final metal particle size and distribution
after the reduction -- 227

 

Schematic representing the metal precursor-metal oxide interaction at
different pH condition and the formation of metal particle size ........... 228

xvii

 

Fig

Fi

Fi

Fi

Figure 7.7. Force interaction in platinum-platinum (top) and platinum-graphite
(bottom) obtained by Lennard-Jones formuation 230

Figure 7.8. Main successive processes representing the formation of metal
nanoparticles on the basal plane of xGnP 232

Figure 7.9. Suggested RTIL locations and TEM morphology of Pt/xGnP-

bmimCH3C02 sample when RTIL 18 added (a, b) before and (c, d) after
Pt precursor _ 233

Figure 7.10. FT [R of (a) Pt/xGnP-bmimCH3COz from sequence-A, (b) xGnP, (c)

Pt/xGnP-bmimCH3COz from sequence-B (d) ethylene glycol, and (e)
bmimCH3C02 234

xviii

CHAPTER 1

BASICS AND RESEARCH CHALLENGE IN LOW

TEMPERATURE FUEL CELLS

1.1 Introduction

Fuel cells are the electrochemical device continuously converting chemical energy
into electrical energy [1]. Fuel cells are similar to batteries in that both of them generate
electrical energy. However, fuel cells differ from batteries in that fuel cells need a
continuous supply of fuels and the continuous removal of products and in that the fuel
and oxidant are not contained within the fuel cell compartment. Hence, fuel cells do not
store energy as batteries do, but must be refilled with fuel after use.

Fuel cells are considered as an ideal next generation energy conversion device that
can meet the increasing concerns about green environment and energy security for the
future and the tendency to increase the flexibility of electricity generation. Fuel cells are
expected to reduce the dependence on fossil fuels and produce less greenhouse emissions
into the atmosphere. The main advantages of fuel cells over conventional heat engines are
high electrical efficiencies, zero or low emissions and less noise pollution [2, 3]. Some
other favorable characteristics of fuel cells that make them considered as next generation
energy conversion devices are good reliability, no need of moving parts in the energy
converter and remote and unattended operation [4]. Thanks to these merits, fuel cells are
being developed for numerous commercial and military applications, such as automobiles,
portable electronic devices, and mobile and stationary power generation. Among the
various types of fuel cell, the proton-exchange membrane fuel cell (PEMFC) and the
direct methanol fuel cell (DMFC) are attractive because they are close to commercial
applications.

PEMFC is the most promising candidate for power application ranging from

micropower to large-scale stationary power for building due to their advantageous

 

 

{can

Opt"

(.1?

hr

features such as low-Operating temperature, high current density, light weight, long
operating life time, low cost, suitability for discontinuous operation, small volume, etc.
[5~7]. On the other hand, DMFC can be potentially applied in clean and mobile power
sources due to their high energy efficiency and quick cold starting and in an electric
vehicle, elimination of the need for fuel reformation [8, 9].

In this chapter, a review on the general background of low temperature fuel cells,
especially PEMFC and DMFC, is provided. The focus is placed on the current status of
the technology and the important technological challenges in carbon-supported metal
catalysts for PEMFC and DMFC. At the end of this chapter, general features of the direct
ethanol fuel cell (DEFC) and the direct formic acid fuel cell (DFAFC) are also discussed
briefly since they also possess excellent characteristics for portable power devices and

use precious metal catalysts on carbon. supports similar to in PEMFC and DMFC.

1.2 Fundamentals of Fuel Cells

1.2.1 Basic principles

The basic structure of all fuel cells consists of two electrodes on opposite sides of an
electrolyte. The electrodes are exposed to fuel or oxidant (e. g. hydrogen or oxygen in the
case of PEMFC) supplied in the form of gas or liquid. The electrodes must be gas or
liquid permeable but the electrolyte should not be permeable to the gas or liquid.
Depending on the types of fuel cells, hydrogen or methanol is oxidized at the anode
liberating electrons and protons which travel through the electrolyte:

H2——>2H++26’ (1.1)

 

The l

L11 1h:

The

\\ ,1]

«Sf.

Fig

The liberated electrons flow in the external circuit producing electrical energy and arrive
at the cathode to participate in the reduction of oxygen:
02 + 4e'——> 20} (1.2)
The oxygen ions recombine with protons to form water:
0” + 2H+ —> H20 with AG = -237 kJ/mol (1.3)
Water can be produced at the anode if an oxygen ion conducting electrolyte is used as in
alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell

(SOFC). A schematic drawing of fuel cells and their simplified reactions are shown in

  
 
   
  
  
   
 
  
 
  
 
 

Figure 1.1.
€02,112 — 02 (air)
(CO removal)
CH3OH —' <— 02 (air)
0 (air
[[2 _ 2 )

(C0, removal)

(:02, Hz —5 ‘— 02(311‘)

‘—' 02 (311')

co, H, —> 4— co2

C09 H2 —’ 02 (air)

 

 

Anode Electrolyte Cathode

Figure 1.1. Summary of fuel cell types.

1.2.2 Important Reactions in Fuel Cells

1.2.2.1 Hydrogen oxidation

Pt-based catalysts are most widely used for the hydrogen oxidation at the anode in
fuel cells. Hydrogen adsorbs on the surface of catalysts and then dissociates to two
hydrogen ions as follows:

2Pt(s) + H; ——> Pt-Had, + Pt-Had, (1.4)
Pt-Had,——> Pt(s) + H+ + e' (1.5)

The overall reaction of hydrogen oxidation is:

H2——-> 2H+ + 2e' 15" = 0v (1.6)
where E” is the equilibrium cell voltage at standard condition.

This reaction is very fast with nearly no activation energy required when Pt is used
as a catalyst or an electrode. Thus, very low loading of a Pt catalyst on the anode can be
achieved [10]. However, the active sites of Pt surface can be poisoned by the adsorption
of CO if impure hydrogen reformed from natural gas or alcohols is used as the fuel. To
resolve the detrimental effect of CO poisoning on Pt activity, the application of air and an
increased operating temperature have been tried to increase CO-tolerance. However. CO-
tolerant catalysts are a fundamentally more attractive alternative and thus the

development of CO-tolerant catalysts is one of the importance research issues [1 1].

1.2.2.2 Oxygen reduction
The oxygen reduction reaction at the cathode proceeds via a direct 4-electron
reduction or an indirect 2-electron reduction shown in the following equations [12].

1) Direct four-electron reduction

a) in an alkaline electrolyte
Pt—Oz + 2H20 + 4e' —> 4OH’ (1.7)
b) or in an acidic electrolyte
PIC—02 + 4H,. + 4e' —> 2H20 (1.8)
2) Indirect two-electron reduction
a) in an alkaline electrolyte
Pt—Oz + H20 + 2e" ———> HOO' + OH' ( 1.9)
followed by either
HOO' + H20 + 2e- ——) 3OH’ (1.10)
or
2HOO'——>20H' (1.11)
b) or in an acid electrolyte
Og+2H++2e'—>HOOH (1.12)
followed by either
HOO' + 2H+ + 2e' ———> 2HzO (1.13)
or
2HOOH ——> 2HgO + O; (1.14)
The direct oxygen reduction in an acid electrolyte is preferable due to its high charge
efficiency and no involvement of peroxide species.
The different reaction pathways result from the different adsorption sites on the
surface of a catalyst, which is illustrated in Figure 1.2 as an example in an acid
electrolyte [13]. If only one side of oxygen adsorbs on the catalyst surface. pathway-II is

favored without the change of O—O bonding. If adsorption takes place on both sides of

oxygen, the 02 molecules are decomposed and four-electron reduction (pathway-I or HI)
is favored, resulting in no peroxide product. Carbon materials are suitable as support
materials for carbon-supported noble metal catalysts for oxygen reduction reaction

because they are inert in this acid environment [14].

 

 

/ 2e‘ 2e'
—* M \ T M ——+ M + 211,0 Pathway-I
.. 2I-I*
o OH
y M + H202
.- + H’
M + 02 ——-> M — o\—“-» M — o\—"» M — o\ Pathway-[l
o o OH _
3e
3“ M + 211,0
M . ,
\0 2c: M - OH 2e'
‘ ' I ——> ——> + 2H20 Pathway-III
O ”H” M - OH 2H‘ M
/
M

Figure 1.2. Pathways of oxygen reduction on catalyst surfaces in an acid electrolyte
[15].

It has been known that many factors influence the kinetics of oxygen reduction
reaction. The factors include particle size and crystal structure of catalysts, d—orbital
vacancy of Pt-based catalysts, pretreatment and activation of electrode and distance
between catalysts on the support surface [16~20]. Additionally, the oxygen reduction at

cathode in fuel cells is very sensitive to the presence of every kind of impurity.

1.2.2.3 Methanol oxidation
Methanol oxidation in a DMFC takes place only at the anodes through a complicated

process with many steps and many intermediates. Pt-based catalysts are most popular due

to their high stability and activity with methanol fuel. The methanol oxidation on Pt-
based catalysts proceeds by a sequence of dehydrogenation steps giving rise to methanol

molecules adsorbed on the catalyst surface, according to the following steps [21]:

i) CH3OH + Pt ——> Pt—CHgOH + H+ + le' (1.15)
ii) Pt-CHzOH + Pt —> Pt—CHOH + H( + 1e' (1.16)
iii) Pt—CHOH + Pt—> Pt—CHO + H+ + le' (1.17)
iv) Pt—CHO ——> Pt—CEO + H" + le' (1.18)

or Pt—CHO+Pt———> ::>C=O +H++ 1e" (1.19)

CO is formed at the end of the methanol oxidation. This CO strongly adsorbs on the
surface of the catalyst and poisons it, preventing further catalytic reaction. Hence, this
CO must be removed by oxidizing the CO with oxygen generated from the dissociation
of water supplied with methanol on the Pt surface or on a secondary metal (e. g. Ru) as

shown below [22].

v) Pt + H20—-—) Pt—OH + H+ + 16' (1.21))

vi) Pt—OH + Pt—CO——> 2Pt + C02 + 1H“ + le' (1.21)
or

vii) Ru + H20 ———> Ru-OH + H“ + le‘ (1.22)

viii) Ru—OH + Pt—CO——> Ru + Pt + C02 + H+ + 1e’ (1.23)

The research challenge is to develop a catalyst which is not poisoned by CO. One of

the most successful catalysts in DMFC is bimetallic PtRu catalyst supported on carbon.

Other promoters such as Sn, Co, Ni, W, Mo and other metals have been studied in the

form of bimetallic or tertiary metallic alloys.

1.2.3 Types of Fuel Cells

Fuel cells can be categorized by either the electrolyte employed in the cell or the
operating temperature for each of fuel cells. If the operating temperature is taken for the
classification, the fuel cells can be divided into low temperature fuel cells and high
temperature fuel cells. Low temperature fuel cells are the Alkaline Fuel Cell (AFC),
PEMFC, DMFC and the Phosphoric Acid Fuel Cell (PAFC). The direct formic acid fuel
cell (DFAFC) and the direct ethanol fuel cell (DAFC) which recently drew attention due
to their merits over PEMFC and DMFC belong to low temperature fuel cells. High
temperature fuel cells are the molten carbonate Fuel Cell (MCFC) and the solid oxide
fuel cell (SOFC).

Typical characteristics and differences of various fuel cell systems are detailed in
Table 1.] along with the advantages and disadvantages of each fuel cell. Pt-based
catalysts or carbon-supported metal catalysts are commonly used in low temperature fuel
cells. As shown in the last row, the development of new catalysts which is less expensive
and more active than the present ones is an ongoing research issue requiring a solution to
commercialize fuel cells. Among the fuel cells mentioned, DMFC and PEMFC are the
target applications for the carbon-supported catalyst synthesized in this research. They
are the most advanced low temperature fuel cells and are attracting much more attention

for their potential as mobile power sources in the future.

 

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10

1.3. Proton Exchange Membrane Fuel Cell (PEMFC)

Owing to high power density and low operational temperature, PEMFC is considered
as the fuel cell of choice for portable and transportation applications. The typical
structure of a basic single cell in PEMFC stack is represented in Figure 1.3. The
membrane electrode assembly (MEA) which is located between a pair of current
collectors or flow field plates is the key component in a PEMF C. MEA consists of two
electrodes separated by proton exchange membrane (PEM). Protons and electrons are
generated by hydrogen oxidation at the anode and oxygen is reduced and then
recombines with the protons to form water at cathode. Both PEMFC electrodes consist of
Pt-based electrocatalysts bonded with Nafion ionomer and/or polytetrafluoroethylene

(PTFE) on a gas diffusion layer.

   
  
           
  
   

Anode

(current collector

Cathode

(current collector)

   

Gas Diffusion
Electrode

Gas DilTusion

Catalyst
Electrode

l ,ayer

H:

Carbon cloth or
Carbon Paper

Nation P'l‘liF.

Carbon
support

Figure 1.3. Typical structure of single cell in PEMFC (modified from [27]).

11

Nafion® as a trademark of DuPont for perfluorosulfonic acid ionomers is a
perfluorinated polymer that contains small proportions of sulfonic functional groups. Its
general chemical structure can be seen below in Figure 1.4 (a). The 803' groups is
permanently attached to the chain, while the hydrogen ion is free to "hop" from one acid
site to another. Electrons are blocked due to the fluorocarbon backbone being
nonconductive. Hence, the Nafion is conductive for solvated hydrogen ions or protons
but nonconductive for electrons because, which allow the Nafion to be qualified for ion-

exchange membrane for applications such as Proton Exchange Membrane (PEM) fuel

cells.
CHJOH + H20 €02
Or
“2
Pt
‘ECFz—CFZH Fz—CFZ} .
x y e-
[O—CFz—TF}O—CF2—CF2\ 4° Carbon
2 //
O OH
CF3
(a) (b)

Figure 1.4. (a) Chemical structure of Nafion ionomer and (b) three-phase boundary

model at gas diffusion electrode.

The gas diffusion layer is normally carbon fiber paper or carbon cloth that acts as a
current collector. The primary purpose of using Nafion is to provide a medium for the
proton conducting pathway within the catalyst layer. The aim of PTFE is to bind together

the electrode particles and to aid in water management.

The gas diffusion layer has to be porous to ensure the supply of the reactant to the
active sites where Pt-based catalyst is in contact with ionic conductor (e. g. Nation) and
electronic conductor (carbon particles) to form “three-phase boundary” which is shown in
Figure 1.4 (b). The three-phase boundary is an important requirement for a good
electrode. The catalyst, Nation and/or PTFE are mixed together with water or alcohol to
form an ink. This ink is deposited onto the diffusion layer by screen—printing, plating,
brushing, filtering, or spraying techniques [23~26]. The ink can also be deposited onto
PEM by the same techniques and then compressed with the diffusion layer.

In a normal PEMFC system, carbon-supported Pt catalysts and carbon supported Pt-
based bimetallic alloy catalysts, especially PtRu, are applied at the cathode and anode,
respectively. The general characteristics of electrodes in PEMFC are briefly presented in

this section, followed by research challenges on the key materials for PEMFC.

1.3.1 Anode in PEMFC

The performance of a Pt catalyst in PEMFC anode is excellent if pure hydrogen is
used. In reality, hydrogen reformed from other fuel sources such as methanol, propane,
and natural gas is supplied but the reformed hydrogen has traces of CO. This CO strongly
adsorbed on the surface of the Pt catalyst and blocks the active sites of the catalysts and
consequently fewer sites are available for further the electro-oxidation of hydrogen, the
so-called the poisoning effect [28]. The water-gas shift reaction or other gas processing
methods are attached to the PEMFC system in order to resolve this problem but they
make the system more complex and expensive [29, 30]. Hence, better catalysts which are

tolerant to CO poisoning and can oxidize CO at low temperature are obviously desired.

13

Unsupported or supported Pt-Ru alloy is the most successful state-of-art catalyst
resistant to CO poisoning. Two possible mechanisms on the positive effect of Ru were
suggested: bifunctional mechanism [31~33] and ligand effect [34, 35]. The former
involves the adsorption of an oxide species onto the Ru surface and then the production
of Ru-OH. This Ru-OH is the source to oxidize CO on Pt to C02. The latter is related to
the change of chemical properties of Pt atoms at the surface due to the presence of Ru in
the catalyst. As a result, CO is bound more weakly to PtRu and thus the CO poisoning
effect becomes weaker.

Other Pt-based alloys catalysts including Os, Mo, W, Co, Ir, and Pd have been
studied by several groups for an improved CO-tolerance of anode catalysts [36~39].
Some of them were reported to show the CO-tolerance exceeding PtRu but still PtRu is

most popular combination until they are firmly confirmed.

1.3.2 Cathode in PEMFC

The oxygen reduction reaction (ORR) on Pt occurs via dissociative adsorption of 02
followed by protonation of the adsorbed species. The bridge model as shown in Figure
1.5 [40] is widely accepted among several suggested and appears to describe this
mechanism. Optimal Pt particle spacing and size are of critical importance [41]. The
complex pathways of the OR result in slow electrochemical kinetics. The rate of an
electrochemical reaction for the OR on Pt is 105 slower than that for hydrogen oxidation
at Pt [42]. This explains why the activity at the cathode strongly influences the hydrogen
fuel cell performance. Therefore. improving the cathode activity is a major focus for

PEMFC electrode development.

14

Ptk ptl+1—0H Pt2
9 2H+ 4e‘

___p —> + 2H20

Ptz + 02 ——-> +
Ptz+1_0H 2” ptZ

Fig. 1.5. The bridge model of oxygen reduction on Pt.

One successful way of increasing catalyst utilization is to employ carbon supported
Pt catalysts. The ideal carbon support should have a suitable surface area, good electrical
conductivity, high chemical/electrochemical stability, and pore size distribution, etc [43].
Vulcan XC-72 (Cabot Corp.) is the most common among a variety of grades of carbon
black. Although carbon is an excellent electronic conductor, it is a very poor proton
conductor due to its hydrophobicity. increasing the hydrophilicity of the carbon support
by creating carbon-oxygen complexes such as phenol, carbonyl, carboxylic, quinine, and
lactone groups through oxidization has been shown to enhance proton conductivity of the
catalyst layer. One serious issue with Pt/carbon catalysts is the sintering of the Pt
particles [44]. Sintering occurs when Pt particles aggregate over their lifetime decreasing
the Pt surface area, and ultimately leading to a decline in performance throughout the
lifetime of operation.

Another approach is to improve the utilization of Pt catalyst. For successful catalyst
utilization, it is important to enhance both the electrical and ionic conductivity of the
carbon support at the same time. A proton conducting polymer like Nafion is added into
the catalyst layer for that purpose. Pt catalyst near or directly in contact with Nafion
provides the best performance. This increases the proton conductivity of the catalyst layer.
It is also reported that carbon-supported Pt catalyst mixed with Nafion outperforms

conventional Pt black electrodes having ten times the Pt loading [45]. When designing an

15

electrode with a carbon-supported catalyst and Nafion, they must be mixed in proper
proportions to form a stable three-phase boundary where the gas, ion conductor, and
catalytically active and electronically conductive phase are all present as mentioned
previously. This requirement limits the amount of Nation that can be added because the
morphology. low gas permeability, and poor electronic conductivity of Nation disrupt
this boundary and adversely affects cell performance [46]. Hence, alternative methods for

providing proton conductivity in the catalyst layer are of interest and importance.

1.3.3 Research Challenges on Catalysts in PEMFC

The key materials in a PEMFC are electrocatalyst, electrode, proton exchange
membrane and bipolar plate. Further increases in the performance of PEMFC in use
today is available from improvements of electrocatalytic activity especially at the cathode
and from appropriate electrode design and from the enhancement of stability, proton
conductivity, and operational temperature range in low cost proton exchange membrane.
N on-noble metal based catalyst and low cost graphite bipolar plates are desired for the
cost reduction of PEMFC system. Choice of hydrogen sources and optimization of
thermal and water management also affect the performance of PEMFC.

The electrocatalyst should have high catalytic activity and high utilization. Pt-based
catalysts are best in hydrogen oxidation at the anode and oxygen reduction at the cathode
in PEMFC. Since hydrogen oxidation is not structurally sensitive, smaller Pt particles
and better their dispersion and utilization, which will reduce the Pt loading and thus the

cost of PEMFC.

l6

There are two methods to overcome CO poisoning of Pt-based catalyst at the anode:
One is to use CO-tolerant catalysts and the other is to introducing bilayer structure. CO-
tolerant PtRu catalysts are commonly used for the case of the reformed hydrogen fuel as

described previously. Metal loading of PtRu catalyst should be higher than that of Pt

 

catalyst. This causes the agglomeration ofcatalytic phases, leading to the progressive loss
of cell potential [47]. Hence, new catalysts with higher C0—tolerance and aggregation—
resistance still need to be developed. Recently, PtSn/C, PtRuM/C (M = Mo, Nb, and Ta)
and PtRu-HMog/C (M = Mo and W) have been investigated or this purpose [48~51]. For
a bilayer anode, a carbon—supported Pt-based catalyst layer is inserted between the PtRu
catalyst layer and the gas diffusion layer [52]. Using this bilayer anode construction
resulted in extended anode lifetime and stable PEMFC performance.

In addition to resolving the CO poisoning of Pt—based catalysts, development of a Pt-
free electrocatalyst is also of great importance to make it PEMFC economically
competitive in market. PdAu-black alloys and PdAu/C catalysts were more resistant to
CO poisoning and had higher catalytic activity at anode potential (0.5 — 0.1V) than
PtRu/C [53]. This result was obtained only at a temperature below 60°C and thus
optimization of PdAu/C is required for its catalytic improvement at relatively severe
condition.

The oxygen reduction at the cathode is very sensitive to the structure of catalysts
which is called as particle size effect [54]. According to the size effect, while a pure Pt
electrocatalyst size should be around 4~5nm for high activity, PtM (M 2 Cr, Ni, Mo, Co,
etc) is less affected by the size of active phases. Although some PtM catalysts have

shown better activity than Pt catalyst, the increase of the electrical efficiency in PEMFC

17

is less than 2 % and their durability is unproven [55]. Hence, recent researches focus on
improving utilization of Pt catalyst and more importantly developing much more active

oxygen reduction catalysts than Pt catalysts.

1.4 Direct Methanol Fuel Cell (DMFC)

DMFC is very attractive fuel cell for commercialization. Since DMFC consumes
liquid methanol fuel which is easily produced from natural gas or biomass, it has some
system-related advantages over PEMFC, making it of interest to fuel cell manufacturer.
The storage and transportation of methanol is less complicated and can be supplied
through the existing gasoline infrastructure. The simplicity of design and cost-
effectiveness are the main advantages of DMFC.

The principle of the DMFC is to directly oxidize methanol to carbon dioxide on the

anode and to reduce oxygen to water on the cathode as shown from (1.24) to (1.26).

 

At anode: CH3OH + H20 —> CO; + 6H+ + 6e' (1.24)
At cathode: 3/202 + 6H+ + 6e' ——) 3HzO (1.25)
Overall: CH3OH + 02 ——-> H20 + Heat & Electricity (1.26)

Although the thermodynamic characteristics of methanol oxidation at the anode are
similar to hydrogen oxidation in PEMFC, the methanol oxidation is a slow process even
on a high loading PtRu electrocatalysts. The thermodynamic cell potential in DMFC is
about 1. 185V. but in reality the cell potential is much lower [56]. Two slow reactions. i.e.

methanol oxidation and oxygen reduction. the presence of a mixed potential at the

18

cathode caused by methanol crossover. and the CO-like poisoning of electrocatalysts are
responsible for the reduction of the fuel consumption efficiency and the power density of
DMFC.

Two important reactions occurring at the electrodes and a unique phenomenon
(methanol crossover) happening only in DMFC will be described next . Research trends

and challenges to resolve the problems of DMFC will also be briefly mentioned.

1.4.1. Anode in DMFC

Methanol oxidation at the anode in DMFC is a complicated process with many steps
and many intermediates. The best catalyst to facilitate this oxidation process is Pt.
However. CO intermediates strongly adsorb and poison the surface of Pt. The CO
intermediates are removed when there are oxygenated species present on the Pt surface as

represented in the following:

CH3OH + Pt ———> Pt—COM, + 4H+ + 4e" (1.27)
H20 + Pt———) Pt-OHM, + H+ + e' (1.28)
Pt—COad, + Pt—OHads ——> C02 + 2Pt + H+ + e' (1.29)

Higher temperatures increase the rate of the oxidation and removal of CO
intermediate, hence DMFC is operated at higher temperature (80 ~ 150°C) than PEMFC.
To decrease the poisoning effect, bimetallic Pt catalysts can be used. Pt-Ru is the most
effective bimetallic catalyst for methanol oxidation. The mechanism for methanol

oxidation using PtRu is as follows:

19

CH3OH + Pt ——> Pt—COad, + 4H+ + 4e' (1.30)
H20 + Ru —> Ru— OH...“ + H+ + e- (1.31)

Pt — COM. + Ru—OHad, —-> C0; + Pt + Ru + H“ + e- (1.32)

Oxygenated species form on Ru at lower potentials and Ru decreases the amount of
CO intermediates formed on Pt [57, 58]. Additionally, Ru is much more stable than other
promoters under the conditions of DMFC operation [59]. Pt-Ru catalysts can be prepared
by numerous methods. such as chemical deposition [60], electrodeposition [61], sputter
deposition [62], and organometallic routes [63]. Each method results in a catalyst with
different properties due to different morphology, dispersion state, alloying state, particle
size, structure, and composition that influence catalytic activity. Catalyst activation also
influences fuel cell performance [64].

On the other hand, the oxidation state of Ru is in dispute. It is claimed that PtORuo,
pure metallic alloy, are much less active for methanol oxidation than a mixed phase alloy
PtRquHy [65] because the presence of hydrous oxides is expected to increase the rate of
methanol oxidation by promoting the formation of Ru-OHMS species that assist the
removal of CO species adsorbed on Pt. RquHy itself is a proton conductor. DMFC
anodes containing RquHy have been shown to require smaller amount of Nation in the
catalyst layer [66]. The optimal atomic ratio of Pt to Ru is another issue. It is commonly
held that a Pt70Ru30 composition is optimal for mixed alloys [67]. Although it is also
claimed by catalyst manufacturer Johnson Matthey that PtsoRu50 is the most active, it has
been found by lattice parameter studies that the mixed phase catalysts with smaller Ru

atom concentrations are desirable [68].

20

1.4.2. Cathodes in DMFC

The Nation membrane used in DMFC is normally the same one used in PEMFC. The
Nafion membrane was not optimized for DMFC but for PEMFC. Owing to the similar
properties of methanol to water, methanol diffuses from the anode to the cathode. This is

the so-called methanol crossover in DMFC. The transported methanol causes mixed

 

potential due to the interference of methanol oxidation with oxygen reduction, resulting
in a loss in cell performance [69]. As a result of methanol crossover, the cathode becomes
water logged or flooded and thus is no longer an efficient structure for gas diffusion. This
is another reason of potential loss in DMFC. Methanol crossover depends not only on the
membrane permeability but also the concentration of methanol and the performance of
the anode itself.

Most importantly, the membrane must be optimized for methanol blocking. This is
possible by controlling the membrane thickness or modifying the membrane to be less
permeable to methanol. For the latter, metal cations can be doped to alter the
microstructure of the membrane [70]. Methanol crossover can be reduced by fueling an
optimum concentration of methanol (1 .5~2M) [71]. Another approach to cope with
methanol crossover is to develop methanol-tolerant cathodes.

Poisoning of the Pt cathode catalyst by CO intermediates also occurs and in turn
reduces the cathode activity. In order to circumvent some of these issues, one can
optimize the electrode composition. High Pt loadings are employed to reduce the mixed
potential loss. A large degree of hydrophobicity is incorporated into the cathode catalyst

layer and backing to aid water removal.

21

1.4.3 Research challenges on Catalysts in DMFC

Carbon-supported metal based electrocatalysts are one of the most extensive research

topics for fuel cell application. The properties of carbon supports play an important role

in obtaining the maximum activity of the catalysts. However, this topic will be discussed

in next chapter.

1.4.3.1 Electrocatalystfor the anode

Since PtRu binary alloy catalysts tolerant to CO-like intermediates at the DMFC

anode was mentioned in section 1.4.2, other types of catalysts which have been studied to

promote the performance of PtRu catalysts or to replace them is described in this section.

1)

2)

One approach to develop new electrocatalysts tolerant to CO-like species is to
formulate PtRu with tungsten and molybdenum [72, 73]. Water is adsorbed
on the oxide form of tungsten and molybdenum and interacts with CO
adsorbed on Pt, thus facilitating CO oxidation to C02. Tungsten oxide and
molybdenum oxides have the ability to easily exchange oxidation states by
adsorbing hydroxyl ions from water and donate these species to methanol
residues adsorbed on Pt. Ternary PtRuW or PtRuMo alloy catalysts also
shows superior activity to methanol oxidation. Therefore, additional research
with these electrocatalysts is required in combination with various carbon
supports.

A novel route to increase methanol oxidation is to use heteropolyacids as an
enhancer [74]. Heteropolyacids was proven to enhance the water discharging

process and the oxidation of CO species strongly bonded to Pt due to a

22

number of factors such as redox behavior of heteropolyacids. They can be
anchored on the surface of carbon supports. Heteropolyacids—anchored carbon
supported PtRu catalysts or PtRu electrocatalysts promoted with
heteropolyacids will be the excellent research topics for improving methanol
electro-oxidation.

3) Non-noble metal catalysts are the most desirable alternative to Pt-based
catalysts in that they can solve the high cost problem of materials employed
in DMFC. Transition metal oxide catalysts are candidates. Mo/MoOx and
W/WOx turn out to have better ability to coordinate water and to generate
mobile hydroxyl species than Pt/PtO [75]. Hence it is worth exploring the
possibility of tungsten oxide and molybdenum oxide as new electrocatalys for

DMFC anodes.

1.4.3.2 Electrocatalyst for the cathode

Methanol oxidation and oxygen reduction should be considered at the same time to
develop the best electrocatalysts for DMFC cathode [76]. Both reactions are affected by
the particle size of electrocatalysts. If the particle size of the electrocatalysts is smaller
than 3nm, the catalytic activity for oxygen reduction will be lower but the catalysts will
be less poisonable, vice versa. Thus, the best compromise is to control the structure and
the particle size in order to increase oxygen reduction and decrease methanol oxidation.
Another is to use a promoter to enhance oxygen reduction and simultaneously hinder

methanol chemisorption.

23

Non-noble metal based catalysts are always desirable for the same reason as in the
anode. Transition metal sulfides [77] and organic transition metal complexes [78, 79] are

proven to be good candidates.

1.5 Direct Ethanol Fuel Cells (DEFC)

DMFC has been actively investigated over many years and now it is regarded as a
promising power source for mobile, stationary and portable application. However,
although methanol is thought as a good fuel for low temperature fuel cells, it has some
drawbacks. Methanol is relatively toxic and has a low boiling point. Therefore, other
alcohols available from biomass resources are being considered as alternative fuels. In
addition, it is still desirable to increase the number of liquid fuels for fuel cell systems in
a bid to extend their practical applications and to facilitate their commercialization.

Low—weight molecular chemicals such as ethanol, propanol, and formic acid are
good candidates for low temperature fuel cell [80, 81]. Ethylene glycol and glycerol are
also considered for fuel cells [82]. Among them, ethanol is the most attractive. Ethanol is
renewable, safe, convenient, and less toxic compared to methanol and it can be produced
in large quantities from sugar—containing biomass sources. DEFC using ethanol as a fuel
appears to be another promising low temperature fuel cell and thus this section is devoted

to introduce the reaction mechanism and the role ofelectrocatalysts in DEFC.
1.5.1 Reaction Mechanism

The complete ethanol oxidation at anode, oxygen reduction on cathode, and overall

reaction in DEFC can be described as follows [83]:

24

Anode: CH3CH20H + 3H20 —> 2C0; + 12H+ + 126—
Cathode: 302 + 12H+ +l2e' —) 6HzO

Overall: CH3CH20H + 302 —> 2COg + 3H20

(1.33)
(1.34)

(1.35)

Standard potential of overall reaction is 1.145V which is similar to DMFC’s potential

(1.185V). The complete anodic oxidation ofethanol involves 12 electrons per molecule

and the cleavages of CC bond [84]. The latter is an important factor of dominating the

fuel efficiency and the yield of electrical energy. Many CO-like intermediates and

products are also involved in anodic oxidation. This CO-like intermediates poisons the

catalyst sites and thus need to be removed.

A detailed reaction mechanism of ethanol oxidation on Pt catalyst depending on

electrode potential can be formulated as follows [85]:

CH3-CH20H ——> CH3-CHO + 2H+ + 2e'

Pt + CH3CHO —) Pt—(CO-CHQW + H+ + e-

Pt + Pt-(CO-CH3)ads -—> Pt-(CO)mr,~ + Pt-(CH3)...,,.
Pt + H20 ——> Pt-Had, + Pt-OHM,

Pt-(CH3)ud_, + Pt-Had, ——> CH4

Pt-(CO).,.1,,. + Pt-OHM, ——> C0 + H+ + e’

(1.36)
(1.37)
(1.38)
(1.39)
(1.40)

(1.41)

Both poisoning species and some traces of methane are produced. Hence, the

development of more active and selective electrocatalysts having the ability to break the

C-C bond and to oxidize CO is required.

25

1.5.2. Electrocatalysts for DEFC

Pt is a good electrocatalyst for DEFC but not the best one. The addition of Ru to Pt
results in the decrease of the electrocatalytic activity towards ethanol oxidation due to the
inactivity of Ru toward ethanol. Other alloys such as PtRuSn and PtSn show very good
efficiency for the anodic oxidation of ethanol. The presence of Sn not only promotes the
cleavage of C-C bond but also oxidizes CO-like poisoning species to CO in DEFC [86].
This is true for other alcohol fuel cell using ethylene glycol, glycerol, and 2-propanol.

For DEFC, the effect of catalyst surface structure, metallic particle size, and other
factors on the ethanol. oxidation at anode must be systematically investigated. For
practical application, carbon supported Pt-based catalysts containing Sn are used in order
to stabilize the catalyst particles and to reduce the amount of precious metal. In this case,
the interaction between metallic particles and supports are a critical parameter on the
efficiency of the anodic oxidation of ethanol and thus is included in one of research
topics.

The negative effect of ethanol crossover on cell performance in DEFC is smaller
than methanol crossover in DMFC. Hence, highly active and effective anode catalysts

will speed up the commercialization of DEFC.

1.6 Direct Formic Acid Fuel Cells (DF AFC)

Formic acid is not explosive and easy to store. Formic acid has high theoretical
potential (1 .45V) and shows negligible fuel crossover due to the repulsion between the
partially dissociated formate anion (HCOOH) and sulfonic groups in the Nafion polymer

membrane. Low contact resistance of formic acid as electrolyte is another significant

26

advantage over other liquid fuels [87-89]. Therefore. formic acid is a potential liquid fuel
for fuel cell application.
Formic acid oxidation at anode based on traditional Pt—based catalysts proceeds via
direct path way or indirect path way (CO path) in the following equations [90, 91]:
1) Direct oxidation
Pt + HCOOH —> X —> Pt + C02 + 2H+ + 2e‘ (1.42)

2) Indirect oxidation (CO path)

Pt + HCOOH —) X —> Pt-CO + H2O (1.43)
Pt + H2O——> Pt-OH + H+ + e' (1.44)
Pt-CO + Pt-OH —> 2Pt + CO2 + H+ + e' (1.45)

In the direct path. HCOOH is directly oxidized to CO2 via one or more active
intermediates and generates two electrons. In the CO path, HCOOH is dehydrated to CO
depending on the applied potential. CO remains on the surface of Pt particles and thus it
must be removed by activating water. However, because the water activation reaction
(1.44) is difficult. Pt catalysts become poisoned by CO and consequently overall activity
is reduced.

Pt catalysts modified with Pd, Bi, and Sb have been studied to reduce CO poisoning
and increase the catalytic activity [92~94]. Among them, Pth catalysts showed the
highest synergistic effect on both CO poisoning and the activity. Very recently, it turns
out that Pd oxidizes HCOOH via a direct pathway, resulting in little or no CO poisoning
effect on Pd particles and higher fuel cell performance than Pt-based catalysts [95].
However, a better synthetic method to prepare tine Pd nanoparticles black or supported

on carbons will provide much potential of DFAFC in real commercialization.

27

l 0.

ll.

12.

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36

CHAPTER 2
BASICS ON MECHANISM OF NANOPARTICLE
FORMATION, IONIC LIQUIDS, MICROWAVE AND

POLYOL PROCESS

37

2.1. Introduction

In addition to the application for low temperature fuel cells, various synthesized
carbon- or metal oxide-supported transition metal catalysts have a wide application
window which includes energy generation/storage systems such as supercapacitors,
rechargeable batteries and hydrogen generation and hydrogen storage. In this research,
unsupported and carbon supported precious metal nanoparticles have been synthesized by
microwave dielectric heating process of metal precursors in a polyol solution in the
presence of a room temperature ionic liquid, which is a fast, simple, cost-effective, and
consistent to produce very fine metal particles. Hence, a review of the formation
mechanism of metal nanoparticles, the microwave heating process, the role of the polyol,

and ionic liquids are necessary to understand the results of this research.

2.2. Formation Mechanisms of Nanostructured Metal Particles

All existing techniques for producing nanostructured metallic particles can be
grouped into top-down methods [1, 2], phase transformation methods [3], or bottom—up
methods [4, 5]. The top-down method consists of reducing the size of bulk metals to the
desired dimensions of metal particles. Mechanical grinding of the solid metals and high
vacuum sputtering are the representative techniques of the top-down methods. Although
it is difficult to obtain monodispersed metal particles especially when high loading is
needed and to achieve a uniform composition among different metal nanoparticles for
specific applications, this method is convenient for rapid screening of mixed metal
catalysts [6]. The phase transformation method is to convert finely divided metal

compounds into metal through mostly thermolysis or pyrolysis. This process is useful to

38

obtain the uniform metal particles of different shapes. The bottom-up method is the wet
chemical reduction of metal salts or the decomposition of metastable organometallic
compounds. The metallic particles are constructed from the metal atoms. The nucleation,
particle growth, and the interaction between particles in this process can be controlled in
a liquid to generate monodispersed metal nanoparticles. Thus, this wet chemical, bottom-
up process is the most versatile, economical, and easiest to perform procedure [7].

The general mechanism for the synthesis of nanosized metallic particles will be
discussed in this section. Another objective of this section is to describe briefly how the
mechanism can be manipulated to control the size, shape, composition, structure, and

surface properties of metal nanoparticles as well as their dispersion.

2.2.1 General Mechanism

The successive process for the formation of metallic particles by wet chemical
precipitation is illustrated in Figure 2.1.The formation of metallic particles begins with
the electron transfer from a reducing agent (R) to the oxidized metallic species (M),
which reduces the metal salt to give zero valent metal atoms. The driving force of the
electron transfer is related to the standard redox potentials (5”) of the metal species and

the reducing agent associated with the reaction according to the following relation.

AG” 2 —nFAE0 (2.1)

where G, n, and F have the usual meaning. The reduction reaction starts only if the redox
potential difference (A5“) of the reducing agent with reference to the metal species

becomes positive. Thus. the reducing agent should have a more negative value than that

39

of metallic species for the reduction reaction and the difference must be larger than 0.3V
to be practical. An increase of AB” results in a more rapid generation of metal atoms in
the liquid phase. leading to higher supersaturation concentration and consequently faster
nucleation.

The metal atoms formed by the reduction reaction are insoluble in solution and their
lifetimes are very short in solution. The metal atoms tend to quickly collide with each
other or with metal ions and coalesce into clusters which are called embryos [8]. As new
metal atoms are generated in the system, embryos eventually reach a critical size and
form the nuclei, which are the frrst stable entities of the metallic phase in the system.
Nucleation can be very rarely the final stage of metal particles unless extremely special
precautions are taken. The number of the nuclei and its size depend on many
experimental parameters such as the concentration of metal salts or complexes, redox
potential of the reduction reaction, reaction temperature, the properties and the
concentration of protecting agents, and the properties of solvent such as viscosity. Nuclei
with diameter less than lnm depend on the difference between the redox potential of the
metal salt and the reducing agent applied and the strength of the metal-metal bonds [9].
Once formed, the nuclei undergo rapid diffusional growth to unstable primary particles
(PP) by further addition of the remaining metal atoms. As more metal atoms are provided
in the system, the metal atoms continuously diffuse onto the primary particles by slow
diffusion to submicrometer sized metal particles or already formed particles undergo fast

aggregation to form larger particles up to several micrometers.

40

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

;' Metal Salts (M+X) l
g Reduction ' Oxidation g
Insolusble E Metal Atom Holnogeneous
Shortllfetime ;' Mn+ + Rm‘-9 M0 + 0x"”' Ephase
Quick collision each 3 ,
other or with metal idns ' I E > Spontaneous
l 5 process
E Embryos E
xMO —»( Mitt)”,
. ...... l[ A,"
. ........ ---- i ““l.
1'7 irst stable entity ‘ Nuclei (~1l1m) Heterogeneous “‘\
lery rarejinal stage ( M x0)em + y M0 _) (M0x+y)nucl phase j E
l 1 Rapid diffusion g
5 Primary Particles
E (Mn0)nud+ mMO —=>(M0m+,,)pp 5
l Slow diffusiWapid coagulation E
Primary Particles (<1 um) Primary Particles (>1 pm) 5
"r. (1140)”, + nMO —» (M0), m(M0)pp a (Mo),

 

 

 

 

 

 

\
\

\ a
h. '
........... — -'

Figure 2.1. The successive process for the formation of metal particles by wet
chemical reaction (modified from [10]).

It is normally expected that the monodispersed metal particles are formed by slow
diffusional growth. However, it has been found that uniform metal particles are formed
by the mechanism of rapid aggregation, not by slow diffusional growth which produces
only amorphous metal particles [1 1-13]. The aggregation process must be captured in the
early stage of particle formation in order to produce nanosized uniform metal particles.
Capping materials such as ligands, polymers, and surfactants are frequently used to
prevent agglomeration as well as stabilize nanosized metals [14-16]. Both mechanisms
also have an effect on the crystallinity, internal structure, density and shape of the metal

particles. The former favors the formation of metal crystal with few irregularities in their

41

lattice and internal grain boundary. The latter creates mostly spherical and polycrystalline

metal crystals with large internal grain boundaries and low density.

2.2.2 How to Control Particle Size?

The ability of producing nanoparticles with a narrow size distribution is a key feature
of modern synthetic methods. Although the kinetics of the particle nucleation from metal
atoms and of the subsequent growth process can not be directly observed by physical
methods, in practice, the size of metal particles can be manipulated by controlling the
essential factors at the nucleation and particle growth stages for the separation of those
two stages. The factors include the nature and concentration of metal salt and reducing
agent [17—20], the presence of protecting agent [21]. the reaction time [22]. and the

reaction temperature [23].

2.2.2.1 Metal salts and reducing agent

It was mentioned in previous section that there are two routes for particle growth: 1)
by the attachment of metal atoms onto already formed particles and 2) aggregation of
already formed particles [24]. Both mechanisms take place at the same time in a real
system. The solute concentration in the nucleation stage can determine the final size of
metal particles [25]. High supersaturation of metal atoms generates a large number of
nuclei consuming a major fraction of the metal salts. If the aggregation is inhibited, the
increase of particle size reflects diffusional growth by exhaustion of the remaining small
fraction of metallic salts or complexes in solution and the final particles remain in the

nanosize range. Thus. strong reducing agents are recommended to create a much large

42

number of nuclei if nanosized metal particles are desired. If a small number of nuclei is
generated with the consumption of a small fraction of metal salts, the particle grows until
the remaining large fraction of metal, salts are used up and thus larger particles are
produced.

The high value of AE affects the spontaneity of the reaction and thus the generation
of nuclei. As AE of the reaction increase, more nuclei are generated, which lead to the
smaller metallic particles. Strong agents are usually recommended. However, a large AE
is not sufficient condition to achieve such uniform small metallic particles. Instead, other
parameters which may affect the aggregation play a more important role in determining

the final size of metal particles and their dispersion.

2.2.2.2. Temperature
The effect of temperature on the size of metallic particles was studied by
synthesizing gold nanoparticles on carbon support and the steady-state size of the particle

is related to the diffusion activation energy given by [23]:

N sinl9 E -
R max = TO——exp[— —0—Q) (2.2)
apsv RT

where 6 is the angle that specified the degree of contact of the particle with the support.
N0 is the steady state concentration of the metal atoms on the surface, arepresents the
fraction of the exposed surface area. ps the surface density of the particle, v is the mean
metal-metal vibration frequency (v = kTD/h, To = Debye temperature), Q is the
desorption activation energy. As expected in Equation (2.2), the size of metallic particles

continued to increase as temperature increased until a certain range was achieved, above

43

which the particle size did not further increase. The nucleation of the small particles
occurred at a specific temperature range which was independent of the supports and the

density of the nucleation site did not increase with temperature.

2.2.2.3. Protecting agents

Ostwald ripening is the phenomenon by which larger particles attracts smaller
particles during the growth process [26-28]. This agglomeration occurs at any stage
during the synthesis of metal particles. To prevent the Ostwald ripening. the stabilization
of metallic particles is required.

There are generally two approaches: 1) electrostatic repulsion resulting from the
chemisorption of charged species (H+ or 0H) at the metal surface and 2) steric repulsion
between particles caused by protecting agents such as surfactants, polymers, or other
organics bound to the metal surface. The stabilization by steric repulsion is more
common due to issues concerning the chemical stability of the metal particles at various
pH values. Protecting agents play a role in stabilizing nanostructured metal particles and
preventing their agglomeration [29]. In this way, protecting materials prevent aggregation

and dictate the final size of metal particles.

2.3 Polyol Process

Because of its simplicity and advantage over most other wet chemical solution
methods, the polyol mediated process becomes one of the most common processes to
prepare nanoscaled metal and metal alloy particles. The polyols have mild reducing

properties and act as solvent with a chelating effect which prevents agglomeration of

particles to some extent during their preparation. Materials obtained from this method

show homogeneous phase composition and narrow size distribution. Owing to these

features, the polyol process was selected as the main solvent for the synthesis of carbon-

supported metal catalysts on which this research focuses. Hence. general aspects of

polyol process will be briefly summarized in this section.

2.3.1. Characteristics of Polyol Process

Polyols such as ethylene glycol, diethylene glycol, triethylene glycol, or mixture of

them) have the following characteristic features for synthesizing nanosized functional

materials composed of noble metals (Au, Pt, Pd, Ru) [30-32], less noble metals (Fe. Co,

Ni. Bi) [33-34], and other nanoscale elemental metals [35—36].

1)

2)

3)

4)

5)

Metal salts and complexes are completely dissolved in the polyols due to the
high polarity of the polyols (8>30) [37].

Nucleation and growth of metal particles can be performed at high
temperatures up to the boiling point of the polyols. Hence, well-crystallized
metal particles can be synthesized [38].

The polyols are low weight molecules. Hence, they may act as weak
stabilizers and can be removed from the surface of metal particles under mild
experimental conditions [39].

The chelation of the solid nuclei by the polyols limits the particle growth as
well as prevents agglomeration of particles [39].

The synthesis in the polyols is comparably easy to perform and suited for

manufacturing large quantities of nanoscale materials.

45

Simply, the polyol-meditated process is a versatile and efficient strategy to produce

nanoscale compounds and/or materials.

2.3.2 General Mechanism

At first, the polyols act as a solvent to dissolve the starting compound due to its high
polarity. Subsequently. the polyols reduce the metal species and then act as the crystal
growth medium for the metal particles.

The formation of metal particles in polyols has two steps [40]. In the first stage,
metal hydroxide crystallizes followed by the precipitation of the intermediate phase by
progressive dissolution of the starting hydroxide and evolution of water. In the second
stage, the intermediate phase dissolves and then is reduced in solution, followed by
homogeneous nucleation and growth of metal particles. The general mechanism of metal
formation in ethylene glycol can be represented by the following reactions.

2CH20H —CH20H —> 2CH3CH0+2H20 (2.3)
2CH3CH0+M(0H)2——>CH3—C0—C0—CH3+2H20+M (2.4)

The reducing action of the polyols can be different for different metal species and the
intermediate solid phase is not a general feature. However, the mechanism of metal
particles via nucleation and growth from the solution is general regardless of meal
species in polyol process.

Recently. a modified mechanism was proposed to explain the stability of noble metal
particles (Pt and Ru) in ethylene glycol in the presence of OH- as follows [41].

2CH,0H — CHQOH —2 2CH3CH0 + 2H,o (2.5)

201,010 +(PtC16)'2 + 60H“ —2 2CH3C00‘ + Pr + 6Cl‘ + 4H,0 (2.6)

46

In this mechanism, hydroxide ions are necessary for the generation of acetate ions which

have an effect on the stabilization of noble metal nanoparticles in ethylene glycol.

2.3.3 Control of Particle Size .

2.2.3.1 Particle size in homogeneous nucleation.

If the nuclei in the nucleation stage form spontaneously without the promoting effect
of impurities, the nucleation is called homogeneous. The formed nuclei grow into larger
particles. This growth continues as long as the meal atoms are provided by the reduction.
To obtain uniform size of metal particles, as described in section 2.1.2, nucleation and
growth must be completely separated. In the polyol process, the polyol itself acts as an
protecting agent for coagulation to some extent.

The final size of the metal particles in polyol process can be varied by mainly
controlling temperature and the content of metal compounds or salts. The particle size
usually decreases as the reaction temperature increases in polyol process. If the reaction
temperature is increased, the number of nuclei formed during the short nucleation stage
increase due to faster dissolution of the intermediate phase and faster reduction in
solution. Consequently the average particle size decreases for a given amount of
precipitated metal.

When the metal compound/polyol ratio increases, the particle size significantly
increases. At a given temperature, the number of nuclei generated in the short and
spontaneous nucleation stage is independent of the quantity of metal compounds added to
the system. When the amount of metal compounds is increased, the number of particles

grown from the same number of nuclei remains constant and their size increases.

47

2.3.3.2 Particle size in heterogeneous nucleation

In order to separate the nucleation step from the growth step more effectively and to
control the number of nuclei in a better way, two modifications have been made in the
polyol process: (1) addition of external foreign particles and (2) formation heterogeneous
nuclei in situ [42, 43]. An example of the former is to introduce 3nm Pd nanoparticles
before the spontaneous nucleation of cobalt in an ethylene glycol-diethylene glycol
mixture. Reduction of a mixture containing a metal precursor of interest with a small
amount of silver nitrate is the example of latter. Because silver nitrate is easily reduced in
polyols, tiny particles of silver metal form more rapidly and then act as nuclei for the

subsequent growth of the metal particles.

2.4. Microwave Process

Recently, microwave dielectric heating process has been applied to organic
chemistry in the presence of catalysts [44-47] and for the preparation of metallic
nanostructures [48-55]. Microwave application is desired for rapid and reliable
production of new organic and inorganic entities as well as for homogeneous and
heterogeneous catalysis. Hence, the principle of microwave heating, the general effects
created in microwaves, and the advantages of microwave heating in the synthetic process

will be described in this section.

2.4.1 Fundamental Theory of Microwave

Microwaves are electromagnetic waves consisting of electric and magnetic field

components. The electric field forces charged particles or materials with molecular

48

dipoles to start to migrate or rotate, which causes their polarization. The combined forces
of electric and magnetic components in microwaves result in rapid rotation of the
molecular dipoles. Since the assembly of molecules cannot respond instantaneously to the
changing direction of the electric field, energy in the form of heat is generated. Heat
generated from materials with the dipoles is a result from internal resistance to the

rotation under microwave irradiation. The principle of microwave heating of a medium,

H2O as an example, is shown in Figure 2.2 [56].

 

 

O9§+§§Q§§ ‘§9’+*§§§§ —

Figure 2.2. Microwave dielectric heating mechanism of H20 [57].

The frequency of microwave for microwave dielectric heating is in the range of
918MHz and 2.45GHz. The latter is used most often and applied in domestic microwave

ovens. The microwave power dissipation in a medium is given by the following equations.
p = 2741512 fs" = 27211112 fe' tan 5 (2.7)
tan 6 = e"/e‘ (2.8)
where f is radiation frequency, 8’ is the relative permittivity which is a measure of the
ability of a molecule to be polarized under an electric field (E), e” is the dielectric loss
which represents the ability of a medium to convert dielectric energy to heat, and tan§ is

the dielectric loss tangent or the energy dissipation factor which indicates the ability of a

material to convert electromagnetic energy into heat energy at a given temperature and

49

frequency. The value of tan 5 depends on the frequency of microwave, the temperature.
and the physical state and composition of the mixture. It is obvious from the equations
that e” or ran 5 is the most important parameter describing the ability of a medium to heat
in microwave [58]. A medium with high tan 6 or e” is preferred for the reaction in
microwaves. The value of 8’, 8”, and tan 5 for a series of common solvents at room
temperature are listed in Table 2.1. Ethylene glycol is one of the best solvents for the
rapid synthesis of metallic nanostructures through microwave dielectric heating due to its

high value of 8” compared to other solvents.

Table 2.1. Physical properties for common pure solvents at room temperature [59]

 

 

Solvent B.p (0C) 8' 8” tan 5
Dichloro methane 40 9. 1 0.38 0.042
Tetrahydrofuran 66 7.6 0.36 0.047
Acetone 57 21 1.13 0.054
Ethyl acetate 77 6 0.35 0.059
Ethanol 78 24.3 4.86 0.2
Methanol 65 32.7 20.9 0.639
Dimethyl formamide 153 36.7 5.87 0.16
Methyl pyrrolidone 202 32 8.86 0.277
Ethylene glycol 198 38 44.46 1.17
Water 100 80 12.56 0.1 14

 

The rate of microwave dielectric heating depends on not only tan 5 but also the size
and quantity of the reaction mixture and on the heat capacity of the medium. Microwaves
impinging on the volume of the mixture undergo a loss of their penetration depth into the

mixture. The penetration depth (d) is related to tan 5 given by the following equation [60].

50

 

0
d = low/8'76” or d = 26;?" (2.9)

where in is the wavelength of the microwaves and e” the dielectric constant in free space.
As mentioned previously, a reaction medium or mixture should have a high tan§ for

efficient absorption and consequently for rapid heating. Tan 5 is proportional to the
polarizability as well as the electrical conductivity of the reaction medium. Thus, the
presence of carbon materials will be helpful to accelerate the rate of microwave dielectric
heating. In addition, the strong absorbing properties of carbons reduce any possible the
loss of microwave penetration into the medium when larger volume is used in the scale-

up of this process.

2.4.2 Effects of Microwave Dielectric Heating

Effects of microwave dielectric heating can be divided into two kinds: thermal
effects and non-thermal effects [61]. The former arise from the different temperature
regime under microwave dielectric heating, the latter is caused by effects inherent to the

microwaves.

2.4.2.1 Thermal Effects

1) Rapid Heating and Superheating

The factors which play an important role in microwave heating are superheating over
boiling points of the medium in a large amount of ions, fast achievement of the reaction

temperature, and efficient mixing and boundary effects.

51

Rapid microwave heating accelerates the reduction of metal salts and the nucleation
of the metal cluster, which provides metallic nanoparticles with a narrow size distribution.
The rate of acceleration can be further increased if microwave energy is absorbed by the
reactant themselves. The uniformity of the temperature and lack of a thermal gradient is a
direct result from the homogeneous microwave power dissipation through a medium and
contributes to the uniform nucleation and growth conditions, leading to monodispersed
metallic particles. The concerted effects of superheating and rapid and uniform heating
make it possible to produce monodispersed small metallic particles with better

crystallinity.

2) Hot spot and hot surface

If solids heated by microwave are present in the reaction, there are dramatic effects
with respect to heating rate. Hot spots are created on the solid-liquid surfaces. Hot surface
can be created on the surface of the metals or other solids by adsorption of organic
materials with large polarizability. The formation of hot spots and hot surfaces
accelerates the reduction of metal precursors and the nucleation of metal cluster, leading

to monodispersed uniform nanostructured materials.

3) Pressure effect

The reaction yield carried out in a sealed reactor under a conventional heat was
comparable to those carried out in a microwave oven. This result illustrates that the
temperature effect due to the build up of pressure in the sealed reactor accounts for the

effects observed during microwave heating [62].

52

2.4.2.2 None-thermal Effects

Non-thermal effects are ones which occur under the same temperature profiles of

solvent between microwave heating and conventional heating during the reaction. There

is a debate concerning if microwave, irradiation of a reaction mixture induces microwave

activation (non-thermal effect) or just thermal acceleration of the chemical reaction. This

is a controversial issue and difficult to resolve [63].

2.4.3. Advantages of Microwave Process

The main advantages of microwave dielectric heating the synthesis of especially

inorganic nanostructured materials can be summarized:

.1 )

2)

3)

4)

5)

6)

Microwave heating increase the kinetics of the reaction up to l~2 orders of
magnitude [64, 65].

Microwave heating induces uniform heating of the solution, leading to more
homogeneous nucleation and a short crystallization time.

Microwave heating process is energy-effective due to initial rapid heating to
reaction temperature [66].

Microwave heating creates localized hot spots at the reaction sites on the surface
of the nanoparticles which enhances the reduction rate of metallic ions.
Microwave process is easy to control due to the absence of convection process
and cost-effective.

Microwave process makes it possible to synthesize nanostructured materials in

one-spot reaction.

53

7) Microwave heating process creates superheating of solvents over their boiling
points which is beneficial to produce highly crystallized materials.

8) Microwave process can be adjusted to control morphologies of nanostructured
materials by changing experimental factors such as the solvents, the reaction time

and temperature. and the concentration of metal precursors and capping materials.

2.4.4 Microwave Process for Metal/Carbon Nanocomposites

There are many recent studies on carbon-supported noble metal or metal oxide
nanoparticles synthesized by microwave-polyol process. Liu et al reported that Pt, Ru,
and PtRu bimetallic nanoparticles uniformly dispersed on carbon black could be prepared
by a microwave-assisted polyol process [67, 68]. The particle size of PtRu nanoparticles
on carbon black was 2~6nm in diameter and they showed a higher and more durable
electrocatalytic activity for methanol oxidation than a commercial catalyst. The authors
concluded that the fast microwave heating accelerates the reduction of the metal
precursor and the nucleation of small metal clusters, resulting in smaller particles with
narrow size distribution on carbon black. It was also proposed that the surface functional
groups such as carboxyl. hydroxyl, and carbonyl groups on the carbon interact with Pt
and Ru ions and thus act as a nucleation precursor that finally reduced to produce PtRu
nanoparticles on the carbon. The function of the carbon presence interrupting PtRu
nanoparticle agglomeration was also emphasized.

Steigerwalt et al reported that PtRu nanoparticles were deposited on graphite
nanofibers by using a single source molecular precursor of PtRu in microwave process

[69]. They insisted that microwave heating process minimized metal diffusion on the

54

carbon supports and thus affords PtRu/graphite nanofiber containing 50wt. % total metal
with average metal particle sizes of only 4-5nm.

Effect of pH on particle size and electrocatalytic activity for methanol oxidation was
evaluated for Pt/CNT nanocomposites prepared by microwave process [70]. The mean
size of Pt particles decreased with the increase of pH. Pd nanoparticles deposited on
carbon black by applying microwave irradiation was also studied [71]. Depending on the
Pd precursors, the size of Pd particles deposited on carbon black ranged from 8 to 10nm
and from 10 to 75nm by 503 and 1003 microwave irradiation, respectively. The carbon
surface can be functionalized with heteroatoms such as oxygen, nitrogen and hydrogen
which are sites suitable for heterogeneous nucleation of metal precursors. Carbons absorb
microwave well. resulting in uniform hot surfaces and hot spots. The presence of carbon
surface interrupts the growth of metal particles, preventing possible coagulation and
flocculation of metal clusters. Hence, microwave dielectric heating can be employed to

produce carbon-supported metal catalysts.

2.5. Ionic Liquid for the Synthesis of Metallic Particles

Ionic liquids are organic salts with low melting points and high polarity [72]. The
ionic liquids are finding an increasing number of applications in catalysis [73],
electrochemistry [74], polymer synthesis [75], and enzymatic reactions [76]. Another
new intriguing application of ionic liquid is the synthesis of functional nanoparticles and
other inorganic nanostructured materials [77]. Since ionic liquids have been used in the
works for this thesis, their characteristics and synergistic effects expected when it is

applied in microwave heating will be briefly described in this section.

55

2.5.1 Characteristics of Ionic Liquids

Ionic liquids (ILs) act as a solvent and stabilizer in the synthesis of very fine Ru and

Ir metal nanoparticles [78]. The Ru and Ir system is very stable and shows high catalytic

activity in catalytic hydrogenation. Thus a lot of advantages of ionic liquids for the

synthesis of metallic nanoparticles can be considered.

1)

2)

3)

ILs is classified as a kind of molten salts. While the melting points of
common salts like NaCl are very high, ILs have low melting points. One
important property of ILs is their low melting points due to low symmetry
[79], week intermolecular interaction [80], an increasing size of anion [81],
and a good distribution of charges in the cation [82]. Hence, without any
special caution or pretreatment, ILs can be directly added to the polyol system
and they are completely dissolved, resulting in a miscible and homogeneous
ILs-polyols solution.

Since ILs have low surface tensions in spite of their high polarity, they can
easily adsorb to other phase, which will weaken Ostwald ripening in the
growth stage of metallic particle leading to the generation of very small
particles. In addition, it is expected to accelerate the nucleation rate due to hot
spot and hot surface effects created at the solid-liquid interface in microwave
heating, resulting in the smaller particles. Low interfacial energy of ILs can
be translated into stabilization of molecular or metal species.

ILs has high thermal stability. Some ILs is stable up to more than 400°C [83].

Hence, reactions to produce nanostructured materials can be conducted at

high temperature beyond 100°C in non-pressurized reactors. Additionally.

56

4)

5)

owing to their thermal stability, ILs is not decomposed even at high
temperature environment created in microwave-polyol heating process and
thus has no detrimental effect on the final products.

High polarity of IL will [increase the heating rate to desired reaction
temperature in microwave process, leading to accelerate the nucleation rate of
metal species and consequently produce smaller particles.

ILs have other unconventional and very rare properties which may be useful
for the synthesis of functional materials and other inorganic materials: the
ability of forming extended hydrogen-bonding and the properties as a
supramolecular solvent [84]. Those features are the molecular basis of most
molecular recognition and self-assembly process, which can be another

application of IL-assisted microwave process.

2.5.2 Application of [LS for Metal/Carbon Nanocomposites

Metal nanoparticles should be stabilized against agglomeration into larger particles.

The usual method is to use capping materials such as polymer, surfactants, or
polyoxoanions for the stabilization of metal nanoparticles through electrostatic or steric
repulsion. Two groups have reported the use of IL as a stabilizer and a polar solvent for
wet chemical synthesis [85, 86]. Very recently, one group used a room temperature ionic
liquid as a solvent for the impregnation preparation of PtRu/carbon black catalyst [87].
PtRu/carbon black catalyst synthesized with ionic liquids showed high electrocatalytic
activity for methanol oxidation. The author insisted that IL can be recycled and synthetic

procedure is simple compared to other impregnation methods.

57

In spite of advantageous features in ILs for microwave heating process, there is no
report on the application of ILs to synthesize supported transition metal catalysts by using

a microwave heating process. Thus. this will be an intriguing research topic.

58

9.

10.

11.

2.6 Bibliography

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E. Matijevic. “Uniform inorganic colloid dispersions. Achievements and challenges”,
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59

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66

CHAPTER 3

CARBON SUPPORTED METAL CATALYSTS

67

3.1. Introduction

Catalysis is of vital importance to not only fine chemical industry but also fuel cell
industry and hydrogen economy. Most chemical processes need a catalyst and
electrochemically active catalysts are inevitable in fuel cell systems. Many techniques of
producing hydrogen from renewable and sustainable resources are conducted only with
the addition of a specific catalyst.

Solid supported catalysts are of special interest. A high activity of a catalyst calls for
a large active surface area and high dispersion of the active phase. Small metallic
particles meet these requirements. However, small particles tend to easily sinter at
relatively low temperature. This problem is resolve by applying small particles onto the
support surface. As a result of decorating the surface of the support with small metallic
particles, they can be thermally stable and maintain a high specific surface are up to high
temperature [1]. Supports allow for the fine dispersion and stabilization of metallic
particles, making it possible for the reactant to access to a much larger number of
catalytically active sites than in the corresponding bulk metals.

Catalytic activities of a supported catalyst depend on a number of factors: the size
and shape of metallic particles and the specific surface area, pore size distribution, pore
volume, density of supports. Mechanical and electrical properties of supports are also an
important factor. Another critical factor which must be taken into account in most
catalyzed reactions is the interaction between metallic particles and supports. Due to this
interaction, the catalytic performance strongly depends on a complex contribution of the

dispersion of the active phases and the changed electronic properties of the metal [2].

68

Hence, a survey of the literature was focused on supported catalysts, especially
carbon-supported precious metal catalysts, for they are a key material in a fuel cell
system, and can be applied for other next generation energy/power sources. In this
section, the basic principles of catalyst synthesis including the surface chemistry of the
carbon supports and interaction of the carbon surface with the metal precursor are
reviewed. Synthetic routes for carbon-supported metal catalysts are also summarized.

followed by catalyst design and recent trends of carbon-supported catalysts.

3.2 Basic Principles of Catalyst Synthesis

Support materials are selected on the basis of desirable characteristics such as
inertness, stability. porosity, surface area. mechanical properties. and physical form [3].
Among a variety of possible supports, only three materials meet these characteristics,
which are alumina, silica, and carbons. Carbons are more convenient than alumina and
silica because of their resistance to acidic or basic media, stability at high temperatures.
low cost compared to other supports, easy recovery of metals from spent catalyst by
burning away the carbon support, and surface flexibility for tailoring catalyst properties
to specific needs [4].

Properties of carbons which are of importance to catalyst preparation and application
include composition, particle size. shape and distribution, surface area, functional groups,
pore shape and size distribution, microstructure, oxidation and corrosion resistance,
electrical conductivity and hydrophobicity [5]. Mineral composition and content are
dependent on the raw material source. This will affect the hardness of the carbon, the

surface area. the redox properties and the acid/base properties. Particle size, shape and

69

distribution can affect filterability and filtration rate as well as catalyst activity. Surface
area. surface groupings, pore size distribution and pore shape affect catalyst preparation
and catalyst activity. Oxidation and corrosion resistance, electrical conductivity and
hydrophobicity are of great importance for fuel cell catalyst application.

The chemical inertness of carbons can sometimes be a disadvantage which makes it
difficult to deposit metal particles. Thus, the surface chemistry of carbons needs to be

changed in order to improve the interaction between metals and carbons [6].

3.2.1 Surface Chemistry

The carbon surface is inert but not as inert as expected. The carbon surface has active
sites which consist of unsaturated valences at the edges and defects of graphene layers.
Heteroatoms such as nitrogen, hydrogen and oxygen on the carbon surface are also active
sites. Among them, oxygen groups are the most important groups influencing the
synthesis of carbon-supported catalysts. Several oxygen groups on the carbon surface are
shown in Figure 3.1 [7]. The portion of oxygen groups can be increased by the treatment
with oxidizing gas or liquids or decreased by heat treatment in inert atmosphere. The
oxygen groups provide chemical reactivity toward the metal precursor. Physical
interaction with the metal precursor is also possible due to the amphoteric character of
carbon. However, the chemical accessibility of the metal precursor to the oxygen groups
is more important in obtaining maximum dispersion of active sites and the catalytic
activity of supported catalysts [8].

The oxygen groups on the carbon surface can be divided in acidic, neural and basic.

Carboxyl, quinone, and lactone groups are acidic. Due to their negative charge, they

70

improve the interaction between the carbon surface and positively charged metal
precursor. The metal precursor in aqueous solution can easily access the carbon [9, 10].
Phenol, carbonyls, and ether's are weekly acidic to neutral. These groups increase the
interaction of the metal precursor or metal particles with the carbon, leading to reduce

sintering of metallic particles [10].

Carboxylic acid

     
  
 
 

O§ OH 0 Lactone

Carboxyl
anhydride

Cyclic peroxide

OH O

Phenol Quinone

Figure 3.1.Surface groups containing oxygen and localized 1: sites (*) on a graphene
layer.

Heat treatment of the carbon in inert atmosphere further graphitizes it, increasing
crystalline order and crystallite size. The basicity of the carbon is also increased, which
can not be explained by the creation of basic groups [11]. The basicity of carbon surface
mostly originated from the delocalized 1t sites and the localized 1t sites. The former is on
aromatic systems corresponding to the basal plane of the carbon and the latter is created
in between two non-neighboring oxygen atoms mostly at the edge of the carbon. The

delocalized 1c electrons have a great effect on the basicity of the carbon because the

71

surface area of the basal plane is much larger than that from the edge. Nitrogen can be
introduced onto the carbon surface by treating the carbon with ammonia at high
temperature [12]. It was found that the carbon with nitrogen increase the activity of the
metallic particles [13]. Amide, imidres, and lactams can be created and converted to

pyridines in an inert atmosphere at higher temperature.

3.2.2 Surface - Precursor Interaction

The oxygen surface groups and It sites on a carbon surface play a very critical role in
interactions between a carbon surface and a metal precursor and consequently in the
interaction of metallic particles with the carbon support.

The 1: sites on the carbon support also act as anchoring sites for the metal particle.
The basal plane of graphite with its planar structure along the c-axis contains a number of
delocalized 1t electrons as shown in Figure 3.2. The strong interaction between a metal
atom and the 1: sites on the xGnP is the reason that exfoliated graphite nanoplatelets
(xGnP) are expected to be decorated by the metallic particles. The 11: sites on the carbon

basal plane interact with water, followed by the electrostatic attraction with the metal ions

as follows [14]:

C,t + 2H2O S CnH3O+ + 0H“ (3.1)

CnH30+—MC16'2 (M = Pt, Pd...) (3.2)

C,t represents surface carbon with delocalized 1t electrons. As a result of this reaction, the
dispersion of metallic particles and the interaction between metallic particles and the

carbon are increased, leading to the increase in the resistance to sintering of the metallic

72

particles [15]. As the strength of the 11: sites on the support increases, the probability of

smaller and well-dispersed metal particles on the carbon increases [16].

0.67lnm

 

 

Figure 3.2. Three dimensional schematic of layered graphite structure (0: carbon
atom).

The oxygen groups make the carbon surface more hydrophilic and thus the metal
precursor in polar solution can easily access to the carbon as mentioned in the previous
section. This means that the more oxygen groups are created on the carbon, the more
metal ions interact with it. The oxygen groups act as anchoring centers for the metal ions.
However, after the complete reduction of the metal precursor, the dispersion of metallic
particles decreases as the oxygen content on the carbon surface increases [17]. After the
metal—organic compound reduction, the oxygen groups have a detrimental effect on the
interaction between the metallic particles and the carbon support. Sintering of the
metallic particles easily takes place regardless of the high initial dispersion of the metal

ions, resulting in larger particles with worse dispersion.

73

[Pt(NH3)41+2
_ . - - (a)

t’ f ?
o=p c=o o-Fw-—>

   

 

?' i" j” 0’)

 

 

 

 

 

PtCl6'2
oH3+ H3(|)+ qH3+ —

 

 

Figure 3.3 Schematic representing the common interactions between the active sites
of a carbon surface and metal ions and their effect on particle size and distribution.

The possible interactions between the metal ions and the oxygen functional groups

on the carbon and the formation of metal particles after the complete reduction reaction

74

carbon surface groups are illustrated with Pt precursors as an example in Figure 3.3. If
the acidic oxygen groups are dominant on the carbon surface, the metal cations are
preferred to the metal anions to produce fine and well-dispersed particles. In contrast, the
metal anions are preferred in the case of the carbon support which is abundant with basic
groups or delocalized 1: sites. Fine metal particles widely dispersed can be achieved by
either increasing the 7: sites on the basal plane of the carbon to increase the electrostatic
attraction with the metal anion or by oxidizing the carbon which will favor the

electrostatic attraction with the metal cations.

3.3 Synthetic Routes of Carbon-Supported Catalysts

The preparation procedure of supported metal nanoparticles can also be divided into
physical or chemical methods. High vacuum sputtering is a representative example of the
former method. Although it is difficult to obtain the dispersion of metal particles with a
narrow size distribution especially when high loading is needed and a uniform
composition is required among different metal particles for supported bimetallic particles.
this method is convenient for rapid screening of supported mixed metal catalysts
compared to the chemical methods [18].

A number of examples for chemical methods have been applied for the synthesis of
supported catalysts as listed in Table 3.1. Chemical changes of a metal precursor either
in an ionic or a molecular state are initiated by reducing the metal precursor to metal
atoms, which then grow to form the metal particles. The chemical synthesis of the metal
particles from the metal precursor dissolved in an aqueous or organic liquid is usually

affected by the introduction of a reducing agent. This wet chemical routes can be initiated

75

by physical mixing, microwave irradiation [19] or, radiolytic [20], sonochemical [21], or

electrochemical [22. 23] methods.

Table 3.1 Examples of chemical method for synthesis of carbon-supported metal

 

 

particles
Metals Supports Loading Size Method Precursor Ref.
(%) (nm)

Pt CNT° 10 2 ~ 5 Colloid H2PtC16 24
12 ~13 2 ~ 4 Colloid K2PtCl4 25

CB" 20 ~ on 2 ~ 3 Radiolytic HzptC16 26

40 2.9 Colloid H2PtCl6 27

SWNHC 20 ~ 40 2 Impregnation H2PtCl6 28

PtRu CNT 1.5 ~ 1.6 Impregnation H2PtC16, RuCl3 29
CB 40 4 ~ 20 Microemulsion HthClo. RuCl3 30

CB 1() ~ 90 3 ~ 15 Impregnation Na6Pt(SOg)4vNa6Pt(SO3)4 31

0di so 4 ~ 5 Microwave (n-C2H4)C1Pt(rl-Cl2)RuCl 32

GNF 42 7 Impregnation (n-C2H4)C1Pt(tt-Cl2)RuCl 33

CNCC 60 2.3 Colloid H2PtCl6, RuC13 34

PtSn CB 20 3 ~ 4 Impregnation Pt/C, tetrabutyltin 35
PtCr CB 20 3 ~ 4.5 Impregnation Na2PtC16, Cer 36
Pd Cpr 2.5 4 Deposition— Pd(NH3)4Cl2 37

reduction
CB, Grg Deposition— K2PdC14 38
reduction

 

aCarbon N anotube, bCarbon Black (Vulcan XC-72R), CSingle-Wall Carbon Nanohorn,
dGraphite Nanofiber, cCarbon N anocoil, fCarbon Nanofiber, gGraphite.

As explained in the previous chapter, the metal atoms collide to form a nucleus in the

early stage of wet chemical processes. Nuclei that grow beyond a critical size are stable.

but a mechanism is needed to control the growth of particles and to achieve a narrow size

distribution. The addition of a support or a capping material prevents particle growth.

Depending on the growth control mechanism and strategies. chemical methods to

76

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synthesize supported metal catalysts can largely be divided into five categories such as
deposition-precipitation deposition-reduction, impregnation-drying, colloid method and
microemulsion. Thus, the discussion on these five synthetic routes will be of use to
understand distinct differences in the strategy of the growth control of metal particles
supported especially on the carbon materials. Figure 3.4 shows schematic procedure for

those chemical routes.

3.3.1 Deposition-Precipitation

Carbon supports are pretreated with an oxidizing agent in order to introduce oxygen
surface groups, followed by the induction of M(OH)2 (M = Pd, Pt, Ru, ...) resulting from
alkaline hydrolysis of a metal halide precursor and the reduction of the precipitated
M(OH)2 with an reducing agent [40]. Normally, the reduction by hydrogen results in

higher metal dispersion in a range of 50 ~ 70 %.

3.3.2 Deposition-Reduction

Procedures of this method are very similar to those of deposition-precipitation
method. They consist of 5 steps: 1) pretreatment of carbon supports with an oxidizing
agent, 2) suspension of the treated carbons in distilled water, 3) addition of an aqueous
solution of a metal precursor, 4) reduction of metal ions by adding a reducing agent into
the system, and as a final step, 5) filtration of the suspension. The catalyst obtained is
dried in an inert atmosphere at room temperature or an elevated temperature. In order to
avoid sintering of metal particles, the catalyst must be heated slowly. This method is an
appropriate method for carbon supports with wide pores due to the fast reduction reaction

[41]. For carbons with smaller pores. the impregnation-drying method is preferred.

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3.3.3 Impregnation Method

This method is simple and the most common. A dissolved metal precursor is
absorbed into the pores of a carbon support and any liquid phase is removed by
evaporation, followed by reduction to produce supported metal particles dispersed on a
support. The reduction step can be chemical or electrochemical. Hydrazine, borohydride,
formic acid and hydrogen are used as the common reducing agents. The pore size
distribution and the morphology of the substrate play an important role in penetration of
the precursor into the support and also providing the confinement for metal particle
growth. Reaction time and kinetics and mass transfer of the reducing agent affect the
nucleation and growth of the nanoparticles. In the case of the electrochemical reduction,
the growth of the particle can be controlled by the amount of current. The major
drawback of this method is the lack of size control of metal particles except for porous

substrates with a narrow pore size distribution, e. g. in highly ordered mesoporous carbon

[42].

3.3.4 Colloid Method

The metal precursor is chemically reduced to form metal nanoparticles in the
presence of a protective agent like PVA, PVP, PPhg, and NR1”. A narrow size distribution
is achieved as the colloidal metal particles are stabilized either by steric hindrance or by
electrostatic charges. Steric stabilization is provided by coating the metal core with
organic molecules. On the other hand, protection from aggregating into larger particles is
provided by the electrostatic repulsion of like charges in the case of adsorbed ions or

charged colloids. Though a narrow size distribution of metal nanoparticles is obtained

79

from this method, the major drawback is the presence of the protecting agent hindering
the catalytic activity of the nanoparticles.

An alternative route of preparing metal particles without the use of protecting agents
was developed. This is achieved for certain metals by proper combination of precursor.
solvents, reducing agent and electrolytes. The use of sodium hydroxide dissolved in
ethylene glycol as a solvent as well as protecting agent is a representative example [43].
The glycol is removed by electro-oxidation during usage as an anode or by
decomposition at 160°C. The size and dispersion of metal particles is controlled by the
process parameters. e. g. pH and water concentration. This glycol colloidal process is

appropriate for large-scale synthesis of metal nanoparticles.

3.3.5 Microemulsion

This method offers unique flexibility in the simultaneous control of size and
composition of mixed metal nanoparticles. The microemulsion is uniformly dispersed in
a continuous medium, which is immiscible with the precursor containing liquid phase.
Normally, oil and water are a dispersed liquid phase and a continuous medium,
respectively. The introduction of a reducing agent into the microemulsion is achieved by
stirring. The chemical reaction is confined within a microemulsion which is a tiny drop of
precursor containing liquid engulfed by surfactant molecules. Since chemical reaction
takes place within the microemulsion, a narrow particle size distribution can be obtained.
After the reduction step, nanoparticles are protected from agglomeration by the surfactant
molecules. Similarly to the colloidal method where protecting agents are used, the

nanoparticles should be adsorbed onto a porous support before the surfactant molecules

80

are removed. The size distribution of metal particles can be further controlled and
improved by a two—component microemulsion method with the reducing agent confined
in a separate emulsion [44, 45]. The final composition of the mixed metal nanoparticles
can be easily controlled by the ratio of the metal precursor solutions. Nevertheless, the
microemulsion method requires the use of costly surfactant molecules with extra washing

steps and thus is not economical for a large scale synthesis.

3.4 Requirements of Carbon Support

The properties of carbon-supported catalysts are affected by those of carbon supports.
The carbons with high surface area are necessary for a high dispersion of metal catalysts,
excellent crystallinity, a pore structure suitable for maximum contact with reactants and
release of byproducts and good interaction with metal particles are generally required.
Other properties of carbon supports required especially for fuel cell application are
surface inertness in a strongly oxidizing environment, good electrical conductivity to
facilitate electron transport, and availability at low cost [46, 47].

High surface area and a well developed porosity are essential for accomplishing
large metal dispersion, which usually, but not always, results in a high catalytic activity.
However, there are many examples showing that neither dispersion nor catalytic activity
is a function of the porosity and surface area of the carbon support. One of such examples
is the result of Rodriguez-Reinoso et al. which showed that iron loading on the carbon
support is a function of the metal precursor used, showing the access of the metal solution
was related not just to the increasing porosity of the carbons, but also to the chemical

character of its surface and its interaction with the different solutions [48]. On the other

81

hand, the surface area of carbon supports influences their corrosion rates in an oxidizing
environment. Carbons with the higher surface area show higher corrosion rates [49]. Thus.
although carbons with high surface area are good for high loading and dispersion of metal
catalysts. they may have a detrimental effect on long-term stability of fuel cell system.

Surface inertness or corrosion resistance of carbon supports in an oxidizing
environment means that carbon materials must be highly graphitized. Less graphitized
carbons have a wider interlayer spacing and poorer alignment than graphitized ones. This
less aligned carbons is more reactive to oxidation and corrodes from the inside outwards
[49]. Thus, most carbon supports must be graphitized especially for fuel cell cathode to
which oxygen or air is supplied.

Excellent electrical conductivity is a major requirement of carbon supports for fuel
cell application. Although the graphitization of carbons can result in a decrease of their
electrical conductivity, their variation is not wide [50]. Hence, if carbons have inherently
high electrical conductivity, their electrical properties are not an important factor to be
considered before any treatment to optimize carbon properties.

We‘ll-adjusted carbon properties can contribute to improve the performance of fuel
cell system through enhanced catalytic activity of carbon-supported catalysts. However,
significant advances have been made in the performance of fuel cell catalysts by
combining well characterized, corrosion resistant, graphitic carbon supports with

platinum based metal or metal alloy electrocatalysts.

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3.5 Catalyst Design - Metal location

The size of metal particles deposited on a support affects the performance of a
heterogeneous catalyst. In general, increased metal dispersion and decreased metal
particle size result in increased catalytic activity. The size of a reactant is also considered.
If the reactant is bulky, the higher catalytic activity is obtained from the catalysts with the
metals located on the external surfaces. The other important factor which has an effect on
the catalytic performance of a supported catalyst is the location of metal particles on the
support material [51]. This is related to the effect of pressure and temperature in reaction
systems. If the metal is deposited within the pore structure (intermediate and interior
locations), it will not all be accessible to reactants at low pressure and thus a surface-
loaded catalyst is preferred. As the pressure increases, an increasing proportion of the
metal will become accessible and thus exhibit a higher activity than the catalyst with
metal deposited only on the external surface. Consequently, when operating a catalytic
system free from mass transport limitations and catalyst poison, one would choose a
catalyst with an exterior metal location at low pressure, an intermediate location at
medium pressure and an interior location at high pressure. The effect of metal location on
thermal sintering resistance must be considered for the reaction carried out at different
temperature. At higher temperature, catalysts with an interior metal location have the
greatest sintering resistance and show higher activity.

Catalysts can be designed with different metal locations for reactions which take
place under different conditions of pressure and temperature. Exterior location of metal

particles on carbon supports is preferred in fuel cell systems because they are operated at

83

low pressure and low temperature. xGnP may be an excellent carbon support for fuel cell

applications because it generates the highest surface-loaded catalyst.

3.6 Examples of Catalysts Supported on New Carbon Nanostructures

Recently, new carbon forms such as graphite nanofibers (GNF) and carbon
nanotubes (CNT), carbon nanohom and carbon nanocoil have attracted a great deal of
attention as catalyst supports in fuel cell electrodes to replace the classically used carbon
black [52~56] because they are believed to be breakthrough materials for energy storage,
electronics. and catalysis due to their exceptionally high adsorption, electronic,

mechanical and thermal properties.

3.6.1 Graphite Nanofiber (GNF)-Supported Catalysts

GNF consists of extremely small graphite platelets perfectly arranged in various
orientations with respect to the fiber axis, giving rise to assorted conformations.
Depending on the morphology of the graphite layers, there are four types of GNF: 1)
herringbone structure (H—GNF) where the graphite layers are stacked obliquely in respect
to the growth axis, 2) a platelet GNF (P-GNF) where the layers are oriented
perpendicular to the fiber axis, 3) a spiral GNF (S-GNF) where the graphite platelets in
the helical nanofibers are oriented parallel to the fiber axis, and 4) finally a ribbon-like
form (R-GNF) where the platelets are aligned parallel to the fiber axis [57, 58].

GNF-supported platinum or platinum/ruthenium catalysts have been synthesized by
the impregnation method [59~61] and the combination of impregnation and microwave

irradiation [62]. The results of their performance as anode catalysts of DMFC prove that,

84

as metal supports in fuel cell electrodes, GNF can replace the classically used carbon
blacks. Catalysts consisting of 5wt. % platinum supported on P-GNF and R-GNF
exhibited activities comparable to that displayed by about 25wt. % platinum on Vulcan
carbon black. While P-GNF and ReGNF supports showed 400~500% improvement in the
current yield compared to the Vulcan carbon support, H-GNF-supported Pt exhibited
much less electrocatalytic activity toward methanol oxidation. The superior catalytic
reactivity of GNF-supported Pt over Vulcan-supported Pt results from (1) increased
electrical conductance of the GNF, (2) decreased impurities, (3) efficient mechanism for
the removal of the adsorbents, and (4) interaction between the highly ordered GNF
substrate and crystallo graphically oriented platinum particles.

The fuel cell performance of GNF-supported Pt-Ru catalysts was compared with
SWNT— and MWNT-supported ones. The highest performance was achieved when Pt-Ru
catalyst on narrow tubular H-GNF was used. An enhancement of up to ~64% relative to
that recorded for an unsupported Pt-Ru colloid anode catalyst of similar surface area was
reported. The samples having some degree of Ru metal phase separation from Pt-Ru alloy
have given the highest electrocatalytic performances. Relatively small particle sizes were
measured on all supports even at very high meal loading (40~60wt. %). Surprisingly, a
low surface area was reported for SWNT (109m2/ g) and especially MWNT (8m2/g) that
could be related to the poor quality of the materials. No explanation for these different
behaviors has been proposed. The use of microwave irradiation process in synthesizing
makes it possible to develop larger-scale preparations of metal carbon nanocomposites

without the expense of catalytic reactivity.

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3.6.2 Carbon Nanotube (CNT)-Supported Catalysts

Many studies as to the synthesis of CNT-supported catalysts and their application in
heterogeneous catalysis have been reported. Li and coworkers [63, 64] explored oxidized
CNT as supports for platinum catalyst in order to solve the slow reaction rate of the
cathode oxygen reduction reaction (OR) in DMFC. They obtained Pt/CNT catalyst with
a metal loading of 10 wt. % by chemically reducing chloroplatinic acid in ethylene glycol.
The average diameter of Pt ranges from 2~4nm. The results show that CNT as supports
for cathode catalyst in a direct methanol single cell produces better performance
compared to XC-72 carbon black (CB). The increase in OR activity of Pt/CNT over
Pt/CB is attributed to (l) the unique structure and better electrical properties of CNT, (2)
a small amount of organosulfur impurities of CNT, and (3) the specific interaction
between Pt and CNT. Matsumoto et a1 [65] also investigated the cell performance with
two electrodes comprising Pt/CNT catalyst fabricated by chemical reduction of KthCl4
in ethanol. The Pt content was estimated to be ~12wt. %. This 12wt. % Pt-deposited CNT
electrode showed 10% higher voltages than a 29 wt. % Pt/CB and reduced the Pt usage
by 60% in polymer electrolyte fuel cells with hydrogen and oxygen. The results indicate
the importance of the high conductivity of supports for high performance with a low Pt
load. High population of the triple-phase boundary (gas-electrolyte—electrode) of Pt/CNT
catalyst is another contribution for higher efficiency over Pt/CB with multiple-phase
boundary. Platinum nanoparticles 1~5nm in diameter were deposited onto MWNT by an
electroless plating procedure [66]. The surface of MWNT was functionalized by HNO3
and K2Cl’207/H2804 before the deposition. The electrode comprising this Pt/MWNT

catalyst showed higher electrocatalytic activity for OR in a single stack fuel cell test.

86

Template method was applied to prepare Pt/MWNT and Pt-Ru/MWNT catalyst which is
tested as the anode [67]. The current density of methanol oxidation for a Pt supported on
MWNT membrane electrode was found to be 20 times higher than that of a bulk platinum
electrode. This material can also be used to electrocatalyze ORR. It is found that the
electrochemical activity follows the order: Pt-WOglCNT > Pt-Ru/CNT > Pt-Ru/CB >
Pt/CNT > Pt/CB > bulk platinum [68]. The higher electrochemical response of CNT

based materials is attributed to the higher available electroactive surface area.

3.6.3 Other Carbon Nanomaterials-Supported Catalysts
Single-wall carbon nanohorns (SWNH) which are a new type of carbon materials

having a hom-shaped sheath of single-wall graphitic sheets were used as a support
material for Pt deposition to demonstrate the suitability of its application for the
electrodes of Polymer electrolyte membrane fuel cell [69]. Pt oxide colloids were
adsorbed on the SWNH surface and dried and reduced in H2. Pt particle size was about
2nm which was less than half of that supported on the conventional CB. The current
density for the SWNH cell was larger than that for the CB cell.

The electrocatalytic performance of 60wt. % PtRu alloy catalysts supported on
newly synthesized carbon nanocoil (CNC) with excellent crystallinity and large surface
area was reported [70, 71]. CNC composed of nanometer-thick graphitic fibers was
synthesized by simple heat treatment of mixtures of a carbon precursor, silica, and a
transition-metal salt. Resorcinol-formaldehyde sol and Co/Ni salt were used as a carbon
precursor and a catalyst. A silica sol was added in order to obtain high surface area

carbon. Depending on the quantity of metal salts, the synthesized CNS has high surface

87

area varying from 200 to 500 mz/g. In addition, the CNT has well-defined porosity which
favors the diffusion of fuel and the removal of the by-product C02 gas. CNC-supported
PtRu catalyst showed excellent electrochemical stability over the potential range of the
methanol oxidation process for DMFC and a capacitance over ten times higher than that
of Vulcan XC—72R CB. The current density (20mAcm‘2) of the CNC-supported catalyst
was four times higher than that (5mAcm'2) of Vulcan CB. The maximum power density
of the CNC-supported catalyst was 150% higher than the Vulcan CB. The remarkably
enhanced performance of this catalyst is attributed to the high electrical conductivity, the
high surface area, and the high crystallinity of CNC and the superior dispersion of the

metal catalyst on CNC.

3.7 Summary

From chapter 1 to 3, the general background of low temperature fuel cells, especially
PEMFC and DMFC, the current status of the technology and the important technological
challenges in electrocatalyst for PEMFC and DMFC and basic features of the direct
ethanol fuel cell (DEFC) and the direct formic acid fuel cell (DFAFC) as alternative fuel
cells have been described. Brief review on the formation mechanism of metal, operational
principle of the microwave heating, and the role of the polyol and ionic liquids have also
provided to assist understanding the results of this dissertation works. So-far developed
Synthetic routes for carbon-supported metal catalysts, the surface chemistry of carbons
and its effect on the interaction between carbons and metal precursors, and catalyst
design and recent trends of carbon-supported catalysts appropriate for fuel cell systems

have also been included.

88

3.8 Research Objectives

The electrocatalysts are supported on carbon materials with high surface area, high
mesoporous distribution, good electronic conductivity and graphitic character. Carbon
black (CB) is the most widely used carbon support for the preparation of fuel cell
catalysts because it meets those requirements well. However, highly active catalysts with
novel carbon material as a support have been explored in order to enhance the methanol
and hydrogen oxidation in the anode and the rate of ORR in the cathode. Carbon
nanotube (CNT) and graphite nanofiber (GNF) have drawn much attention from both a
fundamental and an applied perspective due to their good mechanical and unique
electrical properties. As reviewed in chapter 3, CNT-supported Pt nanoparticle showed
superior activity in OR in a half-cell test and CNT consequently reduced Pt usage
compared to CB-supported electrocatalysts. GNF is also considered to be a potential
support, which has high activities for the electrochemical oxidation of methanol. Pt/GNF
as an anode catalyst greatly increased the performance of the DMFC over Pt/CB or Pt-Ru
colloid catalyst. These studies strongly suggest the possible application of GNF and CNT
as novel supports in fuel cell electrodes.

However, although GNF and CNT satisfied the characteristics which are required to
be an ideal support material for the electrocatalyst in a fuel cell, their main drawback is
the cost of commercial production caused by harsh synthetic condition and low
production yields. There are no available synthetic techniques that can produce CNT of
reasonable purity and quality in large quantities. The market price of CNT is still too high
(~$200/g) at present for realistic commercial application, even though CNT may be

available to consumers for less than $100/pound in a few years. Another disadvantage is

89

in the manipulation of CNT . CNT is needed to be purified for the bulk applications
because as-produced CNT contains a lot of undesirable metal contaminants (Fe, Co.

Ni. . .). Again. the purified CNT should be oxidized in harsh acidic condition, which is
inevitable for better dispersion in solvent. This surface oxidation of CNT may cause the
loss of its original electronic properties which negatively affect the performance of the
catalyst. Additionally, it is revealed that high loading of Pt is not achieved without the
surface oxidation of CNT. Instead of its surface oxidation, CNT can be functionalized
covalently and non-covalently to be dispersed in solvent but the loss of the conductivity
of CNT is always accompanied. The case for GNF is not much different from that of
CNT. Hence, CNT and GNF have many impediments to their wide spread
commercialization because of these inherent problems.

Exfoliated graphite nanoplatelets (xGnP) produced by MSU proprietary method is a
much more cost-effective alternative and in addition has superior properties such as
excellent mechanical and electrical properties comparable to CNT and GNF. More
importantly, xGnP possesses higher corrosion and oxidation resistance and higher
crystallinity than CNT and GNF as will be described in chapter 5. Hence, it is worthy
while to investigate xGnP as a support material for the electrodes of a fuel cell.

T he overall goal of this research is to develop and understand a method for
depositing metal nanoparticles on xGnP as a support for an electrocatalyst for a fuel cell
system. This is of great importance because Pt-related catalysts mounted onto carbon
supports are applied as heterogeneous catalysts and electrodes in fuel cell. Another
reason is that the specific activity of the platinum catalyst in OR in both PEMFC and

DMFC is also highly related to the carbon support which functions to allow the reactant

90

to easily get to the electrocatalyst, show good corrosion resistance in the cathode under
strongly oxidizing conditions, and have adequate water-handling capability at the cathode.
This research will be accomplished by 1) understanding the fundamental
physical/chemical mechanism controlling the dispersion and size of metal nanoparticles
deposited on xGnP and 2) evaluating the performance and relating it to morphology of
xGnP-supported pure Pt and/or bimetallic PtRu alloys which will be highly dispersed on
the xGnP. The catalytic activity of xGnP-supported Pt-based electrocatalyst will also be
compared to that of commercially available XC-72 Carbon black-supported Pt catalyst
from E-TEK.

On the other hand, there are many studies underlining the difficulty of using
conventional methods to prepare Pt catalysts with high metal loadings ( > 30wt.%) and
small particle sizes at the same time. Metal particle sizes for a catalyst with 10wt.% and
30wt. % Pt loading can be produced at 2.0nm and 3.2mm, respectively, but increase to
8.8nm for a 60wt. % Pt catalyst. Therefore, the synthesis of highly dispersed and highly
loaded nanosized metal catalysts is a challenge. Hence, ionic liquids will be used in this
microwave process in order to control the size of metal particles even at highly loaded
and supported metal catalysts. High concentration of polar molecules in ionic liquid will
be a good additive to accelerate the reduction rate of metal precursor, resulting in the

synthesis of small and monodispersed metal particles.

91

10.

ll.

12.

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98

CHAPTER 4
THE SYNTHESIS OF UNSUPPORTED AND SUPPORTED
NOBLE METAL NANOPARTICLES VIA MICROWAVE
PROCESS IN THE PRESENCE OF ROOM TEMPERATURE

IONIC LIQUIDS (RTIL)

99

4.1 Introduction

The synthesis of nanoscale metal colloids or clusters has been the object of much
research in modern nanotechnology due to their great impact on new technological
applications [1, 2]. Nanoscale particles possess unique optical, thermal, electronic,
magnetic properties and chemical reactivity which comes from their quantum size effect
which is not observed in the bulk [3~8]. Most of their unique physical and chemical
properties strongly depend on the size and shape of the metal nanoparticles [9]. Therefore, an
effective synthetic method is required to obtain uniform small metal powders with a
controlled size and a narrow size distribution. Examples of advanced techniques to
produce such nanoparticles are sol-gel processes, microemulsion methods,
hydrothermal/solvothermal processes, electrochemical reduction, sonochemical routes,
and microwave-assisted processes, all of which involve the reduction of the
corresponding metal ions [1 0-1 6].

There are two important points to be considered in the synthesis of metal
nanoparticles. One is to control particle size, particle size distribution, and shape of
particle at the growth stage of metal nanoparticles. The other is to achieve the
stabilization of the metal nanoparticles in solutions. Generally, the control of particle size
and the particle stabilization are achieved by using capping materials such as polymers,
surfactants, coordinating ligands and changing their content in solution [17-19]. The
reduction rate of the metal ions is another way of controlling the size of metal particle
[20]. This chapter is devoted to reporting a process to synthesizing fine precious metal

nanoparticles by a new and simple strategy of reducing metal ions in a liquid polyol

100

under microwave irradiation in the presence of ionic liquids and illustrating how effective
this process can control particle size and uniform size distribution.

Nanocomposites consisting of either metals or metal oxides supported on carbons or
metals dispersed on metal oxides can be advantageous for applications in automobiles
[21, 22], batteries [23-26], supercapacitors [27-30], fuel cells [31-33], hydrogen storage
and production [34, 35], as well as for sensors[36]. Platinum-based metal nanoparticles
are especially crucial components for low temperature fuel cells in which they are the
most active catalysts for the oxidation of fuels supplied and the oxygen reduction reaction
at their anode and cathode, respectively. However, the biggest obstacle to commercialize
these fuel cells is the cost of precious metals and thus considerable efforts are expended
to reduce the amount of the precious metals and obtain their maximum performance at
the same time. The reduction of precious metal catalysts can be accomplished by
enhancing the utilization of the metal catalyst on both the anode and cathode [3 7, 38],
which is possible by introducing a high degree of dispersion of fine platinum-based
catalysts onto carbon supports having a high surface area and conductivity.

Carbon supports play an important role in the properties of supported metal particles.
They have a great effect on not only metal particle size and its dispersion but also
electrocatalytic activity due to metal-support interactions, mass transport and electronic
conductivity of the catalyst layer, and the stability of the metal particles [39, 40]. Many
different novel carbons such as graphite nanofibers [41, 42], carbon nanocoils [43, 44],
carbon nanotube [45, 46], mesocarbon microbeads [47], and carbon nanohoms [48] have
been studied and compared as supports for precious metal catalyst. However, there is no

known report in which carbon will be the best support for the maximum fuel cell

101

performance. This is because of the multiple influences of carbon supports with various
physical properties, different size of metal particles and their degree of dispersion, as well
as different extent of alloying bimetallic catalysts.

In this chapter, a simple approach is introduced to synthesize different metal
catalysts supported on various carbons with an almost consistent particle size and
dispersion and similar degree of alloying in bi- or multi-metallic catalysts. This makes it
possible to investigate systematically the effect of carbon supports and compare their fuel

cell performances directly.

4.2 Experimental Details

4.2.1 Materials

A reagent grade of ethylene glycol (EG, bp = 197.30C) was purchased from J .T.
Baker and used as a reducing agent for unsupported and carbon-supported platinum-
based metal catalysts. Chloroplatinic acid hexahydrate (HthC16-6HzO), ruthenium
chloride hydrate (RuCl3 ° tzO), palladium nitrate hydrate (Pd(NO3)2'tzO) were
purchased from Aldrich and used as metal precursors. Room temperature ionic liquids
used in this work are l-butyl-3-methylimidazolium acetate (bmimCH3COz), l-butyl-3-
methylimidazolium methyl sulfate (bmimCHgsO4), 1-butyl-3-methylimidazolium
thiocyanate (bmimSCN), and 1-butyl-3-methylimidazolium hexafluorophosphate
(bmimPFé) and their chemical structures are shown in Figure 4.1. All of them were
obtained from Aldrich and used as received. Multi-walled carbon nanotubes (MWNT),
carbon black (CB, Vulcan XC-72R), and graphite nanofiber (GNF) were purchased from

Aldrich, Cabot Co., and Nanomirae Inc., respectively and used as carbon supports.

102

Purified single wall carbon nanotube (SWNT) from Southwest and exfoliated graphite
nanoplatelets (xGnP) made in-house were also investigated. Two commercial metal
oxides (TiOz and CeOz) from Aldrich were investigated as non-carbon supports.

[,CHa ,CHs

My 1 [16% :fi Q+scn' [7?th PF;

I.“ if“ f I

(a) (b) (C) (d)

Figure 4.1 Chemical structures of room temperature ionic liquids for the synthesis
of catalysts (a: bmimCH3COz, b: bmimCH3SO4, c: bmimSCN, d: bmimPF6).

4.2.2 Preparation of Unsupported and Supported Metal Particles

The typical experimental procedures for producing unsupported metal particles are
described as follows: First, a predetermined amount of a metal precursor was dissolved in
2g of EG in a 10mL beaker. Second, a controlled quantity of a RTIL were mixed with
18g of EG in a separate beaker and then combined in one beaker. The mixture was
mechanically stirred for 30min, placed in a microwave oven (National, 2.45GHz,
1300W), and then heated for 503. The resulting suspension was centrifuged and washed
with acetone. The solid product was stored in a re-dispersed state in acetone.

Pt particles supported on various carbon supports and metal oxides consist of the
following steps: In one beaker, carbons or metal oxides were ultrasonically dispersed in
18mL of EG for 30 min followed by cooling to room temperature under mechanical

stirring. In another 10 ml beaker, a specific quantity of the Pt precursor was added to 2 ml

103

EG and completely dissolved. Contents in those beakers were mechanically mixed in a
IOOmL beaker for another 30 min, transferred into the microwave oven and irradiated for
a total of 55 ~ 60 seconds to effect the precursor reduction. After cooling to ambient
temperature, the resulting suspension was centrifuged and the residual slurry was washed
with acetone several times and dried in a vacuum oven at 393K overnight. The weight
ratio of Pt and support was controlled according to the targeted metal loading on supports.
The catalysts obtained are called as the Pt/C-N. C can represent XC-72R, MWNT,
SWNT, or GNF.

For Pt/C nanocomposites synthesized in the presence of the RTIL, the controlled
amount of RTIL was added and dissolved in 18mL ethylene glycol prior to the dispersion
of carbons. The molar ratios of RTIL to Pt precursor were seat at 1, 2.5, 5, 10, 20, and 50.
Other steps are the same as for Pt/C-N catalysts. The Pt/C nanocomposites synthesized
with the addition of, bmimPFé, bmimCH3COz, bmimCH3SO4, and bmimSCN are denoted
as Pt/C-bmimPFb, Pt/C- bmimCH3COz, bmimCH3SO4, and Pt/C- bmimSCN, respectively.
For example, Pt/MWNT- bmimPF6 represents the MWNT-supported Pt particles

synthesized by microwave irradiation in the presence of bmimPFb.

4.2.3 Characterization Methods

The Pt colloidal formation process was investigated by UV-vis spectra with a Perkin
Elmer Lamda 900 spectrometer in the range of 220~700nm with 1cm path length quartz
cuvettes. The Pt precursor of 1.0 x 10'6 M was completely dissolved in ethylene glycol
before the measurement. Pt particles dispersed on various carbons and metal oxides were

characterized by recording their X-ray diffraction (XRD) patterns on a Regaku Rotaflex

104

2008 X-ray diffractometer using Cu-Kor radiation with a curved graphite monochromator
in order to identify the phases present in the catalyst and to evaluate the particle size of
the dispersed metal crystallites. The 20 region between 100 and 1000 was explored at
45keV and 100 mV at a scan rate of 5°/min. The average metal particle size was
calculated using the Scherrer equation according to the XRD broadening Pt (220)
reflections which was re-scanned at a rate of 1°/min. The transmission electron
microscopy (TEM) investigation was carried out with J EOL lOOCX operating at a
voltage of 100keV to determine the morphology of the dispersed Pt particles and the
determination of the mean Pt particle size. Specimens were prepared by ultrasonically re-
dispersing the sample powders in acetone for 15 min, applying the power suspension onto
lacey carbon coated Cu grids and drying it in air. X-ray photoelectron spectroscopy
(XPS) results of carbon supported Pt catalysts were obtained from a Physical Electronics
PHI 5400 ESCA system and used to determine the presence of the oxide phases and the
interaction between metal and supports. A non-monochomatic Mg source (with a KG], 2
wavelength at 1253.6eV) was used with a take-off angle of 45degrees. Data was collected
by a multi-channel detector with a Omni VI lens assembly. The instrument was operated
with a pass energy of 93.90eV for survey scans and 29.35 eV for regional scans. The
regional XPS of Pt (4f) was de-convoluted to recognize different oxidation states of Pt on
carbons. Semi quantitative atomic concentrations were calculated using predetermined
sensitivity factors. XPS samples were prepared by applying the catalyst powders directly

to double-sided copper tape on a stainless steel stub.

105

4.3 Results and Discussions

4.3.1 Formation of Pt Nanoparticles

4.3.1.1 Pt formation in the absence of R T IL

The formation process of Pt nanoparticles in ethylene glycol (EG) by microwave
(MW) heating in the absence of RTIL was investigated by UV-vis spectroscopy and the
results were plotted in Figure 4.2. The ethylene glycol solution containing HthCl6-6HzO
before microwave reduction was pale yellow and showed a peak at 266nm in its UV-vis
spectrum due to the ligand-to-metal charge transfer transition of the [PtCl6]-2 ions [49].
As the reaction time under MW heating increased, the peak at 266nm in the UV—vis

spectrum decreased and disappeared after 203 reduction in the MW, suggesting that all

 

 
   
 
  
  

—— Befor MW reduction

— — — After 103 MW reduction

—— — -— — After 153 MW reduction

—————— After 203 MW reduction
- After 303 MW reduction

— _._.__ After40sMWreduction

- — After 503 MW reduction

Absorbance
N
/
/

~

‘\
\
‘
\-
\
‘
K
“
E

“
“
“
‘5
~‘_.
“

~

 

 

 

200 250 300 350 400 450

Wavelength (nm)

Figure 4.2. UV-vis absorption spectral change of HthCk-6H20-ethylene glycol
solution in the absence of RTIL before and after microwave reduction.

106

[PtChg]2 ions were reduced to P0. The color of the solution turned from pale yellow to
dark brown. The adsorption from the ultraviolet to the visible region increased, which
indicates that the band structure of Pt nanoparticles was formed [50]. This broadening tail
peak appeared after 203 reduction in the MW and increased with increasing irradiation
time as a result of particle growth or agglomeration. No further change was found after

505 MW reduction, suggesting the completion of Pt nanoparticles formation and growth.

4.3.1.2 Pt formation in the presence of R T ILs

The high polarity of solvents is one of the most important properties to be considered
in microwave chemistry. RTIL is good microwave absorbent due to large positive
organic cations with a high polarizability and appropriate anions, leading to very high
heating rate and further reduction in reaction time in microwave irradiation. Therefore,
RTIL possesses some advantages in synthesizing inorganic nanomaterials such as
tellurium nanowire, gold nanosheet and metal sulfides by microwave dielectric heating
[51~53]. However, the application of RTIL for microwave chemistry to produce
nanostructures has not been blossomed in spite of such advantages. Reduction
experiments utilizing microwave irradiation in the presence of RTIL have been
conducted and results on the reduction of H2PtCl6-6HZO in EG by microwave irradiation
to determine the influence of RTIL such as bmim CH3SO4, bmimPFb, and bmimCH3C02

on formation of Pt nanoparticles are reported.

Reduction in the presence ofbmimCH3C02. In the first set of experiments, the

influence of bmimCH3Oz on Pt formation from HthCl6-6HzO-EG solution was

107

investigated. A constant amount of Pt precursor of 1.54 x 10”1 M was used. The effect of
MW reduction time was studied with a fixed composition of bmimCH3Oz/ HthCl6-6HZO
= 20 in molar ratio. The results are shown in Figure 4.3. When bmimCH3Oz was
introduced into EG containing Pt, salt, the peak intensity at 267nm corresponding to
[PtCl6]'2 ions in the UV-vis spectrum decreased slightly with 53 MW irradiation but
completely disappeared after 103 MW reduction as clearly seen in Figure 4.3. The
broadening tail peak in the visible region indicating the formation of Pt nanoparticles was
almost uniform in spite of increasing MW irradiation, suggesting that Pt nanoparticle
formation and growth can be completed within 10s in the presence of bmimCH3Oz. It can
be concluded in this set of experiments that the reduction reaction in the presence of
bmimCHgOz is at least twice as fast as that in the absence of RTIL and the growth and
agglomeration of Pt particles is restricted by the presence of bmimCH3COz, which may
result in the formation of smaller Pt nanoparticles in RTIL-assisted microwave process.
The mechanism of the protecting agent can be measured by UV-vis spectroscopy. If
bmimCH3C02 acts as a protecting agent, the peak at 267nm shifts to higher wavelength
due to the expansion of the ligand field splitting of PtSd orbital as a result of the
coordination of N and/or O atoms of bmimCH3COz to Pt4+[54]. No peak shift was found
in this work after the addition of bmimCH3COz, indicating that bmimCH3C02 does not
act as a protecting material. However, TEM images of Pt nanoparticles synthesized in the
presence of bmimCH3COz indicates that it has a great effect on preventing Pt
nanoparticles from agglomerating with each other. The details on this will be discussed in

the next section.

108

 

 

 

 

 

 

 

5 l
1
1
; Before MW reduction
4 1 l — — — After 53 MW reduction
i —————— After 103 MW reduction
. —- — . —- —- After 153 MW reduction
at 1 ~- -- After 203 MW reduction
0 3 d
c
«I
.D
L
O
3
< 2 ‘
1 ..
‘2‘ -._.
O
200 450

 

Wavelength (nm)

Figure 4.3. Typical UV-vis absorption spectral change of HthCk-6H20-ethylene
glycol solution containing bmimCH3C02 before and after microwave reduction.
The effect of bmimCH3Oz content on the process of Pt particle formation was also
monitored through UV-visible spectrometry. The concentration of bmimCH3Oz was
varied so that the molar ratio of bmimCH3Oz to Pt precursor (1.54 x 10'4 M) was eitherl,
5, 10, or 20. Figure 4.4 was plotted with UV-vis adsorption spectra obtained after 105
MW reduction, which provided clear difference on the reduction rate. The Figure clearly
showed that the peak at 267nm quickly decreased with increasing the content of
bmimCHgOz. When the molar ratio of bmimCH3Oz/Pt > 10, the peak at 267nm
completely disappeared even after 105 MW irradiation. The adsorption spectral patterns

above bmimCH3Oz/ HgPtCl6-6HZO = 20 were not affected in spite of the further increase

109

of bmimCH3Oz concentration, suggesting that no further addition of bmimCH3Oz to Pt
precursor-EG solution is needed to increase the reduction rate of Pt ions at the present

condition of MW irradiation.

 

  
 
  
 
  

 

 

 

 

5
Before MW reduction
— -— After 103 MW reduction-No RTIL
4 - a — — — — After 103 MW reduction-BMIMCHSCOZU)
______ After 108 MW reduction-BMIMCHSCOJS)
After 103 MW reduction-BMIMCH3C02(1O)
§ 3 e After 103 MW reduction-BMIMCHJCOJZO)
(U
.0
h
0
B
< 2 ‘
1 a
o . . . . *_T___T
200 250 300 350 400 450 500

Wavelength (nm)

Figure 4.4. UV-vis absorption spectral change of HthC16-6H20-ethylene glycol
solution containing different amount of bmimCH3C02 before and after microwave

reduction.

Reduction in the presence of bmimCH 3S04 and bmimPF6. The influence of other two
RTIL, bmimCH3SO4 and bmimPFb, on the formation of Pt particles was investigated at a
fixed composition of the RTIL/ HthC16-6HZO = 20 in molar ratio. The concentration of
both RTIL was determined on the basis of previous results obtained by bmimCH3COz. As

shown in Figure 4.5 obtained in the presence of bmimCH3SO4, the peak corresponding

llO

to Pt ions drastically decreased after 105 MW irradiation and Pt nanoparticles completely
form after 205 MW irradiation, suggesting that the reduction reaction of Pt ions and the
formation of Pt can be completed in 205 through MW heating. The similar behavior was

obtained in the presence of bmimPF6 (not shown in here).

 

3 - 1 —— Before MW reduction

1 — — — After 108 MW reduction

l —————— After 203 MW reduction

—— » -— - —- . —— After 303 MW reduction
- - After 408 MW reduction

 

 

Absorbance

—~“-._
~‘ “g
‘_ -—K._ .
~ — .‘_
~__'—_ ......
_—~ ' ‘ ‘ ____
—‘-—_‘——
—__

—_~
‘—_
‘-—-—
——*—.

 

 

 

 

200 250 300 350 400 450

Wavelength (nm)

Figure 4.5. Typical UV-vis absorption spectral change of HthCk-6H20-EG solution
in the presence of bmimCH3804 in MW irradiation.

The reduction rate of [PtCl6]-2 in the presence of bmimCH3COz, bmimPF6, and
bmimCH3SO4 were compared in Figure 4.6. The molar ratio between RTIL and Pt salt
was 20. The resulting data were recorded afier 105 MW irradiation. As clearly seen in the

Figure, the peak height of Pt ions rapidly decreased in the following order; bmimCH3COz

111

> bmimPF6 > bmimCH3804. This order may have resulted from the different polarity of

RTIL because the reaction has been conducted in the microwave oven. All of the RTIL

,1

 

I After 103 MW reduction-No RTIL
After 103 MW reduction-bmimCH‘fiO4

A After 103 MW reduction-bmimPF,
9 After 103 MW reduction-bmimCH3002

e3» " l‘f"ulflll$'
e

 

2*.

Absorbance (a.u)
O

  

 

 

 

200 250 300 350 400

Wavelength (nm)

Figure 4.6. Comparison of UV-vis absorption spectra obtained in the presence of
three RTILs such as bmimCH3SO4, bmimPFg, and bmimCH3COz after 10s MW

reduction.

have the same cation, l-butyl-3-methylimidazolium but different anions. Thus, only a
difference in the polarity originates from anion groups such as CH3COz, CH3SO4, and
PF6. CH3COz is expected to have the highest polarity due to unsymmetrical arrangement
by oxygen atoms among them. Hence, the decrease of the peak intensity was most
dramatic in the presence of bmimCH3COz. Meanwhile, since both CH3SO4 and PF6 have

fairly symmetrical molecular geometry, it is hard to tell which one has higher polarity. It

112

has been reported that for the bmim+ ionic liquid, the polarity correlates with anion size
and large anion results in a decrease of the polarity [55, 56]. Based on this report, PF6
seems to have slightly higher polarity than CH3SO4 because the former has a smaller size
compared to the latter. Therefore, the introduction of bmimPF 6 resulted in faster
reduction than bmimCH3SO4. Overall the decreasing order of the reduction rate seems
reasonable. It is also expected based on the results in Figure 4.6 that the size of Pt
nanoparticles may be in the following order of bmimCH3SO4 > bmimPF6 >
bmimCH3COz because the faster reduction produce smaller particles, which is supported

in section 4.3.2.

Reduction in the presence of bmimSCN.

Another set of UV-vis measurements has been conducted to monitor the formation
process for bmimSCN/ Pt = 20 in ethylene glycol via microwave irradiation. As shown in
Figure 4.7, UV-vis spectra of mixed solutions of Pt precursor with bmimSCN were
totally different from those obtained for the Pt solutions with three RTIL studied
previously. The mixed solution of Pt precursor and bmimSCN initially showed
significant absorbance enhancement of the Pt ion peak at about 280nm. Within 105, the
peak became broader and a shoulder formed at about 370nm which seems to correspond
to another type of Pt ions [5 7]. After 205, the peak completed to shift to a higher
wavelength at 300nm proving the coordination of bmimSCN to Pt ions, which in turn
provides the stabilizing action of bmimSCN to Pt metal clusters. As MW irradiation
proceeded, the peak intensity at 300nm decreased and disappeared completely, indicating

the formation and the growth of Pt clusters and the completion of Pt growth.

113

The ability of bmimSCN to reduce Pt ions in ethylene glycol without MW irradiation
was also studied. The mixed solution of HthCl6-6HzO-EG-bmimSCN was stored in a
vial at room temperature for a day and then UV-vis measurements for the solution were
conducted. Surprisingly, the peak at 280nm corresponding to Pt ions disappeared
completely and only a broader shoulder was found after 1 day without MW irradiation as
shown in Figure 4.8. This phenomenon was not found with other RTIL used in this work
The result suggests that bmimSCN itself can act as a weak reducing agent. Consequently,

bmimSCN acts as a strong protecting agent as well as a reducing agent, which is expected

to have a synergic effect on the reduction of Pt size.

 

 

 

 

 

5 r l
| It
i ; It: —— BeforMW reduction
.1” -—- — — After 103 MW reduction
4 4 K 2‘1: —————— After 203 MW reduction
\I 1,1 ----------- After 303 MW reduction
\‘lt f1. ~~ After 403 MW reduction
as i“; \ —— -——--—-4~ After 503 MW reduction
3 3 ‘ ' 0 Before MW reduction-No SCN
3 ‘\\-‘ ’\\
'5 ‘33 \ \ \\
.3 g \\
a 0 . ‘o.
< 2 8 5 '°. \. \
". 5 '°. \ \
°-. 5 '.\. "\\
‘2 '-‘.\\
1 ‘ .1., \ _
\a. grass...“
0 I I I I F I __
200 250 300 350 400 450 500 550
Wavelength (nm)

Figure 4.7. UV-vis absorption spectral change of HthCk-6H20-EG solution
containing of bmimSCN(20) before and after microwave reduction.

ll4

 

 
   
  

Il —— Before MW reduction

_ _. —- — Before MW reduction-after1day
+- ————— 508 MW reduction

503 MW reduction-after1day

Absorbance
N

 

 

 

200 250 300 350 400 450 500

Wavelength (nm)

Figure 4.8. UV-vis absorption spectral change showing the ability of bmimSCN
reducing Pt ions.

4.3.2 Unsupported Metal Nanoparticles

Three samples of unsupported Pt nanoparticles were synthesized in the absence and
the presence of bmimPF6 and bmimCH3C02 by MW irradiation in ethylene glycol in
order to know if there is the ability of RTIL as a stabilizing agent. Figure 4.9 show TEM
micrographs of Pt nanoparticles and corresponding Pt size distribution histograms of the
samples. The size and the size distribution of the metal particles was obtained by
measuring the size of 150 ~ 200 randomly selected particles in TEM images.

Pt particles reduced by MW irradiation in ethylene glycol without the addition of the

RTIL were easily precipitated. It was observed as in Figure 4.9 (a) that, most of the Pt

115

nanoparticles were agglomerated and connected to form a network, resulting from the
absence of capping materials which terminate particle growth and prevent Ostwald
ripening (also referred to as coarsening) of the metal particles. The corresponding
histogram revealed that the average particle size was 6.22 i 0.87 nm and it size
distribution was rather wide. Unlike Pt particles prepared without the RTIL, no
immediate precipitation of Pt nanoparticles synthesized in the presence of bmimPF6
occurred but some precipitated Pt particles were found 2 day later. They seemed to exist
in the form of agglomeration as in Pt-N but actually were well separated individually and
3-dimensionally as in Figure 4.9 (b) with the mean size of 2.05 i 0.27nrn. On the other
hand, there was no precipitation of Pt particles prepared with the addition of
bmimCH3COz even after long storage for days, demonstrating high stability of the Pt
particles. TEM image of the Pt metals in Figure 4.9 (c) shows a remarkably uniform and
high dispersion of Pt metals on a copper grid. Pt particles have the average diameter of
1.62 um i 0.27 with a much narrower particle size distribution compared with Pt particles
obtained in the absence of RTIL. Similar results were obtained from other RTIL used in
this work.

It is generally agreed now that the size of metal nanoparticles depends on the
reduction rate of the metal sources [5 8]. The faster the reduction rate, the smaller the
metal particles are generated. It has also been proved in UV-vis measurements that the
reduction rate of the Pt precursor increases with the addition of RTIL and is proportional
to RTIL content. Therefore, the formation of much smaller Pt particles in the presence of
RTIL results from the further accelerated fast heating due to the high polarity of RTIL

under microwave irradiation. Additionally, the homogenous MW heating reduces the

116

temperature and concentration gradients in the ethylene glycol and thus provides a
uniform environment for the nucleation and growth of metal particles, as well as to
produce a narrow distribution of metal particles. Another important factor for the
formation of the smaller unprotected Pt particles in the presence of RTIL is its low
interface tension. RTIL is easily adsorbed onto the surface of metal particles which
reduces the tendency to undergo Ostwald ripening [59]. As a result, very small particles
are generated.

Although there was no obvious evidence from UV-vis spectrometry investigation,
how can the high stabilized Pt nanoparticles prepared in the presence of bmimPF6 and
bmimCH3COz be explained? The plausible answer may be the fact that the intrinsic high
charge and the steric bulk of RTILs can create an electrostatic and steric colloid-type
stabilization of transition-metal nanoparticles, which was proved by the Dupont group
[60, 61]. When the metal-nanoparticles have been stabilized or capped by ionic liquids, it
will be quite meaningful to define them as metal-doped or metal—containing ionic liquids
which have potential for many chemical reactions [62, 63]. It is worth noting that RTILs
can stabilize and facilitate the formation of small Pt particle with a narrow size

distribution from the simple Pt precursor via MW reduction.

117

 

 

3o . ,

dave = 6.22 nm
std = 0.87

 
   

25

20

Distribution (%)
G

 

 

4~5 5~5.5 5.5—~6 6~6.5 6.5~7 7~7.5 7.5~8

Particle size (nm)

(a)

Figure 4.9. TEM analyses of Pt nanoparticles in the absence (a) and the presence of
(b) bmimPF6 and (c) bmimCH3C02.

118

 

    

Distribution (%)

 

1.0~l.4 l.4~l.8 l.8~2.2 2.2~2.6 2.6~3 3~3.5

Particle size (nm)

(b)

Figure 4.9. Continued

119

Distri ution (%)

40

30

20

 

 

dave = 1.62 nm
std = 0.27

 
   

 

 

 

0.5~l l~1.3 l.3~l.6 l.6~1.9 1.9~2.2 2.2~2.5

Pt partilce size (nm)

(C)

Figure 4.9. Continued

120

TEM analysis of Ru nanoparticles synthesized with the addition of bmimCH3C02 via
microwave irradiation is shown in Figure 4.10. Like Pt nanoparticles produced at the
same reaction condition, Ru particles were separated and stabilized by the RTIL. The

mean size of the particles was 1.08nm and the size distribution was very narrow.

 

 

35 dave = 1.08nm
30 std = 0.24nm

     

 

Distribution (%)
N
O

 

 

0.4~0.6 0.6~0.8 O.8~1 l~1.2 1.2~l.4 l.4~l .6

Ru particle size (nm)
Figure 4.10. TEM analysis of Ru nanoparticles with the addition of bmimCH3C02.

121

Pd nanoparticles were also prepared with the addition of bmimSCN and seen in
Figure 4.11. The extremely small size of Pd particles (lnm in diameter) existed in a

similar form as Pt particles synthesized with bmimPF6.

 

Figure 4.11. TEM image of Pd nanoparticles with the addition of bmimSCN.

The Pt precursor was reduced only in bmimCH3C02 by MW heating process at the
same condition for the synthesis unsupported Pt metals to investigate the ability of RTIL
reducing the Pt precursor. After 30 second in a microwave, light gray powders
corresponding to irregular Pt networks as in Figure 4.12 were precipitated, suggesting
that bmimCH3C02 itself can also reduce the Pt precursor to Pt nanoparticles via
microwave heating. High polarizability of RTIL due to large positive ions makes it a very

good medium for absorbing microwave radiation [64]. Hence the use of RTIL for the

122

synthesis of inorganic nanostructures may lead to interesting research such as for the

large-scale production of gold nanosheets in bmimPF6 by microwave reduction [65].

 

200nm
_

Figure 4.12. TEM micrograph of Pt reduced via microwave heating of lmL
bmimCH3C02 containing HthC16o6H20.

The crystalline structure of unsupported platinum nanoparticles was obtained by
using powder X-ray diffraction (XRD). Figure 4.13 shows the XRD patterns of the
platinum nanoparticles prepared in the presence of various amount of bmimCH3C02. The
four clear peaks, namely (111), (200), (220), and (31 1), identified by Miller indices were
shown in the sample (Figure 4.13 a) synthesized without the addition of bmimCH3C02.
The peaks were consistent with face centered cubic (fcc) crystalline for Pt. The four
characteristic peaks became broader as the content of the RTIL increased (Figure 4.13 b

and c). When the molar ratio between bmimCH3C02 and Pt precursor became over 20

123

(Figure 4.13d), the diffraction peaks disappeared. The peak broadening in the XRD
pattern indicates the formation of Pt particles in the nanometer scale [66] and validates
the efficiency of the present experimental technique to produce nanosized metal particles.
XRD also proves that the average size of Pt particles decreases as the concentration of
bmimCH3C02 increase, which is consistent with results from UV-vis measurements and

TEM investigation.

(1“) (200)

‘1 (22") (311) (a)

’3 -i "

5.”; ‘ ‘ l" ‘9‘ ‘ I : ‘ i ,.

:3 [ll]. (b)

a ll I

E (c
I,,,,-,__ ._ MlluLIi.....-.u.l..-.iuu....aman.n-....-. .....I.- 4...... .. . .-
30 40 50 60 70 80 90

2 theta (0)

Figure 4.13. X-ray diffraction patterns of Pt nanoparticles synthesized in EC at (a)
bmimCH3C02/Pt precursor = 0, (b) 2.5, (c) 5, and (d) 20.

4.3.3 Carbon Supported Pt Catalysts

4.3.3.1 Multiwalled carbon nanotube (M WN D-supported Pt particles

 

 

Pt nanoparticle supported MWNT were s, “ 'zed via urierc..aw-polyol process in
the presence of all of the RTIL used in this work. The molar ratio of RTIL to Pt precursor

was fixed at 50. Their TEM images are shown in Figure 4.14. The largest Pt was

124

./—_

produced in the absence of any RTIL and the smallest Pt in the presence of bmimSCN.
The size of Pt particles is in the decreasing order of Pt-N > Pt-bmimCH3SO4 > Pt-
bmimPF6 > Pt—bmimCH3C02 > Pt-bmimSCN as expected from the results of UV-vis
measurement in which the reduction rate was in the increasing order of Pt-N < Pt-
bmimCH;SO4 < Pt-bmimPF6 < Pt-bmimCHgCOz. The smallest Pt with the addition of
bmimSCN is resulted from the synergic effect of bmimSCN as a good microwave

absorbent, strong protecting agent as well as weak reducing agent.

 

Figure 4.14. TEM images of (a) Pt/MWNT-N and (b) Pt/MWNT- bmimCH3SO4, (c)
Pt/MWNT-bmimPFs, (c) Pt/MWNT- bmimCH3C02, and (e) Pt/MWNT-bmimSCN.

125

 

Figure 4.14. Continued.

126

 

Continued.

14

Figure 4.

127

The result of TEM analysis for Pt particles dispersed on the surface MWNT is shown
in Figure 4.15. As a result of the introduction of RTIL, the average size and standard

deviation of Pt particles drastically decreased, indicating the improved size distribution.

 

      

10— ~ ~ ~
E
5
a, 8
a
T: 6
t:
:3
n. 4
0.)
%‘3
g2
< .
0i...._’ _m_ s

No RTIL BMIMCH3SO4 BMIMPF6 BMIMCH3C02 BMIMSCN
Figure 4.15. Average size of Pt nanoparticles supported on MWNT synthesized in
the presence of different RTILs.

A few reports on the deposition of Pt nanoparticles on SWNT [67~69] was found
very recently. However, the size and dispersion of Pt particles seemed to have yet to
reach at the satisfactory level in those papers because they were not uniformly dispersed
on SWNT and their size distribution was quite broad. Before Pt deposition, it is also
required to oxidize SWNT to some degree to improve the anchorage of Pt particle on the
surface of SWNT, which makes the procedure complicated. Hence, in the present work,
Pt/SWNT has been prepared via MW irradiation in the presence of bmimCH3C02. As in
Figure 4.16, the extremely small size of Pt particles (1 .3~1 .6nm in diameter) was highly
dispersed on SWNT. It is believed that it is possible to further reduction of their size with

the addition of bmimSCN.

128

 

Figure 4.16 TEM images of Pt particles dispersed on SWNT synthesized in the
presence of bmimCH3C02.

129

4.3.2.1 Graphite nanofiber (GND-supported Pt nanocomposites

TEM morphologies of Pt/GNF-N, Pt/GNF-bmimPFé, and Pt/GNF- bmimCH3COz
nanocomposites with 20wt. % metal loading and the effect of RTIL on the mean size of
Pt phase corresponding to each sample are shown in Figure 4.17. It has been known that
there are difficulties in depositing Pt onto carbon nanofibers or graphite nanofibers via
colloidal and conventional routes. Hence, multistep deposition/decomposition of Pt
precursor is applied to increase Pt metal loading on them [70, 71]. The problem continued
even in microwave process as in Figure 4.17a. It was found that only approximately 10
wt.% of Pt was deposited onto GNF in final product, even though Pt precursor
corresponding to 20wt. % metal loading for Pt/GNF-N was added to the starting mixture.
The above fact confirms that Pt can not be efficiently supported on GNF even using
conventional microwave-polyol process. However, it was obvious from Figure 4.17b and
4.17c that the samples prepared in the presence of RTILs were highly covered with
smaller Pt particles compared to those synthesized in the absence of RTIL. Therefore, it
is clearly seen that the addition of RTIL in microwave-polyol process for the preparation
of GNF-supported Pt particles contributes not only to increase Pt loading deposited onto
GNF but also to generate the smaller Pt particles with narrow the size distribution. It is
worth noting that the agglomerates of Pt are rarely found in the case of Pt/GNF-
bmimCH3COz. The effect of RTIL on the reduction of Pt size for GNF-supported
catalysts is clearly shown in Figure 4.17d. The average sizes of Pt particles in Pt/GNF-N,

Pt/GNF-bmimPFé, and Pt/GNF-bmimCH3COz catalysts are 3.3 i 1.1nm, 2.46 i 0.7nm,

and 1.53 d: 0.4mm, respectively.

130

 

(b)

Figure 4.17. TEM morphologies of spiral graphite nanofiber (Gm-supported Pt
(20wt.%) nanocomposites by microwave heating process (a) in the absence of RTIL
and in the presence of (b) bmimPFg and (c) bmimCH3C02 and (d) the mean size of
Pt particles obtained from TEM analysis for all the samples.

131

Average particle size (nm)

 

 

 

 

 

Pt/GNF-N Pt/GNF-PF6 Pt/GNF-CH3 C02

(d)
Figure 4.17. Continued.

132

4.3.2.3 Carbon black (CB)-supported Pt Nanocomposites

Figure 4.18 and 4.19 show TEM analysis of Pt particles supported on XC-72R
carbon black which was fabricated by microwave dielectric heating in the presence and
absence of bmimPF6 and bmimCH3COZ. Nanosized Pt particles were quite uniformly
dispersed on CB in all of the samples. However, the mean particle size of Pt in Pt/CB-N
nanocomposite was about 3.38 i- 101 nm, while the size of Pt in Pt/CB-bmimPF6 and
Pt/CB-bmimCH3C02 were 1.75 i 0.74 nm and 1.28 i 0.24nm, respectively. In addition,
the Pt particle distribution of Pt/CB-N was much broader than that of Pt/CB-bmimPF6
and Pt/CB-bmimCH3C02. The results explain the addition of RTIL in synthesizing CB-
supported Pt catalyst is very effective to reduce the Pt size and improve Pt dispersion on
the surface of CB as in the Pt dispersed on MWNT and GNF, which is expected to

enhance the electrocatalytic activity for fuel cell applications.

 

Figure 4.18. TEM morphologies of (a) Pt/XC-72R-N, (b) Pt/XC-72R-bmimPF6 and
(c) Pt/XC-72R-bmimCH3C02 composites.

133

 

Figure 4.18. Continued.

134

 

Average particle size (nm)

 

 

Pt/CB-N Pt/CB-PF6 Pt/CB-CI-B C02

Figure 4.19. The average size of Pt particles in Pt/XC-72R-N, Pt/XC-72R-bmimPF5
and Pt/XC-72R-bmimCH3C02 composites.

4.3.2.4 Highly loaded Pt/carbon nanocomposites

It is difficult to achieve the synthesis of Pt catalysts with high metal loadings ( >
30wt.%) and small particle sizes at the same time by applying conventional methods [72-
74]. Metal particle size for a catalyst with 10wt.% and 30wt. % Pt loading can be
produced at 2.0nm and 3.2mm, respectively, but increase to 8.8nm for a 60wt. % Pt
catalysts. Therefore, the synthesis of highly dispersed and highly loaded nanosized metal
catalysts is a challenge. However, the problem can be resolved by a simple method of
introducing RTIL in microwave process. Figure 4.20 shows TEM images various
carbons-supported Pt nanoparticles with 60wt. % metal loading which were produced
without or with bmimCH3C02 in microwave heat process. The molar ratio of the RTIL to
the Pt salt was set at 50. As seen in Figure 4.20a, without RTIL, large Pt particles over
9nm in average size were highly agglomerated and the coarsening of the particles by
Ostwald ripening was clearly shown. The corresponding size histogram (not shown here)

showed very broad distribution. However, when Pt dispersed on the carbons was

135

synthesized in the present of RTIL, the mean size of Pt particles decreased strikingly
below 2.0nm with a narrow size distribution and they were very well dispersed on the
carbons (Figure 4. 20 b and c). Agglomerates of Pt particles were rarely found in spite of
high loading metals. This result suggests a simple and effective way of tuning active
surface area of Pt and catalytic activity of carbon supported Pt catalyst by controlling the
particle size and dispersion, no matter how much Pt is loaded and no matter what kinds of

carbon support is used.

 

(a)

Figure 4.20. TEM images of Pt nanoparticles highly loaded on (a) Pt/xGnP-N, (b)
Pt/Vulcan XC-72R-bmimCH3C02 and (c) Pt/MWNT-bmimCH3C02.

136

 

Figure 4.20. Continued.

137

The average particle size diameter and the standard deviation obtained for all of the
Pt/carbon nanocomposites synthesized in this work are summarized in Table 4.1. The
surface area (S) of the nanocomposites was calculated from the mean diameter of the
particles obtained from the TEM images by using the following equation:

_ 6000
pd

s (4. 1)

where S is the surface area (mZ/g), d is the mean diameter of Pt particles (nm), and p is
the density of Pt (21 .4g/cm3). Assuming that all Pt particles are spherical, the volume-
surface mean diameter (d,_,) of each nanocomposte could be obtained from the particle
size distribution according to the following equation:

d _ 2nd?

I

Then the surface average dispersion (D), which is defined as the ratio of the surface
atoms to the total atoms within the particles, was also calculated using the relationship

between relative particle size (d,,.1) according to the following equation [75].

d
d = i 4. 3
re] dat ( )
D : —————2°640 81 (4. 4)
(drel) '

where da, 5 0.278nm as the atomic diameter of Pt. These equations provide more accurate
than other one because they consider the particle size effects. The calculated D values for

all nanocomposites are also included in the Table 4.1.

138

Table 4.1 Information of Pt particles on various carbon supports synthesized via

RTIL assisted MW heating process

 

Pt content

 

 

 

 

 

 

 

 

 

samples (wt. %) (1 (nm) S (mz/g) D (%)
Pt/MWNT-N 20 7.10 i 1.47 39.47 18.22
Pt/MWNT-bmimPF6 20 1.92 i 0.40 145.80 51.72
Pt/MWNT-bmimCH3COz 20 1.55 i 0.25 181.03 63.08
Pt/MWNT-bmimCH3C02 60 1.65 i 0.26 169.72 59.94
Pt/MWNT-bmimCH3804 20 3.23 i 0.63 86.80 34.27
Pt/MWNT-bmimSCN 20 0.93 i 0.17 301.48 94.20
Pt/CB-N 20 5.56 i 0.90 50.36 22.45
Pt/CB- bmimPF6 20 1.75 i 0.29 160.18 56.88
Pt/CB- bmimCH3COz 20 1.28 :1: 0.24 218.98 76.08
Pt/CB- bmimCH3C02 60 1.57 i 0.32 181.03 61.44
Pt/GNF-N 20 3.29 r: 0.53 85.22 34.33
Pt/GNF- bmimPF6 20 2.46 i 0.41 l 13.97 43.33
Pt/GNF- bmimCH3C02 20 1.52 i 0.36 184.46 61.05
Pt/xGnP-N 20 3.74 i 0.64 74.93 30.94
Pt/xGnP-N 60 8.25 i 1.59 33.97 18.32
Pt/xGnP-bmimPF6 20 1.95 i 0.37 144.05 51.51
Pt/xGnP-bmimCH3COz 20 1.68 i 0.30 166.81 56.17
Pt/xGnP-bmimCH3COz 60 1.58 i 0.33 175.15 59.89

 

It can be seen from the Table that Pt/C nanocomposites prepared via MW irraidiation
in the presence of RTIL generally exhibit surface area and dispersion of Pt particles than
Pt/C prepared in the absence of RTIL. Especially, the surface area and dispersion of Pt

particles in Pt/C-bmimSCN are extremely high over 300m3/g and 90%, respectively, so

139

that bmimSCN may provide the chance to synthesize the excellent catalysts with high
catalytic activity. Therefore, the present preparation method developed in this work
seems to be a great way to obtain the supported nanosized noble metal nanoparticles with

a narrow size distribution and thus a great dispersion on the supports.

4.3.3 Metal Oxide Supported Pt Catalysts

Nanocomposites composed of noble metal nanoparticles and metal oxides such as
T102, CeOz, ZrOz, etc, have attracted much attention because they possess extraordinary
catalytic, optical and electronic properties [76, 77]. Such nanocomposites are of great
importance as catalysts for next generation power/energy conversion devices. Pt/TiOz
nanocomposite is most attractive among various combinations because it is a key material
in biomass conversion, solar energy conversion, photocatalytic cells, and photochatalytic
water splitting [78]. Their catalytic properties are influenced by interaction between metal
and support and the size of metal phase. Hence, it is challengeable to develop new
method of manipulating the size and dispersion of metal particles by which metal-support
interactions is affected. Pt nanoparticles dispersed on Ti02 nanopowders fabricated via
MW process in the presence of bmimCH3COz were compared to those in the absence of
it in Figure 4.21. Pt/TiOz-bmimCH3COz nanocomposite consisted of much smaller
particles with narrow size distribution and better dispersion than Pt/TiOz-N. Similar
results were obtained with CeOz and Ale3nanopowders as shown in Figure 4.21 (c) ~
(f). Therefore, MW process assisted with RTIL can also be an effective way to produce

supported noble metal particles on various metal oxides.

140

 

(b)

Figure 4.21. TEM images of Pt particles in (a) Pt/TiOz—N, (b) Pt/TiOZ-
bmimCH3C02, (c) Pt/CeOz-N, (d) Pt/CeOZ-bmimCH3C02, (e) Pt/A1203-N, and (f)
Pt/AJZO3-bmimCH3C02.

141

 

 

4.21. Continued.

Figure

142

 

 

(f)
4.21. Continued.

igure

F

143

4.4 Conclusion

A simple and effective RTIL-assisted microwave process to synthesize uniform
small metal powders with a narrow size distribution and to fabricate nanocomposites
consisting of metals dispersed either on carbons or on metal oxides has been developed.
It has demonstrated that RTIL can act as a promoter to increase the reduction rate of
metal salt, a protecting material to prevent metal particles from growing and
agglomerating each other, and a reducing agent to reduce metal ions to metal particles in
microwave heating process. It has also shown that metal particles can be highly dispersed
on various carbon and metal oxides with the help of RTIL. Depending on the
combination of metal particles and support materials, such novel method is expected to
have promising applications in a wide variety of fields including industrial catalysts,
automobiles, rechargeable batteries, supercapacitors, low temperature fuel cells,

hydrogen storage and production as well as for sensors.

144

~

10.

11.

12.

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151

CHAPTER 5
THE EVALUATION OF EXFOLIATED GRAPHITE NANOPLATELETS
(xGnP) AS A SUPPORT FOR Pt CATALYSTS

IN DIRECT METHANOL FUEL CELL

152

5.1 Introduction

Fuel cells have attracted attention because they can provide an alternative way to
reduce the dependence on fossil fuels. Direct methanol fuel cell (DMFC) seems to be the
most promising among the various types of fuel cells. It is expected to be used in portable
power sources and transportation in the near future [1~3]. However, DMFC has critical
shortcomings which include methanol crossover from anode to cathode, slow kinetics at
both anode and cathode, and the cost of the precious metal catalysts [4~6]. One strategy
for last two problems is to achieve maximum utilization of the precious metal catalysts
which is possible by dispersing small metal particles on support materials without any
agglomeration. Platinum-based nanoparticles supported on conductive supports with high
surface area are currently and widely used as electrocatalysts in DMF C [7~11].

The catalytic activity of supported Pt catalysts is determined by many factors such as
the size and size distribution of Pt particles, their shape, and the crystallographic
orientation of Pt [12, 13]. This implies that the catalytic performance of supported Pt
particles depends on the synthetic route used to produce them. It is difficult to achieve a
high degree of dispersion of nanosized platinum particle with narrow size distribution on
the surface of supports through a conventional synthetic method. Therefore, it is still
desirable to develop an alternative simple and effective technique to manipulate Pt
particle shape, size, and distribution. The ideal method should be applicable to all kinds
of supports without any pretreatment and should produce consistent catalytic activity in
spite of large scale synthesis.

The catalytic activity of supported Pt particles is also related to the nature of the

supports [14]. Carbons are ideal for supporting nanosized Pt particles because they are

153

the only material having all of the essential properties such as high electrical conductivity,
high surface area, excellent crystallinity, good corrosion resistance, appropriate pore
structure, surface properties conducive to water-handling capability, and low cost. Those
properties are critical requirements which a good support should have. Carbon black,
especially XC-72 carbon, has been widely studied as a support but it has been shown that
the presence of micropores and sulfuric acid groups on XC-72 carbon black lead to a
poor utilization and aggregation of Pt particles, respectively [15, 16]. Hence, a variety of
novel carbon nanostructures such as single wall carbon nanotube (SWNT) and multiwall
carbon nanotube MWNT), graphite nanofiber (GNF), carbon nanohom (CNH), carbon
nanocoil (CNC) have been investigated and have shown promising results [l7~23].
Among them, SWNT and MWNT appear to be the most promising due to their excellent
thermal, mechanical and electrical properties [24, 25]. As an additionally advantage of
both MWNT and SWNT, Pt particles dispersed on the external walls of them are highly
effective since it is easier to make contact with the reactant gas or liquid than those in the
pores of other carbons.

Exfoliated graphite nanoplatelets (xGnP) have been produced in MSU by expanding
the graphite layers from graphite intercalation compounds via a specific thermal shock
and downsizing the expanded layers through ultrasonication and/or milling process.
xGnP meets the key properties required as an excellent carbon support. It has high
surface area, excellent corrosion resistance and superior mechanical and electrical
properties [26, 27]. xGnP is a much more cost-effective than SWNT and MWNT. In
addition, if Pt nanoparticles are well dispersed on the surface of xGnP, they can make

easy contact with the reactant as CNT and thus utilization of the catalyst is expected to

154

increase. In spit of such great benefits, the potential application of xGnP as a carbon
support has not been fully investigated.

In this chapter, the superior properties of xGnP to other carbons are manifested and a
simple technique to prepare xGnP supported Pt nanoparticles is reported. The method is
related to the reduction of Pt precursor in ethylene glycol solution in the presence of
room temperature ionic liquid (RTIL) and xGnP via microwave irradiation. The influence
of preparation conditions such as water and RTIL content were investigated. XRD, XPS,
and TEM characterizations were carried out to determine the particle size and distribution.
Cyclic voltammetry measurements for xGnP supported Pt catalysts were performed to
evaluate their catalytic activity. The results were compared with those of a commercially

available Pt catalysts deposited on XC-72R carbon black from E-TEK.

5.2 Experimental Details

5.2.1 Materials

Chloroplatinic acid hexahydrate (HthC16-6H20) was purchased from Aldrich and
used as a Pt precursor. Ethylene glycol (EG, bp = 197.3°C), diethylene glycol (DEG, bp =
245°C), and triethylene glycol (TEG, bp = 285°C) from J .T. Baker were used as agents to
reduce the Pt precursor. Two different RTILs, 1-butyl-3-methylimidazolium
hexafluorophosphate (bmimPF6) and l-butyl-3-methylimidazolium acetate
(bmimCHgCOz), were used in this work. Their chemical structures are shown in Figure
4.1 of chapter 4. bmimPF6 is denoted as RTIL-l and bmimCH3C02 as RTIL-2 in this
chapter for convenience. Both of them were purchased Aldrich and used as received.

xGnP produced in MSU was used as a carbon support. Figure 5.1 shows typical

155

dimension of xGnP which has average thickness of 12nm and average diameter of 1pm.
MWNT, carbon black (CB, Vulcan XC-72R), and GNF were purchased from Aldrich,
Cabot Co., and Nanomirae Inc., respectively and used for comparison. Purified SWNT

from Southwest was also used for that purpose.

 

Figure 5.1 Typical TEM pictures of xGnP.

5.2.2 Preparation of xGnP Supported Catalysts

Typical procedures to fabricate Pt/xGnP catalysts with 20wt. % Pt loading in the
absence of RTIL consist of the following steps: In a glass beaker, xGnP is ultrasonically
dispersed in 18mL EG for 30 min and the EG solution containing xGnP is cooled down
to room temperature under mechanical stirring. In another 10 ml beaker, a specific
quantity of the Pt precursor is added to 2 ml EG and completely dissolved. Contents in
those beakers are mechanically mixed in a 100mL beaker for another 30 min, transferred
into a commercial microwave oven (National, 2.45GHz, 1300W), and irradiated under air

for a total of SOseconds to reduce the Pt precursor. After cooling down to ambient

156

temperature, the resulting suspension was centrifuged and the residual sltury was washed
with acetone several times and dried in a vacuum oven at 373K overnight. The weight
ratio of Pt and xGnP was controlled according to the targeted metal loading on the
support. The catalysts obtained in this way are called as the Pt/xGnP-N.

For Pt (20wt. %)/xGnP catalysts synthesized in the presence of the RTIL-l and
RTIL-2, various contents of the RTIL are added and dissolved in 18mL ethylene glycol
prior to the dispersion of xGnP. The different molar ratios of RTIL/Pt precursor are
introduced. Other steps are the same as for Pt/xGnP-N catalyst. The Pt/xGnP catalysts
synthesized with the addition of bmimPF6 and bmimCH3C02 are denoted as Pt/xGnP-

RTIL-l and Pt/xGnP-RTIL-2, respectively.

5.2.3 Characterization Methods

Pt catalysts dispersed on xGnP were characterized by recording their X-ray
diffraction (XRD) patterns on a Regaku Rotaflex 200B X-ray diffractometer using Cu-Ka
radiation with a curved graphite monochromator in order to identify the phases present in
the catalyst and to evaluate the lattice parameter and the particle size of the Pt crystallites
dispersed on xGnP. The 20 region between 10° and 100° was scanned at 45 keV and 100
mV at a scan rate of 5°/min. The average metal particle size was calculated using Scheller
equation according to the XRD broadening Pt (220) reflections which was scanned at a
rate of 1°/min again. Transmission electron microscopy (TEM) investigation was carried
out with JEOL 100CX and J EOL 2200FS Operating at a voltage of 100keV and 200keV
for the morphology of the dispersed Pt electrocatalysts and the determination of the

average size of Pt particles. Specimens were prepared by ultrasonically re-dispersing the

157

sample powders in acetone for 15 min, applying the power suspension onto lacey carbon
coated Cu grids and drying them in air at ambient temperature. Thermo-gravimetric
Analysis (TGA 2950, TA instrument) was performed to compare the thermal-oxidative
stability of various carbons, evaluate the degradation of xGnP supported Pt catalysts and
determine the Pt loading. Samples weighing 3~4mg were tested over temperatures
ranging from room temperature to 800°C (for supported Pt catalysts) or 1000°C (carbons)
at 20°C/min under flowing air. X-ray photoelectron spectroscopy (XPS) results of
Pt/xGnP catalysts were obtained from a Physical Electronics PHI 5400 ESCA system and
used to invesitage the presence of the oxide phases and the interaction between Pt and
xGnP. The regional XPS of Pt (4f) was de-convoluted to recognize different metallic and
oxidation states of Pt on the xGnP.

Electrochemical investigation was performed by cyclic voltammetry (CV) using a
potentiostat (Gamry, FAS2 F emtostat). A conventional three-electrode cell equipped with
a platinum wire counter electrode, an Ag/AgCl reference electrode, and glassy carbon
working electrode was used. The glassy carbon electrode with 3mm diameter was
polished to a mirror finish with a 0.05 pm alumina suspension before each experiment.
The catalyst ink was prepared by ultrasonically dispersing 5mg of Pt/xGnP catalysts in
0.8m] of alcohol containing 0.1m] of 0.1wt % Nafion solution for 20 min. Then 20 ”L of
the catalyst ink was micropipetted on the top surface of the glassy carbon electrode.
Finally, the electrode was dried at 40°C to yield a Pt loading of about 0.3142mg/cm2. The
CV experiments were performed in 1M H2SO4 solution in the absence and the presence
of 2M CH3OH at the scan rate of 50mV/s. All the solutions were prepared by using

ultrapure water. N2 gas was purged for 15 min before starting the experiment. The

158

Chronoamperometry tests were also performed in 2M CH3OH/1M H2SO4 at 0.45V for the

period of 30003 to study the catalyst stability.

5.3 Results and Discussions

5.3.1 Comparisons of Various Carbon Supports

Supported electrocatalysts used in the fuel cells have to meet severe performance
requirements of long-term stability over 40,000 hours. Oxygen is supplied on the cathode
side for oxygen reduction reaction (OR) and water generated in OR has to be rapidly
removed from the electrode. This environment has the potential for unwanted surface
oxidation of carbon supports on the cathode in PEMFC and DMF C. Support corrosion
may occur due to the presence of oxygen, which is responsible for the performance loss
of PEMFC and DMFC. Therefore, the carbon support inevitably influences the catalytic
activity of carbon supported catalysts in OR at the cathode in PEMF C and DMFC [14].
Good corrosion resistance or oxidation resistance is one of important characteristics that
an ideal support should possess for applications in PEMFC and/or DMFC under strong
oxidizing conditions. Kinetic stabilization of the support towards corrosion can be
accomplished by increasing the graphitization of the carbon supports which in turn
facilitate the easy water removal by hydrophobization of the carbon surface.

Figure 5.2 shows the thermo-oxidation stability of various carbon supports measured
in an air environment by TGA. Three carbons such as MWNT, XC-72R and xGnP started
to decompose above 600°C, while SWNT and GNF were completely decomposed before
reaching at 600°C. Thus, MWNT, XC-72R, and xGnP appear to be more appropriate

supports than SWNT and GNF on the basis of long-term stability in the oxidizing or

159

corrosive environment in fuel cell systems. xGnP showed the highest oxidation resistance
among all of the support candidates. This is an expected result because the graphite has
the highest degree of graphitization among all of the carbons, which is proven from XRD
data. Interestingly, the XC-72R CB which is the most widely used in fuel cell showed

higher onset temperature for decomposition than either GNF or SWNT, which may

suggests a heat treatment of the CB to increase its graphitization.

 

 

 

 

 

 

 

 

 

120
100 a E_ - -----m..——w-‘wur
\ %‘W.
I. I.
80 - l 1‘:
7 l l l,
25 ' i '\
E 60 i i °'
3’ '. i ‘-
3 I l ‘.
40 ~ “ l
‘ e
______ r |
20 2 .......
o .
0 200

 

Temperature (°C)

Figure 5.2. T hermo-oxidative stability of various carbon supports in an air
environment.

Since the Pt particles loaded on carbons can act as a catalyst for chemical
combustion of the carbon support which causes a decrease of fuel cell performance, it is

interesting to compare combustion resistance of each carbon loaded with Pt particles.

160

Hence, XC-72R CB was selected because it is most widely used in fuel cell at present.
The combustion stability of xGnP and XC-72R CB loaded with 20wt. % Pt particles is
compared in Figure 5.3. When both catalysts were exposed to air at elevated temperature,
XC-72R lost its weight more rapidly at about 230°C lower temperature than the bare CB.
However, the decomposition temperature of xGnP loaded with Pt particles shifted 1000C
lower temperature compared to bare xGnP. This result indicates that xGnP support for Pt
particles possesses much better stability in oxidizing environment of fuel cells than the

commercial one.

 

 

 

 

 

 

120
100 - m: ........ _‘_'
80 -
‘2?
E. 60 .
H
.c
.9
g 40 -
20 -
xGnP
______ 20wt. % PtlenP \._._,___._._.-._
0 ‘ _.—.——-— XC-72R
— -------- 20wt.% PtIXC-72R
0 200 400 600 800 1000

Temperature (°C)

Figure 5.3. Comparison of TGA curves for xGnP and XC-72R with and without
20wt. % Pt particle. Scan rate 25°C/min.

161

The degree of crystallinity of carbons can be compared from X-ray diffraction
analysis. Figure 5.4 shows X-ray diffraction patterns measured on all the carbon supports
used in the present work. There is a common major peak at between 20 = 24~ 26°
corresponding to (002) plane for all. carbons and some of carbons have minor peaks at
about 20 = 44° and/or 55°. SWNT shows the lowest graphitization with a (002) plane d
spacing of 3.49 A among all carbons, while xGnP has the highest graphitization with a d-
spacing of 3.37A. It is found on the basis of peak height at (002) plane that the degree of
crystallinity is in the order of xGnP > GNF > CB > MWNT > SWNT. This provides

reason why xGnP showed the best thermo-oxidative stability among all of the carbons.

 

 

 

 

 

Counts (a.u)

 

 

 

20

Figure 5.4. X-ray diffraction patterns of various carbon supports (a: SWNT, b:
MWNT, c: CB, d: GNF, and e: xGnP) used in the present work.

162

The crystallite size of individual carbon materials perpendicular to the basal plane
can be calculated from the (002) peak according to Debye-Scherrer formula as shown

below:

_ 0.91m,
B(26) cos Bmax

 

(5.1)

where D is the crystallite size, Ego, is the X-ray wavelength (0.154138nm), 19”,“, is the
maximum angle of the (002) peak, and B(26,m,,) is the full width half maximum peak
(FWHM) in radian. The results calculated are listed in Table 5.1. xGnP has the smallest
d-spacing and the largest crystallite which also explain why xGnP showed the highest
oxidation and combustion resistance. Consequently, it is proved from TGA and XRD

data that xGnP can be an excellent carbon support with good corrosion resistance.

Table 5.]. Characteristics of carbon supports calculated from their XRD pattern

 

 

Carbons 0max (°) d-spacing F WHM 8(28max) D (nm)
SWNT 25.20 0.349 6.06 0.1058 1.31
MWNT 25.45 0.350 4.69 0.0819 1.74
CB 24.50 0.363 6.90 0.1204 1.18
GNF 26.02 0.342 2.20 0.0384 3.71
xGnP 26.45 0.337 0.24 0.0042 34.02

 

5.3.2 XRD analysis of Pt/xGnP
Figure 5.5 shows the X-ray diffraction patterns measured on microwave synthesized
Pt/xGnP-N and Pt/xGnP-RTIL-2 with the addition of various molar ratio of RTIL-2/Pt.

The peaks at about 20 = 26.48° (002) and 546° (004) are attributed to the graphite

163

structure (002) and (004) of xGnP, respectively. For Pt particles, XRD pattern display
diffraction characteristic of a face-centered cubic (fcc) crystal structure with major peaks
at 20 = 39.74° (11 1), 46.32° (200), 67.6° (220), and 81.62°(311). The diffraction peak at
20 = 39.74° for Pt (111) corresponds well to the d-spacing = 0.227nm at Pt (111) plane.
While the sharp reflections are found when low molar ratio of RTIL-2/Pt are introduced,
the four characteristic peaks become broader at RTIL-2/Pt = 10 and disappear at RTIL—
2/Pt =50. This large extent of peak broadening suggests the formation of very small Pt
nanoparticles and their good dispersion [28]. XRD intimates that the average size of Pt
particles decreases as the concentration of the RTIL-2 increase and visible evidences are

in section 5.3.3.3.

 

Intensity

 

 

20 40 60 80
2 theta

Figure 5.5. X-ray diffraction patterns of Pt/xGnP prepared (a) without RTIL-2 and
with the addition of (b) RTIL-2/Pt = 2.5, (c) 10, and (d) 50.

164

5.3.3 TEM analysis of Pt/xGnP

5.3.3.1. Effect of water

There are two experimental factors to produce metal nanoparticles with different
average size dispersed on carbon supports in microwave heating process of EG
containing metal precursor. One is to control pH value of EG solution. It has been
reported that Pt particles became smaller and more unifome dispersed on the surface of
CNT when the synthesis pH increased [29]. The other is the ratio of EG to distilled (DI)
water. This is probably another key factor in controlling the particle size and distribution
of Pt on carbon supports in the synthesis of metal particle via microwave heating process,
even though it has not been reported. Li et al. reported that, in conventional EG method,
smaller and more homogeneous Pt particles are obtained by increasing the concentration
of EG or decreasing water content [30]. The mean size of Pt particles was 2.0 nm when
no water was added and increased to 4.5 nm when the water content was 70 vol. % in EG.
Wang et al. also reported a polyalcohol route for the preparation unsupported noble-metal
(Pt, Ru, Rh, etc) nanoclusters, in which they found that the metal particle size is very
sensitive to the water content of the solvent [31]. The size of Pt particles was about 1.1nm
without the addition of water and about 2.4 nm with the water content of 9 vol. %.

TEM analysis of Pt/xGnP catalysts prepared via microwave irradiation by
controlling DI water content in EG solution clearly showed the effect of water content on
the size and dispersion of Pt particles supported on xGnP as in Figure 5.6 and 5.7.
Figure 5.6a is a representative TEM image at a low magnification, showing that the
whole surface of a single xGnP was very well covered with Pt nanoparticles. As seen in

Figure 5.6 b, c, d and Figure 5.7, the average size of Pt decreased from about 3.8nm to

165

about 2.6mm as the content of DI water increased from 0 to 50 vol. % in EG solution.
This result was contrary to the reports mentioned above. This may result from the
different mechanism in synthesizing Pt particles deposited on xGnP. Li et al. and Wang et
a1. prepared Pt particles through conventional heating method of EG-metal salt solution,
which is different from microwave process for this work. It is known that the size of the
metal particles is determined by the reduction rate of the metal precursor. The fast
heating by microwave irradiation accelerates the reduction of metal precursor and the
nucleation of the metal clusters. Hence, the introduction of a high dielectric solvent, DI
water (£= 80.1 at 298K), into a relatively low dielectric EG (a: 41 at 298K) will help
the fast heating which contributed to the formation of small Pt particles by microwave
dielectric heating. However, it is worth noting that the presence of a large quantity of
water hinders the dispersion of Pt particles on xGnP because DI water has low chemical

affinity to the basal plane of xGnP which is highly hydrophobic.

 

166

 

 

tinned

. Con

6

5

igure

F

167

 

(d)

Figure 5.6. TEM images of Pt/xGnP (a) at low magnification and at high
magnification synthesized with the addition of (b) 0, (c) 5, and (d) 50 vol. % DI
water, respectively.

 

 

 

 

E 5
E.
E 4
o
o 3
N
.3 2
C
8
E 1
2
,_ 0
0 5 50
Water content (vol.%)

 

 

 

Figure 5.7. The effect of water content in EG on Pt particle size determined in TEM.

168

5.3.3.2. Effect of R T IL

There are several advantages of using a RTIL in the synthesis of supported metal
nanoparticles [32]. First, since RTIL produces a low interfacial tension which results in
high nucleation rates, it is possible to generate very small particles which undergo weak
Ostwald ripening. Second, it will accelerate the solvent heating in microwave process
enough to ensure a sufficient rate of reaction due to the high polarizability of RTIL. Third,
RTIL has a strong affinity to the basal plane of xGnP due to its hydrophobicity and thus
most of RTIL molecules will be adsorbed on the xGnP surface. Finally, owing to the high
thermal stability of RTIL, reactions can be conducted at temperatures well beyond 100°C
safely. All of these advantages synergistically accrue to generate well dispersed small Pt
particles on the surface of xGnP. Figure 5.8 and 5.9 show the result 3 of TEM analysis of
Pt/xGnP-N, Pt/xGnP-RTIL-l, and Pt/xGnP-RTIL-Z. Compared to Pt particles of
3.74nm in average size obtained in the absence of RTIL (figure 5.8a and 5.9), the mean
size of Pt reduced to about 2.0nm when RTIL-1 is introduced to EG (figure 5.8b and 5.9)
and further decreased to about 1.68nm when RTIL-2 was added (figure 5.8c and 5.9).
The size distribution of Pt particles also was improved by the introduction of RTIL-1 and
RTIL-2. As proven by data from UV-vis spectroscopy in chapter 4, RTIL-2 increased the
reduction rate of the Pt precursor more rapidly than RTIL-1. Therefore, the smaller Pt
particles on the surface of xGnP generated by the addition of RTIL-2 are in good
agreement with the result in previous chapter. Consequently, as a result of the presence of
RTIL, it is possible to achieve the very high heating rate by microwave, which results in

the increase of reduction rate of the metal precursor and thus producing smaller Pt

169

particles well dispersed on xGnP. RTIL also act as a capping materials of preventing the

Pt particles from experiencing Ostwald ripening

 

Figure 5.8. TEM images of (a) Pt/xGnP—N, (b) Pt/xGnP-RTIL—l, and (c) Pt/xGnP-

RTIL-2.

170

 

(C)

Figure 5.8. Continued.

 

 
     

5 dave = 3.74nm
std = 0.90

’E‘ 4
c
‘5' dm = 1.95nm _
.5 3 std = 0.37 dave ‘ 1.68nm
a std = 0.24
2 2
.2
‘1:
E 1

0

 

PtIXGnP-N

 

PtlenP-RTIL-1 PtIxGnP-RTIL-Z

Figure 5.9. The average particle size and size distribution of Pt particles in (a)
Pt/xGnP-N, (b) Pt/xGnP-RTIL-l, and (c) Pt/xGnP-RTIL-Z.

171

5.3.3.3. Effect of R T IL content

The effect of RTIL content on the particle size and distribution of Pt particles was
investigated. As shown in Figure 5.10. Addition of both RTIL results in the reduced size
of Pt particles with narrow size distribution. While 3.74nm of Pt particles with large
standard deviation were formed without any RTIL, the mean size of Pt particles was
reduced below 3nm and their size distribution became narrower when the RTIL
corresponding to the molar ratio of RTIL/Pt precursor =1 was added. The size of Pt
particles further decreased with increase of the RTIL content but asymptotically approach
1.5nm beyond RTIL/Pt = 10 in molar ratio. Therefore, the RTIL content beyond RTIL/Pt

= 10 does not seem to have a great effect on further size reduction of Pt particles on xGnP.

 

 

5
T I PtIxGnP-RTlL-1
4 . d e PtlenP-RTlL-2
E
5
fl 3 - "
"a;
2
.2
t:
R 2 -
1 -

 

 

 

0 20 4o 60 80 100 120
Molar ratio (RTIL/Pt)

Figure 5.10. Effect of RTIL content on the size of Pt particles supported on xGnP.

172

5.3.3.3. Effect of reducing agents

The size of the Pt on xGnP synthesized by the present method is dependent on the
nature of the reducing agents. Employing DEG or TEG as reducing agents, the change of
Pt size was studied and the results are shown in Figure 5.11. When Pt/xGnP was
prepared in DEG or TEG by microwave heating process without the addition of RTIL-2,
the larger Pt particles with irregular shapes were formed on xGnP compared to those in
EG (Figure 5.11a, b and Figure 5.8a) and they were agglomerated. However, the
particle size was drastically reduced below 2nm and monodispersed on the xGnP surface
when Pt/xGnP catalysts were fabricated in the presence of the RTIL-2. No significant

effect of reducing agent on Pt particle size was found in this case.

 

(a)

Figure 5.11. TEM images of Pt/xGnP-N synthesized in (a) DEG and (b) TEG,
respectively and Pt/xGnP-RTIL-Z prepared in (c) DEG and (d) TEG, respectively.

173

 

(C)

Figure 5.11. Continued.

174

 

(d)

Figure 5.11. Continued.

Generally, the metal size is influenced by the reduction temperature and reduction
rate. The increase of reduction temperature accelerates the reduction rate of the metal
precursors, resulting in smaller metal particles with a narrow size distribution. The
boiling points of all the reducing agent in the present work are in the order of TEG >
DEG > EG. Hence, if Pt/xGnP nanocomposites were prepared through reflux at high
temperature, the particle size should be in the order of increasing boiling point. However,
since the most important factor dominating the particle size in the microwave heating
process is the reduction rate which is determined by the MW-adsorbing properties of the
reducing solvents, dielectric constant of each solvent must be considered. Dielectric
constant of all the solvents here is in the following order; EG > DEG > TEG [33] which

is reverse to the order of boiling point but will be the order of fast reduction rate in

175

microwave process. Therefore, it is expected that the particles size decreases in the order
of TEG > DEG > EG which is well consistent with TEM analysis as in Figure 5.8 and

5.11.

5.3.3.4. Comparison of morphological data

In Table 5.2, the averages sizes of Pt particles in Pt/xGnP catalysts prepared at
different molar ratio of RTIL/Pt precursor measured from TEM images were compared
with those calculated from the XRD peak at (220) by using Equation (1). The results
from TEM and XRD agreed well. The surface areas of those catalysts are calculated from
the mean diameter of the particles obtained from the TEM images by using the following
equation:

_ 6000
pd

where S is the surface area (mz/g), d is the mean diameter of Pt particles (nm), and p is

S (5-2)

the density of Pt (21.4g/cm3). Among all the catalysts, the surface area is found to be
increased with increasing molar ratio of RTIL/Pt. Dispersion (D) of the metal particles is
an important parameter used to calculate the activity of the catalyst per surface metal
atom. D is defined as the mean fraction of the total metal atoms exposed at the surface. D
of Pt particles deposited on xGnP roughly calculated from d = 1.08/D (nm) with TEM
results [34] were also listed in Table 5.2. The data showed that RTIL-2 is more effective
in reducing Pt size and thus increasing Pt dispersion on xGnP than RTIL-1. It is also
interesting that the surface area and dispersion of Pt particles in the highly loaded catalyst
with 60wt. % of Pt synthesized in the presence of RTIL-2 were 5 and 3 times higher than

those in the catalyst prepared without RTIL.

176

Table 5.2 Comparison of morphological data of XC-72R and xGnP supported Pt

(20wt. %) catalysts prepared at different molar ratio of RTIL/Pt

 

 

 

 

 

samples RTIL/Pt in d from d from S D

inolar ratio TEM (nm) XRD (nm) (m /g) (%)

Pt/XC-72R - 2.23 i 0.55 2.54 125.73 48.43
Pt/xGnP-N 0 3.74 i 0.64 3.94 74.97 30.94
03 8.25 i 1.59 6.65 33.97 18.32

Pt/xGnP-RTIL-l 2.5 2.88 i 0.46 3.01 97.35 37.50
5 2.54 i 0.41 2.56 110.38 42.52

10 2.10 i 0.40 2.43 133.51 51.43

50 1.95 i 0.37 2.09 144.05 55.38

Pt/xGnP-RTlL-2 2.5 2.64 i 0.41 2.81 106.20 40.91
5 2.01 :1: 0.39 2.28 139.49 53.73

10 1.78 i 0.37 2.05 157.51 60.67

50 1.68 i 0.30 1.99 166.81 64.29

50b 1.58 i 0.33 1.83 175.15 68.35

 

a'hThe catalysts loaded with 60wt. % Pt.

5.3.4 XPS analysis of Pt/xGnP

Pt/xGnP catalysts prepared without and with the addition of RTIL—2 were analyzed
by XPS to determine the oxidative state of Pt particles as well as the interaction between
Pt and xGnP. Figure 5.12 shows the Pt 4f signal in Pt/xGnP-N consisting of three pairs
of doublets. The two strong energy band at 71.66 and 74.99 eV are due to Pt particles in
metallic state, while two weak energy bands observed at 73.09 and 76.42 can be assigned
to Pt+2 and Pt+4 in oxidation state. XPS study explains the electron donation by Pt to a
carbon support [34]. The Pt 4f7/2 peak for the Pt/xGnP catalyst shifted to higher values by
0.56eV with respect to 71.1eV for Pt metal in the literature [36, 37], indicating the strong
Pt-xGnP support interaction through electron transfer from Pt to the surface of xGnP.
This shift can be interpreted as a small particle effect as reported earlier in [3 8, 39]:

particles in the l~2 nm range have not yet attained the normal bulk band structure, so the

177

binding energy for the particles shifts to higher values. On the other hand, it is worth
noting as in Figure 5.13 that the XPS spectrum did not show the peak at l98eV
corresponding to the presence of Cl', indicating the perfect reduction of Pt precursors.
This is a significant advantage because of the necessity of Cl-free electrode preparation in
order to avoid the catalytic loss or degradation of Pt/xGnP catalysts.

Deconvolution of the XPS spectrum was carried out for other RTIL-2 /Pt precursor
molar ratio = 2.5, 5, 10, and 50 catalysts. The percentage of Pt particles in metallic and
oxidative state was calculated and listed in Table 5.3. The results for unsupported Pt
blacks synthesized in the absence and the presence of RTIL-2 and a commercial Pt/XC-
72R from E-TEK Inc. were also included for comparison. While Pt/xGnP-N and the E-
TEK catalyst showed similar content of metallic Pt particles, higher percentages of
metallic Pt were present in Pt/xGnP-RTIL—Z. However, the content of metallic Pt
species decreased with increase of RTIL-2 content. It is not clearly known if the partial
oxidation of Pt takes place in the middles of synthetic steps of supported catalysts and/or
if the reduction reaction of the Pt precursor is incomplete. Pt oxidation can occur when
the supported Pt is exposed to air because oxygen can easily be chemisorbed on the
surface of the Pt clusters. The presence of Pt oxides may be responsible for the detection
of oxygen-containing species found on the oxygen groups of xGnP support. The content
reduction of Pt species in oxidation is possibly due to the lower susceptibility of
oxidation of Pt nanoparticles which are adsorbed by RTIL-2. However, the decrease of
metallic Pt at high molar catalysts may result from the increase of RTIL-2 on Pt particles.

The electronic interaction between Pt and xGnP can be recognized by the shift of

metallic Pt peak location for Pt/xGnP with respect to unsupported Pt [40, 41]. As listed in

178

Table 5.3, the metallic Pt peaks for xGnP-supported RTIL-2 /Pt precursor molar ratio = 0,
2.5, 5, and 50 catalysts shifted to higher values by 0.28, 1.16, 0.87, and 1.68eV with
respect to corresponding unsupported Pt blacks. This result can be interpreted as related

to the presence of Pt-xGnP electronic effect. The Pt-xGnP interaction is through electron
transfer from Pt to the surface of xGnP. In the absence of mechanical interlocking, the
adhesion arises from molecular, electrostatic, chemical surface forces acting across the

interface of two solids [42]. The shift of Pt peaks for Pt/xGnP-RTIL-2 samples is bigger

than the commercial Pt/XC-72R, indicating that the interaction between Pt-xGnP is

stronger than that between Pt-XC-72R carbon black. The small size effect is excluded

here because the Pt size for supported samples is similar to that for corresponding

unsupported particles.

Table 5.3 Peak location of Pt4f and content of different Pt species determined from
Pt/xGnP-N and Pt/xGnP-RTIL-Z samples

 

 

 

 

 

 

 

 

Samples RTIL/Pt in Metallic Pt Pt oxide
molar ratio (peak locations, eV) (peak locations, eV)
Pt 0 64.57 % (71.38, 74.72) 35.43 % (72.52, 75.96)
Pt-RTIL-2 2.5 65.80% (70.67, 74.03) 34.20% (71.60, 74.03)
5 71.13% (71.09, 74.39) 28.87% (72.17, 74.39)
50 73.64% (70.54, 73.87) 26.36% (71.67, 75.50)
Pt/XC-72R — 65.55 % (71.78, 75.10) 34.45 % (73.35, 76.91)
Pt/xGnP-N —— 65.45 % (71.66, 74.99) 34.55 % (73.09, 76.42)
Pt/xGnP-RTlL-2 2.5 82.68 % (71.83, 75.16) 17.32 % (73.40, 76.73)
5 73.81 % (71.96, 75.29) 26.19 % (73.45, 76.78)
10 72.42 % (72.17, 75.50) 27.58 % (73.40, 76.73)
50 67.80 % (72.12, 75.45) 32.20 % (73.60, 76.93)

 

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5.3.5 Electra-oxidation of Methanol

The electrochemical performance of Pt/xGnP catalysts synthesized in the absence
and the presence of RTIL-l and RTIL-2 was studied for methanol oxidation in 1M H2804
+ 2M CH3OH solution at 50mV/s scan rate at the room temperature. A commercial Pt
/XC-72R catalyst from E-TEK was also tested for comparison. All of the catalysts have
approximately 20wt. % of Pt particles. Figure 5.14 shows the results of cyclic
voltammetry measurement for Pt/xGnP and Pt/XC-72R catalysts. The methanol oxidation
is represented by the peak in the current at around 0.8 V in the forward scan, while the
other oxidation peak at around 0.57 V in the reverse scan is associated with the removal
of the residual carbon species formed in the forward scan [43, 44]. The magnitude of the
peak current in the forward scan is directly proportional to the amount of methanol
oxidized at the anode in DMFC. As clearly shown in Figure 5.14, all of the Pt/xGnP
catalysts showed higher oxidation current than Pt/XC-72R, indicating that the former is
more active for methanol oxidation than the latter. Especially, the oxidation current
obtained from Pt/xGnP-RTIL—Z catalysts increased almost twice as high as Pt/XC-72R.
This enhancement of the catalytic activity for methanol oxidation may be attributed to the
improved utilization of Pt phase on the surface of xGnP. This better Pt dispersion and
utilization on the Pt/xGnP catalysts are due to the higher accessible surface area provided
by the surface of xGnP. Pt particles on XC-72R can be trapped in its pores where the
reactant can not access to or Nafion can not form a three phase boundary but there are no
such Pt regions on xGnP. This difference explains the efficiency of Pt/xGnP catalysts

over the commercial catalyst. The stronger interaction between Pt particles and xGnP

182

may be another possible reason of explaining the better catalytic performance of

Pt/xGnP-RTIL-Z, which was proved by data from XPS spectra of .Pt 4f.

 

Pt(2.2nm)lXC-72FI

— — — Pt(3.7nm)IxGnP-N
_0 03 . 534 ------ Pt(2.6nm)/xGnP-RTIL-2
° .; I, l| ................... Pt(2.0nm)IxGnP-R11L-2

— -------- - Pt(2.9nm)IxGnP-RTIL-1

-0.02 4

i (A)

-o.01 -

 

0.00 ‘

 

 

 

0.2 0.4 0.6 0.8 1.0 1.2 1.4
E (v vs. AglAgCl)

Figure 5.14. Comparison of catalytic activity for methanol oxidation on Pt/XC-72R,
Pt/xGnP-N, and Pt/xGnP- RTIL-1, Pt/xGnP- RTIL-2 catalysts.

The difference between peak potentials for the forward and backward scan indicates
the tolerance of the catalysts toward CO [45]. The lower value means the better CO-
tolerance. The peak potential differences for Pt/XC-72R, Pt/xGnP-N, PtlenP-RTIL-l,
and two different Pt/xGnP-RTIL-2 catalysts with 2.6 and 2.0nm of Pt particles were 268,
232, 253, 225, 253, and 239mV, respectively. Even though all of the Pt/xGnP catalysts
had lower potential difference, it seems hard to insist the better CO—tolerance of the
Pt/xGnP than the commercial Pt/XC—72R catalyst but they seemed to have similar CO-

tolerant properties.

183

Table 5.4 Mass activity of Pt particles for Pt/xGnP catalyst prepared via microwave
irradiation at the various molar ratios of RTIL/Pt

 

Samples Pt size Pt loading Mass of Pt Mass activity at Mass activity at
(nm) (wt.%)a (mg) peak in forward peak in reverse
(mA/mg Pt) (mA/mg Pt)

 

Pt/XC-72R 2.23 19.79 0.0218 577.98 828.44

Pt/xGnP-N 3.74 19.75 0.0217 788.01 1025.81
Pt/xGnP-RTIL-l 2.88 19.57 0.0215 840.47 1 135.81
Pt/xGnP-RTIL-2 2.64 19.36 0.0213 1090.14 1485 .92
Pt/xGnP-RTlL-2 2.01 18.90 0.0208 999.04 1487.02

 

a Measured from TGA in air environment at 25°C/min ramp rate.

Mass activity (the current/quantity of the catalyst), which is the way to express the
catalytic activity of Pt particles dispersed on carbon supports, is particularly important
because the cost of electrodes in fuel cells depends on the amount of Pt-based catalysts. If
the catalysts have high mass activity, it is possible to reduce the loading of precious Pt
metal. Mass activities of Pt/xGnP catalysts calculated at peaks in the forward and the
reverse scan were compared with those of the Pt/XC-72R catalyst in Table 5.4. All of the
Pt/xGnP catalyst showed better activity than Pt/XC-72R and two different Pt/xGnP-
RTIL-2 samples with 2.64 and 2.01nm of Pt especially provided much higher mass
activity than Pt/XC-72R with 2.23nm Pt. They showed an 80% increase in mass activity
compared to Pt/XC-72R, resulting from much better utilization of the Pt particles on
xGnP. It was reported that 3nm Pt-based particles exhibited higher mass catalytic activity
for methanol oxidation and oxidation reduction [46, 47]. However, that report does not

agree with the present result. According to the result of mass activity, a more critical

184

factor influencing catalytic activity of support Pt catalysts seems to be the utilization of Pt

particles rather than the Pt size. The structure of xGnP which allows the active phase to

make an easy contact with reactant and the proton conductive materials simultaneously

also contributes to the increase in the utilization of Pt phase in Pt/xGnP catalysts.

 

 

 

 

 

-0.004
Pt(2.2nm)lXC-72R-ETEK
A , — — — — Pt(3.7nm)IxGnP-N
\, ' ‘ - -. ---------------- Pt(2.6nm)IxGnP-RTlL-2
-0003 ‘1'\ \ — — — — Pt(2.0nm)lenP-RTlL-2
l'\. '\.i — —- ----- Pt(2.9nm)IxGnP-RTlL-1
'\-.
A \
S. -o.002 _
-0.001 -
0.000 I l l l l 17
0 500 1 000 1 500 2000 2500 3000
Time (s)

3500

Figure 5.15. Chronoamperometry curves for methanol oxidation at 0.45V on Pt/XC-
72R, Pt/xGnP-N, and Pt/xGnP- RTIL-l, Pt/xGnP- RTIL-2 catalysts.

The catalytic stability is another important evaluation of carbon supported Pt

catalysts for methanol oxidation. The current-time curves at 0.45V are shown in Figure

5.15. The initial methanol oxidation currents for Pt/xGnP catalysts were higher than that

of Pt/XC-72R and their order of the initial currents was in good agreement with the

results of . CV measurements. The currents for all the catalysts gradually decayed with

time, which is caused by the adsorption of intermediate species on the surface of Pt

185

particles during the methanol oxidation reaction [48]. The current values for Pt/xGnP
catalysts synthesized in the present of RTIL-1 and RTIL-2 were always higher than that
of Pt/XC-72R for the period of the measurement, although initial relatively fast drop in

the first 5005. This suggests the better long-term stability of Pt/xGnP-RTIL catalysts.

5.4. Conclusion

The purpose of this work is to evaluate xGnP as a support for the catalytic oxidation
of methanol in DMFC. xGnP has suitable properties such as good electronic conductivity,
high corrosion resistance, high surface area, high mechanical properties, high accessible
area for the reactant, as well as low resistance between the interface of electrolytes and
xGnP. The thermo-oxidative stability and combustion resistance of xGnP have been
studied with TGA and compared with other novel carbon nanostructures. TGA results
proved that xGnP has the highest stability among other carbon candidates for fuel cell
application. The 20wt. % Pt/xGnP catalysts have been prepared in the presence of various
molar ratios of RTIL to Chloroplatinic acid hexahydrate in EG, DEG, and TEG as
reducing agents via microwave technique. TEM and XRD results reveal that, regardless
of Pt loading level, highly-dispersed Pt nanoparticles below 3nm with a narrow size
distribution were synthesized on the xGnP by introducing RTIL to the solution and the
size of Pt nanoparticles were tuned by changing the molar ratio of RTIL/xGnP. Pt/xGnP
catalysts exhibit higher catalytic activity than the commercial Pt/XC-72R catalysts due to
higher dispersion and utilization of the Pt particles as well as stronger Pt-support
interaction. All the results presented in this chapter indicate the use of xGnP as a support

with a great potential not only for DMFC but also for other low temperature fuel cells.

186

10.

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191

CHAPTER 6
PtRu NANOPARTICLE FORMATION BY RTIL-ASSISTED
MICROWAVE PROCESS AND EVALUATION OF PtRu/xGnP
ELECTROCATALYSTS FOR DIRECT METHANOL FUEL

CELL (DMFC)

192

 

6.1 Introduction

Fuel cells have many benefits such as elimination of air pollutions, less noise
pollution, high electrical efficiencies, good reliability, and possibility of remote and
unattended operation [1-3]. They are able to meet requirements as potent and lightweight
power and energy sources for a variety of applications. The best choice of fuels for fire]
cells is pure hydrogen but it has inherent problems which include hydrogen production
and storage, hydrogen distribution, and safety. Liquids fuels such as methanol, ethanol,
ethylene glycol and formic acid are considered as alternatives for pure hydrogen. They
can be produced in a large quantity and additionally, they are easy to be stored and
distributed. Methanol has been most widely studied among them.

DMFC has a critical problem resulting in low overall performance. The problem is
caused by the sluggish methanol oxidation at the anode and the high overpotential for
oxygen reduction reaction at cathode resulting from methanol crossover [4]. Pt is the only
metal catalyst showing a significant activity for methanol oxidation but Pt alone is not
sufficient due to CO-poisoning. There is a need to achieve facile oxidation of CO
chemisorbed on the surface of Pt at the anode. A first popular approach is to produce Pt
alloys with different metals to form the surface oxide in the potential range for methanol
oxidation. A second method is to introduce surface adatoms on the surface of Pt [5, 6]. A
third type is to deposit Pt or PtRu onto a metal oxide such as CeOz or W03 [7, 8].

It is also required to develop methanol tolerant cathode catalysts for oxygen
reduction reaction for commercialization of DMF C. Three classes of catalysts have been
investigated for this purpose. A first one is to use the macrocyclic derivatives of a wide

range of transition metal compounds [9]. A second class is related to Chevrel-type

193

 

compounds of the composition of M06.xMxX3 (X=S, Se, Te) or transition metal cluster
catalysts such as M ,.,,Mo,,SeOz , which are based on transition metal and non-metallic
counter ions [10-12]. A third is unsupported noble metal catalysts as nanoparticle, binary
alloys, single or polycrystalline surfaces and supported noble metal catalysts. Among
them, PtRu and PtSn alloys as methanol oxidation catalysts are particularly effective in
the high and the low potential region, respectively [13, 14] and Pt-based alloys with
transitional metals with carbon have been extensively investigated for methanol tolerant
oxygen reduction catalysts [15, 16].

Pt and Pt-based alloys, especially PtRu, are the best materials for oxygen reduction
and methanol oxidation at the cathode and anode in DMFC, respectively [17]. However,
there have been demands to reduce amounts of Pt because the high cost and low
availability of Pt is one of obstacles against practical implementation of DMFC. One way
is to employ electrically conductive and high surface are carbons for Pt dispersion,
resulting in fine Pt nanoparticles supported on carbons with a high catalytically active
surface area. Catalytic properties of Pt particles are strongly influenced by carbon
materials. They affect the size and size distribution of metal particles, the extent of
alloying in multimetallic catalysts, the stability and the utilization degree of supported
metal particle [18-19]. Catalytic activity of Pt-based catalysts is also affected by the
fabrication techniques [20]. There are many different methods to synthesize carbon
supported catalysts for DMFC, which includes impregnation, colloids, sol-gel,
electrodeposition, thermal decomposition, microwave irradiation, and supercritical
processes. It is desirable to develop a low cost, easy, rapid and consistent fabrication

method which guarantees highly active catalyst.

194

 

xGnP supported PtRu catalysts synthesized in a similar way as described in chapter 5
were investigated and the results presented in this chapter. The influence of xGnP as a
new support on the dispersion and the performance of PtRu catalyst was investigated and

compared with a commercial carbon black supported catalyst.

6.2 Experimental Details

6.2.1 Materials

A reagent grade of ethylene glycol (EG, bp = 197.3°C) from J .T. Baker was used as
reducing agent for xGnP supported PtRu catalysts. Chloroplatinic acid hexahydrate
(HthC16-6HZO) and ruthenium chloride hydrate (RuCl3 ' tzO) from Aldrich were used
as Pt and Ru precursors, respectively. RTIL used in this work are l-buty1-3-
methylimidazolium hexafluorophosphate (bmimPF6) and l-butyl-3-methylimidazolium
acetate (bmimCHgCOz) and their chemical structures are shown in Figure 4.1 of chapter
4. bmimPF6 is denoted as RTIL-l and bmimCH3C02 as RTIL-2 in this work. Both were
purchased from Aldrich and used as received. xGnP with 12nm of thickness and 1pm of
diameter made in MSU was used as a carbon support. The morphology of xGnP can be

seen in Figure 5.1 of chapter 5.

6.2.2 Preparation of Unsupported and Supported Catalysts

The typical experimental procedures for unsupported bimetallic PtRu particles are
described as follows. In two separate 10 ml beakers, a specific quantity of Pt and Ru
precursors was dissolved in 2mL EG, respectively. In another 100mL beaker, RTIL-1 or

RTIL-2 was mixed with 16g of EG solution vigorously to which two EG solutions of Pt

195

and Ru precursor was added and stirred vigorously for 30min. The mixture was
transferred into a microwave oven (National, 2.45GHz, 1300W), and irradiated for
SOseconds for the reduction of Pt and Ru precursors at the same time. The resulting
suspension was centrifuged and washed with acetone several times. The PtRu particles
was re-dispersed in acetone and stored in a vial.

For PtRu(3O and 40wt. %) /xGnP catalysts synthesized in the presence of RTIL,
various weight ratios of EG to RTIL were added and dissolved in 18mL ethylene glycol
prior, followed by the addition and the ultrasonic dispersion of xGnP. Other steps are
similar to PtRu nanoparticles in the presence of RTIL. The PtRu/xGnP catalysts
synthesized with the addition of bmimPF6 and bmimCHgCOz are denoted as
PtRu/xGnP-RTlL-1(X) and PtRu/xGnP-RTIL-2(X), respectively, in which X
represents the weight ratio of EG to RTIL. PtRu/xGnP-N represents PtRu bimetallic

particles deposited on xGnP, which was prepared in the absence of RTIL.

6.2.3 Characterization Methods

PtRu catalyst samples dispersed on xGnP were characterized by recording their X-
ray diffraction (XRD) patterns on a Regaku Rotaflex 2003 X-ray diffractometer using
Cu-Ka radiation with a curved graphite monochromator in order to identify the phases
present in the catalyst and to evaluate the particle size of the PtRu particles. The 20
region between 10° and 1000 was explored at 45 keV and 100 mV at a scan rate of 5°/min.
The average metal particle size was calculated using Scheller equation according to the
XRD broadening PtRu (220) reflections which was scanned at a rate of 1°/min again.

Transmission electron microscopy (TEM) investigation was carried out with JEOL

196

 

100CX and JEOL 2200FS operating at a voltage of 100keV and 200keV for the
morphology of the dispersed PtRu electrocatalyst and the determination of the mean size
of PtRu particles. Specimens were prepared by ultrasonically re-dispersing the sample
powders in acetone for 15 min, applying the power suspension onto lacey carbon coated
Cu grids and drying them in air. The particle size and distribution of the metal particles
was obtained by measuring the size of 150 ~ 200 randomly selected particles in TEM
images. Thermo-gravimetric Analysis (TGA 2950, TA instrument) was performed to
measure real loading of PtRu bimetallic particles deposited on xGnP. Samples weighing
3~4mg were tested over temperatures ranging from room temperature to 1000°C at
25°C/min under flowing air.

Electrochemical investigation was performed by cyclic voltammetry (CV) using a
potentiostat (Gamry, FAS2 Femtostat). A conventional three-electrode cell equipped with
a platinum wire counter electrode, an Ag/AgCl reference electrode, and glassy carbon
working electrode was used. The glassy carbon electrode with 3mm diameter was
polished to a mirror finish with a 0.05 pm alumina suspension before each experiment.
The catalyst ink was prepared by ultrasonically dispersing 5mg of 30wt. % PtRu/xGnP
catalyst, 0.8ml of ethanol and 0.1ml of 0.1wt % Nafion solution. Then 20 uL of the
catalyst ink was micropipetted out on the top surface of the glassy carbon electrode to
yield a Pt loading of about 0.3142mg/cm2. The CV experiments were performed in 1M
H2804 solution in the absence and the presence of 2M CH3OH at the scan rate of 50mV/s.
All the solutions were prepared by using ultrapure water. N2 gas was purged for 15 min

before starting the experiment. The Chronoamperometry tests were performed in 2M

CH3OH/ 1M HZSO4 at 0.40V for the period of 25003.

197

6.3 Results and Discussion

6.3.1 Unsupported PtRu catalysts

Distinct differences in the morphologies and particle sizes of unsupported PtRu
nanoparticles synthesized in ethylene glycol via microwave heating process with and
without the addition of RTIL-2 are shown in Figure 6.1. While PtRu-N sample prepared
without RTIL-2 showed large particle-aggregated morphology and PtRu particles held
together by an irregular network as in Figure 6.1 (a), PtRu particles fabricated with the
addition of RTIL-2 appeared to have the same particle-aggregated morphology as PtRu-N
but they were much smaller due to rapid reduction rate by RTIL-2 and very well
separated individually and 3-dimensionally due to the stabilization effect by RTIL-2. The

average particle size in PtRu-RTIL-2 was about 1.5nm with very small standard deviation.

 

(a)

Figure 6.1. TEM microscophs of (a) PtRu-N and (b) PtRu-RTIL-Z (400).

198

 

Figure 6.1. Continued.

XRD measurements for Pt-N, PtRu-N and PtRu-RTIL samples were performed and
the XRD patterns acquired are shown in Figure 6.2. All of the particles showed
characteristic peaks at around 40°, 46°, 68°, and 82°, which are attributed to (111), (200),
(220), and (311) crystalline plane of Pt with face-centered cubic (fee) structure. However,
the most dramatic peak pattern was obtained from PtRu-RTIL2(400) particles which have
(111) and (200) reflections merged together and (220) and (31 1) peaks broadened due to
the small particle size. No visible peaks related to either (101) or (102) Ru reflection at
20 = 44 and 583° were found in the Figure. This confirms that Ru is not segregated in a

separated phase but comprised in bimetallic alloy particles.

199

 

 

     

 

w»

30 4o 50 60 70 so 90
29 1°)

Intensity (a.u)

 

 

 

Figure 6.2. XRD patterns of (a) PtRu-N, (b) PtRu-RTIL2(800), (c) PtRu-RTIL2(400)
and (d) Pt-N.

The 20 change of the (220) peaks for Pt-N, PtRu-N and PtRu-RTIL particles is
shown in Figure 6.3. The (220) peak for PtRu particles prepared with and without the
addition of RTIl-2 clearly shifted towards the higher degree compared to Pt peak. The
higher angle shift of the peak represents the formation of PtRu particles homogeneously
alloyed with Pt and Ru atoms and the existence of Pt-Ru interaction. On the other hand,
PtRu-N sample showed the highest shift and the shift degree seemed to decrease as the
content of RTIL-2 increased. The large extent of the peak broadening was also observed
as a result of the very small particle size with the increase of RTIL-2 content. All these
features intimate that the lattice parameters and the composition of Pt and Ru atoms in

PtRu bimetallic particles are distinct from each sample.

200

 

 

Intensity (a.u)

 

 

20 (°C)

Figure 6.3. The change of (220) peaks of (a) PtRu-N, (b) PtRu-RTIL2(800), (c)
PtRu-RTIL2(400) and (d) Pt-N.

Pt and Ru form an alloy with Ru atoms replacing Pt atoms in a Pt fee structure when
Ru atomic fractions is below 0.62 [21]. Above 0.62, Pt atoms replace Ru atoms in a Ru
hcp structure. The composition of Pt and Ru atoms in the PtRu alloy particles can be
determined by the information obtained by the shift of Pt reflections. After fitting with a
Gaussian distribution, the (220) peak was used to calculate the lattice parameter, the
average particle size and the composition of PtRu alloy particles.

The lattice parameter of the PtRu crystal was calculated from 6m, according to
Vegard’s law [22]:

a = filKal

sin (9max

(6.1)
The diameter (D) of PtRu particles was calculated according to the Scherrer’s formula:

201

 

_ 0.9/1Ka]
B(26) cos Bmax

 

(6.2)

where xix“, is the X-ray wavelength (0.154138nm), 6m, is the angle at the position of the
peak maximum and B(2 6”,”) is the full width at half-height (F WHM) of the diffraction

peaks in radian. The Ru atomic fraction in the PtRu alloy, X Ru was calculated from XRD
data using the formula [23].

a = a0 — kXRu (6.3)
where an is the lattice constant of pure Pt and k is a constant = 0.0124nm.. In the case of
unsupported pure Pt, a0 = 0.3923nm [24] but in this work, a0 = 0.3927nm which was
calculated from XRD measurement. The a, D, and X Ru values deduced from XRD data
are listed in Table 6.1. First of all, the particle sizes calculated from XRD measurement
were in good agreement with those obtained by TEM analysis. The lattice constant of all
PtRu particles was smaller than that of Pt particles. The decreased lattice parameters in
the presence of Ru confirm that PtRu particles are not a physical mixture of Pt and Ru but
a solid solution of two metals. The lattice parameter increased with increasing RTIL-2
content. Ru fraction in PtRu alloys showed the reversed trend. It has been reported that
Ru % in PtRu alloys is independent of the particle size but strongly depends on the
preparation method of the metal particle, metal loading and the form of the particles (bulk
or nanoparticles, with or without supports, etc [25, 26]. Therefore, the difference of Ru %
in the alloys in this work appears to result from different reduction rate due to the

presence of RTIL-2.

202

 

Table 6.1. The data from XRD and TEM for the different PtRu and Pt particles

 

 

Ddia from Ddia from
Samples 26mm, a (nm) FWHM
XRD (nm) TEM (nm) XI...
Pt-N 67.45 0.3927 1.75 5.463 6.22 _
PtRu-N 68.28 0.3885 2.26 4.251 _ 0.34
PtRu-RTIL2(800) 67.86 0.3906 2.53 3.767 3.92 0. I 7
PtRu-RTIL2(400) 67.80 0.3909 5.24 1.828 I .47 0.15

 

6.3.2 Supported PtRu

The morphologies and particle size distribution of xGnP supported PtRu

nanoparticles synthesized via microwave irradiation with and without the addition of

RTIL-1 and RTIL-2 are shown in Figure 6.4. The loading of PtRu paritlces was 40wt. %.

For PtRu/xGnP-N, the PtRu particles had an average size of around 5mm and partially

agglomerated compared to PtRu-N. The particles showed poor distribution on the surface

of xGnP. For PtRu/xGnP-RTIL-l and PtRu/xGnP-RTIL-Z samples, the size of PtRu

particles drastically decreased to 2.33 and 1.57nm in average, respectively. The particles

were highly dispersed over the xGnP without agglomeration compared to those of

PtRu/xGnP-N. This result confirms again that the use of RTIL in microwave process

leads to the formation of smaller PtRu bimetallic nanoparticles with a much more

uniform size distribution.

203

 

()
saga

N
M

Distribution it
8

 

3~4 4~5 5~6 6~7 7~8 8~9 9~10
Particle dz. (nm)

 

 

Distribution (%)

    
  

l~l.5 1.5~2 2~2,3 2,3~2.6 2.6~3 3.0«35
Particle size (nm)

 

is

Distribution (%)

0.54 I~I.4 l.4~l.8 l.8~2.2 2.2~2.6 2.6-30
Particle size (nm)

 

Figure 6.4. TEM analyses of PtRu/xGnP-N, PtRu/xGnP-RTIL-l, and PtRu/xGnP-
RTIL-Z catalysts with 40wt. % metal loading (from top to bottom).

204

 

Figure 6.5 shows the effect of RTIL-2 on the dispersion and size of PtRu catalysts
supported on xGnP with 30wt. % metal loading. It was clearly seen that, as the content of
RTIL-2 increased, much smaller particles with narrower size distribution formed and the
dispersion of the particles was greatly improved with little indication of agglomeration. It
is noteworthy that the particle size of PtRu did not increase regardless of increasing metal
loading, which can be compared from the Figure 6.3 and 6.4 (c). A commercial PtRu/CB
from E-TEK was also investigated for comparison (Figure 6.5 d). An average diameter

of the particles of around 1.90nm was obtained but some of them were agglomerated.

 

 

3~3.5 3.54 44.5 4.5~5 5~5.5 5.5~6 6.5~7
Particle dze(nm)

 

Figure 6.5. TEM analyses of (a) PtRu/xGnP-N, (b) PtRu/xGnP-RTIL2(800), (c)
PtRu-RTIL2(400), and a commercial PtRu/CB catalysts.

205

 

s'ss

Distribution (96
'5’ ‘5

2~2.5 2.5~3 3~3.5 3.54 44.5 4.5~5 5~5.5
Particle dze(nm)

 

U
C

Distribution (96'

 

0.5~l 1~l.4 l.4~l.8 l.8~2.2 2.2~2.6 2.6-10
Particle a'ze(nm)

 

 

3.
6

Distribution (%)
8 ‘5

 

 

1.0~1.5 1.5~2.0 2.0~2.5 2.5~3.o 3.o~3.5 3.54.0
Particle izs (nm)

 

Figure 6.5. Continued.

206

Nanosized PtRu particles on xGnP were characterized by high resolution electron
microscopy (Figure 6.6). The “A” area on the TEM image is corresponding to the
layered graphite structure of xGnP of which the interplanar spacing looks bigger than that
of PtRu particle although unclear. The image shows a well-crystallized PtRu structure.
Fringes corresponding to PtRu lattice structure in a polyhedral shape at the edge of xGnP
were clearly seen with the interplanar spacing of about 0.39nm which is in good

agreement with XRD data.

 

Figure 6.6 A High resolution TEM image of PtRu/xGnP-N catalyst (scale bar: 2nm).

The PtRu alloy nanoparticles dispersed on xGnP were characterized by cyclic
voltammetry (CV) in electrolytes of 1M H2SO4 as shown in Figure 6.7. The potential was

scanned between -0.25 and 1.20V at the rate of 50mV/s. The peaks in hydrogen

207

adsorption and desorption region from 0 to 0.33V corresponding to the reductive
adsorption of protons in the cathodic scan and the subsequent oxidation of the hydrogen
adatoms in the anodic scan were clearly shown for all of the samples. The hydrogen
region seemed to be slightly more defined for the PtRu/xGnP-RTI-L2 catalysts than
PtRu/CB-ETEK, suggesting that PtRu/xGnP-RTIL-Z catalysts may have the higher

degree of alloying.

 

 

 

 

 

 

 

40.002
-0.001 3
0.000 4
E
0.001 -
PtRu/CB-ETEK
0.002 - — — — - PtRu/xGnP-N
--------------- PtRu/xGnP-RTIL-Z (800)
-— - — - —- PtRu/xGnP-RTlL-Z (400)
0003 I T I I i f T I

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
E (V vs. AgIAgCI)

Figure 6.7. Cyclic voltammograms recorded at 50mV/s of PtRu/xGnP catalysts via
microwave process and a commercial PtRu/CB-ETEK.

The electrochemical active surface area (EAS) for PtRu/xGnP catalysts and the
PtRu/CB catalyst can be estimated from the integrated charge in the hydrogen

electrosorption region of the cyclic voltammogram according to the following formula

[27]:

208

= QH
[PtRu]

 

(6.4)

where EAS has unit of mZ/g PtRu, [PtRu] represents the PtRu loading in the electrode
and QH = 0.21 mC/cmz. The calculated results listed in Table 6.2. The value of integrated
charge was obtained by using software called ‘Echem Analyst’ which was provided by
Gamry. The active surface area of all PtRu/xGnP catalysts followed the order of
PtR‘u/xGnP-N < PtRu/CB-ETEK < PtRu/xGnP-RTIL2(800) < PtRu/xGnP-RTIL2(400).
Even PtRu/xGnP-RTIL2(800) (3.72nm) which have the larger average size than
PtRu/CB-ETEK (1 .90nm) showed higher active surface area, suggesting better utilization
of PtRu particles on xGnP than on CB. The highest active surface area was obtained from

PtRu/xGnP—RTIL2(400) which has the smallest particle size.

Table 6.2. Electrochemically active surface area of PtRu particles for PtRu/xGnP
synthesized via microwave process and a commercial PtRu/CB catalyst

 

 

Samples QH Sm, (cmz) Catalyst loading (mg) EAS (mz/g)
PtRu/CB-ETEK 2.977 14.18 0.03602 39.36
PtRu/xGnP-N 2.087 9.94 0.03277 3032
PtRu/xGnP-RTIL2(800) 3 .720 17.71 0.02869 61.75
PtRu/xGnP-RTIL2(400) 4.098 19.51 0.02482 78.62

 

Figure 6.8 shows the results of cyclic voltammetry carried out to evaluate the
activities of PtRu/xGnP catalysts for methanol electrooxidation at room temperature
against a commercial PtRu/CB fi'om E-TEK. The peak at around 0.8V during the forward
scan (from 0 to 1.2V) corresponds to methanol oxidation and another peak at about 0.5V

during reverse scan (from 1.2 to 0V) represents the removal of the removal of the residual

209

carbon species formed in the forward scan [28, 29]. The peak current due to methanol
oxidation for PtRu/xGnP-N, PtRu/xGnP—RTIL2 (800), PtRu/xGnP—RTIL2(400), and the
E-TEK sample were 8.77, 13.23, 16.96, and 9.14 mA, respectively. Therefore, the
catalyst activities were in the following order; PtRu/xGnP-N < PtRu/CB-ETEK <
PtRu/xGnP-RTII.2(800) < PtRu/xGnP—RTIL2(400). This result is consistent with EAS
data obtained from hydrogen electrosorption but dose not match with the well-known fact
that PtRu particles with around 3.0nm exhibits most high catalytic activity for methanol
oxidation [30]. Thus, this discrepancy may mainly result from the different degree of

dispersion and utilization of PtRu particles in between PtRu/xGnP and PtRu/CB catalysts.

 

 

 

 

 

-0.025
PtRu/CB-ETEK
— — — — PtRu/xGnP-N
-0020 .1 A .................. PtRu/xGnP-RTlL-2(800)
I: i — -------- PtRu/xGnP-RTlL-Z (400)
: :2: ,. N
0015 . | .-’ '\
la .-/. .r.
5., -0.010 4 ,j'f l .-/ (.5853
-0.005 -
0.000 -
0.005 I I I I I l
0.0 0.2 0.4 0.6 0.8 1.0 1 2 1.4
E (v vs. AgIAgCI)

Figure 6.8. Cyclic voltammograms of the methanol oxidation in 1M H2804 + 2M
CH3OH at 50mV/S for PtRu/xGnP catalysts and a commercial PtRu/CB-ETEK.

210

Chronoamperometric curves were measured at 0.4V for three PtRu/xGnP and
PtRu/CB-ETEK electrodes. As shown in Figure 6.9, the initial methanol oxidation
currents for all of the catalysts were in good agreement with the result of CV
measurements and BSA calculation. However, the decay in the methanol oxidation
currents for each catalyst with time was different. While PtRu/CB-ETEK lost 87% of
initial current after 25005, PtRu/xGnP-RTIL2 (400) and PtRu/xGnP-RTIL2 (800) were
less pronounced with 73% loss, which indicate their long-term stability compared to the
commercial PtRu bimetallic catalysts. PtRu/xGnP-N showed the highest current loss of

94% among all the catalysts measured.

 

 

 

 

 

0.0016
'0-0014 ‘ 4. —— PtRu/CB-ETEK
'\ _‘ — — — — PtRu/xGnP-N
-00012 -.._ \._ ------------------ PtRu/xGnP-RTIL-Z (800)
\-.\ — ..... —-- PtRu/xGnP-RTIL-Z (400)
-0.0010 - '
3. -0.0008 -
-0.0006 1
-0.0004 -
-00002 «
0.0000 . . r .
0 500 1000 1500 2000 2500
“me (3)

Figure 6.9 Chronoamperometry curves for methanol oxidation at 0.40V on
PtRu/CB-ETEK and three PtRu/xGnP catalysts.

211

6.4. Conclusion

xGnP supported or unsupported PtRu bimetallic nanoparticles have been synthesized
via microwave dielectric heating method in the presence of RTIL. It was proved from
TEM investigation that the introduction of RTIL during reduction reaction of Pt and Ru
precursors in microwave process results in the smaller particles with a narrow size
distribution for unsupported samples and high dispersion of the particles on the surface of
xGnP in the case of supported samples. The methanol oxidation of PtRu bimetallic
particles dispersed on xGnP was evaluated using electrochemical measurement and
compared with a commercial PtRu/CB catalyst. PtRu/xGnP catalysts prepared in the
presence of RTIL displayed the higher active surface area than the commercial catalyst,
which seems to come from improved utilization of the PtRu particles on xGnP. Hence,
the enhanced catalytic activity for methanol oxidation was obtained from PtRu/xGnP-
RTIL catalysts. All the results presented in this chapter give credence for synthetic
method to control the size, distribution, and dispersion of Pt-based metal particles and for

the use of xGnP as a support with a great potential for DMFC.

212

6.5 Bibliography

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Chem. Rev., 95(1), p191-207, 1995.

V. Plzak and B. Rohland, Advanced Electrochemical Hydrogen Technologies, in
Modern Aspects of Electrochemistry No. 26, Plenum Press, New York, p105, 1994.

L. Blomen and MN. Mugerwa, “Fuel Cell Systems”, Plenum, New York, 1993.

X.M. Ren, E.T. Springer, T.A. Zawodzinski, and S. Gottsfield, “Methanol Transport
through Nation Membranes. Electro-osmotic Drag Effects on Potential Step
Measurements”, J. Electrochem. Soc, 147(2), p466-474, 2000.

Dianxue Cao and Steven H. Bergens, “A nonelectrochemical reductive deposition of
ruthenium adatoms onto nanoparticle platinum: anode catalysts for a series of direct
methanol fuel cells”, Electrochimica Acta, 48(27), p4021-403 l , 2003.

Dianxue Cao and Steven H. Bergens, “An organometallic deposition of ruthenium
adatoms on platinum that self poisons at a specific surface composition: A direct
methanol fuel cell using a platinum—ruthenium adatom anode catalyst”, J. of
Electroanal. Chem, 533(1-2), p9l-100, 2002.

C. Roth, M. Goetz, and H. Fuess, “Synthesis and characterization of carbon-
supported Pt—Ru—WOx catalysts by spectroscopic and diffraction methods”, J. Appl.
Electrochem., 31(7), p793-798, 2001.

AK. Shukla, M.K. Ravikumar, A.S. Arico, G. Candiano, V. Antonucci, N. Giordano,
and A. Hamnett, “Methanol electrooxidation on carbon-supported Pt-WOH
electrodes in sulphuric acid electrolyte”, J. Appl. Electrochem., 25(6), p528-532,
1995.

R. Jasinski, “Cobalt Phthalocyanine as a Fuel Cell Cathode”, J .Electrochem. Soc.,
112(5), p526, 1965.

10. N. Alanso-Vante, P. Bogdanoff, and H. Tributsch, “On the Origin of the Selectivity

ll.

of Oxygen Reduction of Ruthenium-Containing Electrocatalysts in Methanol-
Containing Electrolyte”, J. Catal., 190(2), p240-246, 2000.

O. Solorza, K. Ellmer, M. Giersig, and N. Alonso-Vante, “Novel low-temperature
synthesis of semiconducting transition metal chalcogenide electrocatalyst for

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multielectron charge transfer: molecular oxygen reduction”, Electrochim. Acta,
39(11-12), pl647-1653, 1994.

H. Tributsch, M. Bron, M. Hilgendorff, H. Schulenburg, I. Dorbandt, V. Eyert, P.
Bogdanoff, and S. F iechter, “Methanol-resistant cathodic oxygen reduction catalysts
for methanol fuel cells”, J. Appl. Electrochem., 31(7), p739-748, 2001.

AS. Arico, H. Kim, V. Antonucci, A.K. Shukla, M.K. Ravikumar, and N. Giordano,
“Methanol oxidation on carbon-supported Pt---Sn electrodes in silicotungstic acid”,
Electrochim. Acta, 39(5), p69l-700, 1994.

M. Watanabe, Makoto Uchida and Satoshi Motoo, “Preparation of highly dispersed
Pt + Ru alloy clusters and the activity for the electrooxidation of methanol”, .1.
Electroanal. Chem, 229(1-2), p395-406, 1987.

N.M. Markovic, T.J. Schmidt, and V. Stamenkovic, and RN. Ross, “Oxygen
Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review”, Fuel
Cells, 1(2), p105-116, 2001.

T. Toda, H. lgarashi, and H. Uchida, M. Watanabe, “Enhancement of the.
Electroreduction of Oxygen on Pt Alloys with Fe, Ni, and Co”, J. Electrochem. Soc.,
146(10), 3750-3756, 1999.

S. Wasmus and A. Kuver, “Methanol oxidation and direct methanol fuel cells: a
selective review”, J. Electroanal. Chem, 461(1), p14-31, 1999.

Y. Takasu, T. Kawaguchi, W. Sugimoto, and Y. Murakami, “Effects of the surface
area of carbon support on the characteristics of highly-dispersed Pt---Ru particles as
catalysts for methanol oxidation”, Electrochim. Acta, 48(25-26), p3 861-3 868, 2003.

V. Rao, P.A. Simonov, E.R. Savinova, G.V. Plaksin, S.V. Cherepanova, G.N.
Kryukova, and U. Stimming, J. of Power Sources, “The influence of carbon support
porosity on the activity of PtRu/Sibunit anode catalysts for methanol oxidation”,
145(2), p178-187, 2005.

Y. Takasu and Y. Murakami, “Design of oxide electrodes with large surface area”,
Electrochim. Acta, 45(25-26), p4135-414l, 2000.

E. Antolini and F. Cardellini, “Formation of carbon supported PtRu alloys: an XRD
analysis”, J. Alloy. Comp., 315(1), p118-122, 2001.

214

22

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28.

29.

30.

D. Chu and S. Gilman, “Methanol Electro-oxidation on Unsupported Pt-Ru Alloys at
Different Temperatures”, J. Electrochem. Soc., 143(5), p1685-1690, 1996.

E. Antolini, L. Giorgi, F. Cardellini, and E. Passalacqua, “Physical and
morphological characteristics and electrochemical behaviour in PEM fuel cells of
PtRu/C catalysts”, J. Solid State Electrochem., 5(2), p131-140, 2001.

E. Antolini, F. Cardellini, L. Giorgi, and E. Passalacqua, “Effect of Me (Pt+Ru)
content in Me/C catalysts on PtRu alloy formation: An XRD analysis”, J. Mater. Sci.
Lett., 19(23), p2099-2103, 2000.

E. Antolini, “Formation of carbon-supported PtM alloys for low temperature fuel
cells: a review”, Mater. Chem. And Phys., 78(3), p563-573, 2003.

Q. Lu, B. Yang, L. Zhuang, and J. Lu, “Anodic Activation of PtRu/C Catalysts for
Methanol Oxidation", J. Phys. Chem. B, 109(5), p1715-1722, 2005.

Z.L. Liu, J .Y. Lee, M. Han, W.X. Chen, and L.M. Gan, “Synthesis and
characterization of PtRu/C catalysts from microemulsions and emulsions”, J. Mater.
Chem, 12(8), p2453-2458, 2002.

R. Mancharan and J .B. Goodenough, “Methanol oxidation in acid on ordered NiTi”, J.
Mater. Chem, 2(8), p875, 1992.

X. Zhang, and K.Y. Chan, “Water-in-Oil Microemulsion Synthesis of Platinum-
Ruthenium Nanoparticles, Their Characterization and Electrocatalytic Properties”,
Chem. Mater., 15(2), p451-459, 2003.

Y. Takasu, H. Itaya, T. Iwazaki, R. Miyoshi, and T. Ohnuma, W. Susimoto, and Y.
Murakami, “Size effects of ultrafine Pt—Ru particles on the electrocatalytic oxidation
of methanol”, Chem. Commun., p341-342, 2001.

215

CHAPTER 7

DETAILS ON MECHANISM FOR NANOPARTICLE

FORMATION IN SOLVENT AND IN THE PRESENCE OF

SUPPORTS

216

7.1. Formation of Nanoparticles in Solution

The formation of metal nanoparticles in solution by a wet chemical route involves
two distinct dynamic processes: the nucleation of primary clusters of atoms in saturated
solution and the aggregation of the primary clusters into secondary or final particles
[1~5]. The primary cluster of atoms forms through successive stages consisting of
reduction of the metal salt, generation of the metal atom, embryos, nuclei formation, and
diffusional growth of the nuclei. The secondary particles form as a result of an
irreversible aggregation between new metal atoms with the primary cluster. The
secondary particle can further grow to larger particles and be coarsened by Ostwald
ripening due to the increased solubility of small particles with decreasing particle size.
The sequence describing the formation of metal particles can be found in section 2.2.1 of
chapter 2. In this section, the thermodynamic or dynamic growth mechanism for the
general formation of metal nanoparticles is presented followed by a hypothesis for the

formation mechanism of metal particles in the presence of RTIL.

7.1.1 Nucleation
The nucleus forms according to the following scheme:
mX, = X,,,
Xm + X1 = Xm+l
X0”) +X, 3X"
X 1 represents a metal atom and X" an embryo which can be generated or disappeared by

the reversible association/dissociation process. The metal atoms are referred to as solutes

217

which act as monomers for the primary-particle nucleation. In classical nucleation theory,

the free energy of the n-solute embryos can be expressed by [6, 7]:
_ 4 3 2
AG——§7rR AF + 47rR 7 (7.1)

AF = kaln(c/co) (7.2)
where R is the radius of nucleus, AF is the bulk free energy difference per volume
between solutes and nuclei, 7is the surface tension at the solute-liquid solvent interface, c

is the solute concentration, co is the equilibrium saturation concentration, ,0 is the density
of solutes, k is Boltzmann’s constant, and T is the temperature. The first term of right
hand side in the Equation (1) is the volume term which favors the formation of the nuclei.
The second term is the surface energy term which works against the formation of the
nuclei. At large R, the volume term plays an important role in creating nuclei but at small
R, the surface energy term will be more important.

As soon as a solution is supersaturated with metal atoms, nucleation begins with the
association of metal atoms to form nuclei. There is the critical size of a nucleus, RC,

which is derived from the maximum value of AG in the derivative of AG with respect to

 

 

R.
RC = 27 = 27 (7.3)
AF kaln(c/ CO)
The energy barrier to nucleation is equal to
3 3
AG — 16717 - 167W (7.4)

 

 

6‘ 3AF —3kaln(c/c0)

Thus, the nucleation rate given by activation over the free energy can be expressed as

218

J = J 0 exp(— AG%T) (7.5)

From classical theory, it can be understood that the increased concentration of the
solute will decrease the critical nucleus size and nucleation energy barrier and increase
the nucleation rate, while the high surface energy of solutes will increase the critical size
of nucleus and decrease the nucleation rate. Thus, in order to create monodispersed small
metal particles, fast supersaturation of metal atoms in solution and low surface energy of

metal atoms are required.

7.1.2 Growth

Primary particles dispersed in a solution present a very high degree of freedom for
rotation and translation motions. Thus the particles experience a high collision rate at a
high velocity due to the Brownian motion. The collision frequency for a single primary

particle can be described by

z _ x/erDzvN
V

V = PIE: (7.7)
m

where D is the particle diameter, N is the total number of particles which can be

(7.6)

empirically calculated [8], V is the total volume occupied by the system, v is the mean
velocity of the dispersed particles, and m is the mass of a particle. As an example, the
mass of a spherical Pt primary particle with a radius of 0.5mm and a density of
21.45g/cm3 is 1.1x10'20g. The Pt particle presents a mean velocity of about 42m/s which

is a very high value. N is approximately 2.45 x 10'8 at Pt concentration of 0.0025M and

219

thus the collision frequency at 473K is estimated to be of about 2x 107 collisions/s for
each particle. This value is significant even if the calculation is approximate. Thus, the
coagulation mechanism based on the effective collision of primary particles can be used
to explain the particle growth and aggregation [9, 10]. The oriented attachment
mechanism which is a simplified coagulation mechanism provides a good prediction of
the behavior of particle growth [11~13]. However, deviation in the fitting of the equation
due to the Ostwald ripening mechanism is observed [14]. Hence, the particle growth can
be described by an equation by combining the oriented mechanism dominant in the initial

stage and the Ostwald ripening mechanism which is prevalent later,

2
8 c
r3_r)3_ Wmoo.

“’ _ 54817an t (7'8)

where C... is the bulk solubility, Vm is the molar volume, and a is the radius of the
solvated ion, N is Avogadro’s number, r is the mean radius of secondary particles, req is
the equivalent radius at a time t, yis the surface energy, and 77 is the viscosity of a liquid
solution. This equation may provide a reasonable explanation on the growth of Pt

particles synthesized in this work. As in nucleation stage, low surface energy of primary

particles is preferred.

7.1.3 Formation of Metal Nanoparticles in the Absence of RTIL

Figure 7.1 shows the schematic process of synthesizing platinum nanoparticles in
ethylene glycol via microwave heating in the absence of RTIL as an example. A metal
precursor is solvated in a solvent which is a reducing agent and then the solution is

supersaturated with metal atoms which result from the fast reduction of the solvated

220

metal ions by the microwave heating and composition of the organo-metallic precursor.
The metal atoms spontaneously collide with each other or with metal ions, resulting in
the formation of embryos. The embryos eventually reach a critical size to form the nuclei.
The nuclei rapidly grow to unstable primary particles by further association with the
remaining metal atoms. Secondary particles form through the process of growth,
aggregation and Ostwald ripening (or coarsening). The formed secondary particles may
agglomerate and grow in size due to the absence of capping agents to terminate further
particle growth and reduce particle coarsening. Hence, when RTIL is not present, large
and highly agglomerated metal particles are synthesized as shown in TEM image (Figure

4.9a) of chapter 4.

~ ~'
. -
Reduction
— 1‘]
00

 

M‘ or M- ions in Metal atoms formation
solution
‘0

y

SC - ' 09
paratron a“ _
N . _ [96% 0‘9”? Metal nuclel

W .. . P360636 formation

 

Cluster formation

Figure 7.1 The simplified process for uniform particle formation synthesized via
microwave process in the absence of RTIL.

7.1.4 Formation of Metal Nanoparticles in the Presence of RTIL
In the case when RTIL is added to the solution, the number of the nuclei and their

size are particularly dependent on the surface energy of the nuclei and reaction

221

temperature since the addition of RTIL results in the significant change of those two
experimental parameters. Figure 7.2 (a) shows how metal nanoparticles are synthesized
in the presence of RTILs, especially bmimPF6, bmimCH3SO4, and bmimCH3COz. As
shown by the results of UV-vis spectroscopy in chapter 4, those three RTILs are not
coordinated with the solvated metal ions but the RTILs promote the reduction reaction
due to a high heating rate resulted from the high polarizability of the RTIL in the
microwave environment. The solution becomes supersaturated with metal atoms more
quickly than in the absence of RTIL. Since the RTILs have low surface energy in spite of
their high polarity, they easily adsorb onto the surface of atoms or embryos. As a result,
the metal atoms or embryos have a low surface energy, which will reduce nucleation
energy barrier and nuclei size based on Equation (4) and (5). As a result, many more
primary particles with low surface energy are formed compared with the case of the
absence of RTIL. The growth and the coarsening of the secondary particles are affected
by the surface energy of the primary particles. On the basis of the Equation (8), it can be
understood that low surface energy of the primary particles leads to the generation of the
smaller secondary particles. In turn, the low surface energy of the secondary particles
weakens Ostwald ripening in the particle growth stage, resulting in the production of very
small metal particles. In addition, low interfacial energy of the RTILs can be translated
into stabilization of molecular or metal species.

Figure 7.2 (b) shows the formation process of metal nanoparticles synthesized in the
presence of bmimSCN. Unlike other RTILs used in this work, bmimSCN coordinates
with the solvated metal ions, which has been shown by the shift of UV-vis adsorption

spectrum in chapter 4. It is also shown in the same chapter that bmimSCN has the ability

222

to reecue metal ions. Hence, bmimSCN seems to participate actively in the formation of
metal particles. As drawn in the Figure, the process of reduction, nucleation, particle
growth and aggregation occurs almost simultaneously in a very short time in the presence
of bmimSCN. The effect of low surface energy of bmimSCN on nucleation, particle
growth and aggregation stage can be explained in a similar way as described for
bmimPF6, bmimCH3SO4, and bmimCH3COz. Consequently, the role of bmimSCN with
its low surface energy functions as a strong protecting agent as well as a reducing agent
for the formation of the smallest metal particles among various RTILs.

v a
0 Add RTIL _

 

 

 

 

 

 

— - Y
.. 00 F I
- - -
M+ or M‘ ions in :3
solution 9 ' . .. . ...
0
v?“ o
\4‘”
0 °

. . . 6 S .
“I eparatron
. . —

‘3}

 

 

Stabilized clusters formation

(a)
a Reduction 0 .
I Nucleation E 3

Separation 6

—. . Q .
Growth *‘*
Aggregation

 

 

 

gr:
5*
at

M” or M‘ ions in . . .
solution StabIIIzed cluster formation

(b)
Figure 7.2 The simplified process for uniform particle formation synthesized via

microwave process (a) in the presence of bmimPF6, bmimCH3SO4, and
bmimCH3C02 and (b) bmimSCN.

223

7.2 Deposition of Nanoparticles on xGnP

Metal nanoparticles have been deposited on the surface of various carbons and metal
oxide supports. Hence, in this section, some details on interaction between metal
particles-supports and the formation and growth of metal particles on xGnP, which were

not covered in previous chapter, will be described.

7.2.1 xGnP Surface-Metal Precursor Interaction

Among various heteroatoms, oxygen-containing groups greatly influence an
acid/base character of the xGnP surface. Several oxygen groups on a graphene layer are
shown in Figure 7.3. Surface oxides such as carbonyl, carboxyl, phenolic hydroxyl,
lactone, and quinine groups are representative of the acidic groups. The basic groups are
associated with ethers but the basicity of xGnP surface originates mostly from the 7: sites
on a graphene layer. There are two sites of TE electrons on the surface ox xGnP: the
delocalized 1r sites and the localized 7t sites. The former are on aromatic systems
corresponding to the basal plane of xGnP. The latter are created in between two non-
neighboring oxygen atoms mostly at the edge of xGnP. The delocalized effect of TI
electrons has a great effect on the basicity of xGnP because the surface area of the basal
plane is much larger than that from the edge.

The oxygen surface groups and 7: sites on xGnP surface play a very critical role in
interactions between xGnP and a metal precursor and consequently in the interaction of
the metal particles with the xGnP support. The xGnP basal plane with its planar structure

along the c-axis contains a number of delocalized 1t electrons as shown in Figure 7.3. The

224

electrostatic interaction of the it sites-metal ions is also presented in Figure 7.4 in which

C,t represents the delocalized 1r electrons and Pt anion is taken as an example metal ion.

Quinone Carboxyl

Carboxyl
an hydride

 

peroxide 0H 0
Phenolic Lactone
hydroxide

Figure 7.3. Schematic representation of oxygen-containing groups and localized 1:
sites (*) on a graphene layer.

C,r + 2H20 —) C,H,o+ + OH-
I. C,H,o+ - (PtCld'z
\

. $¢¢ l.‘

0.671nm

I '
‘gs

:<--*""
0. 1426151n

 

Figure 7.4. Three dimensional schematic of layered graphite structure (O: carbon
atom) and schematic interaction between the delocalized 11: sites and Pt precursor as
an example.

225

The increase of the basic C,t sites on the basal plane of xGnP will maximize the

electrostatic attraction with the metal anion (C 7: — PtCl6— 2) and minimize the

electrostatic repulsion (C 00- — PtCl6— 2 ), resulting in increasing the dispersion of metal

particles. The strong interaction between the solvated metal ions and the TI sites on xGnP
is responsible for metal particles depositing at high density on the surface of xGnP. Metal
particles anchored on the 7: sites of xGnP are more resistant to metal sintering than those
on oxygen groups, indicating the strong adhesion of metal particles on xGnP [15]. The
oxygen groups are mostly located at the edges of xGnP. Thus, the metal particles at the
edge of xGnP result from the interaction of the oxygen groups-metal ions in polar
solvents. However, after the reduction of metal ions, the oxygen groups have a
detrimental effect on the interaction between the metal particles and xGnP because
sintering of the metal particles on oxygen groups easily takes place regardless of the high
initial dispersion of the metal ions.

The possible interactions between the metal ions-the 1r sites on the xGnP basal plane
and the metal ions-the oxygen groups at the xGnP edge and the formation of metal
particles on xGnP after the complete reduction are again illustrated with Pt precursors in
Figure 7.5. Fine and well-dispersed metal particles forms on the basal plane of xGnP as a
result of strong attraction of the metal ions-7t sites, while relatively large particles are
deposited at the xGnP edge site due to less stable oxygen groups at an elevated

temperature and the weak interaction of the metal ions-acidic oxygen groups.

226

PtCI,-2 PtCl6‘2

PtCl6'2 [130+ PtCI‘S‘2 PtClfi'2 0H PtCl6'2 PtCl6'2

,.
I I ........ I I 1.3.1 I - ,5:

E3 Rag—0C1. C. C. C, C, C, “(2,65% g
a 8 ‘ be:

: O O O O ,0 O, ‘0 O! ,0. 0,0, .09 o a:

Figure 7.5 Schematic representing the common interactions between the metal ions
and the active sites of xGnP and final metal particle size and distribution after the
reduction.
7.2.2 Metal Oxide-Metal Precursor Interaction

Surfaces of most metal oxides have hydroxyl groups or become hydroxylated on
contacting with water molecules. The hydroxyl groups can be acidic, neutral, or basic
according to the pH of the surrounding solution. If the pH is below the isoelectric point
(IEP) which is the value when the net surface charge is zero, the metal oxide surface
becomes positively charged due to protonation. If the pH value of the solutions is above
IEP, the surface of metal oxide particles becomes negatively charged due to
deprotonation. Thus, the metal oxides adsorb metal anions like PtC16'2 in solutions with
pH values below IEP and metal cations like Pt(NH4)2+2 at pH values above IEP, which is
schematically represented in Figure 7.6. IEP values of some important metal oxides as

catalyst supports are listed in Table 7.1.

227

Table 7.1 [so-electric point values of some metal oxides [16, 17]

 

 

 

Metal oxides IEP Adsorption
W03 0.5 cations
S102 1.0~2.0

MnOz 4.0~4.5 Cations or anions
SnOz 5.5

Ti02 6
y-FeO3 6.7

ZrOz 6.7

C602 6.8

A1203 7.0~9.0

ZnO 8.7~9.7 Anions
YzO3 9

La203 10.5

MgO 12~13

IEP
(a) pH < IEP (b) IEP < pH
HO\ / OH

PtCIgZ-u-THZO

/

x

l.

\

/0:---pt(NH4);2

J

Figure '7.6 Schematic representing the metal precursor-metal oxide interaction at
different pH condition and the formation of metal particle size.

228

7.2.3 Pt-Pt and Pt-Graphite Interactions

The Lennard-Jones (LJ) potential and the Sutton-Chen (SC) potential can be used to
describe the interaction of platinum atoms as an example [l8~21]. Although SC model is
normally used for a molecular dynamic study for atomic interaction, the LJ model is
more convenient than SC model when the substrate is present and thus involved in metal

atom interaction. The general form of LJ potential and force is given by

12 6
UU=45 f1 — 3 (7.9)
r0: r0:
4 12 ' 6
FL, =—8 12 3 —6 3- (7.10)
r.. r.. r..

IJ 1] '1

where a is an energy parameter, ais a lattice parameter, and ry- is the distance between
atoms 1' and j. The first term in Equation (9) and (10) represents the repulsion between
atomic cores and the second term represents the bonding energy between atoms due to
the surrounding electrons. F0.694eVand 0:0.29471 can be used for platinum-platinum
interaction [22] and 5:0. 0409eVand 0:0.293 6 for platinum-graphite interaction [23].
Figure 7.7 shows the force interaction between platinum atoms and also between
platinum atom and graphite substrate. The result indicates that platinum-platinum and
platinum-graphite are the most attractive when they are located at about 0.37nm. It is also
expected from the plot that platinum atoms may have good mobility on the surface of

xGnP due to their much higher attraction force than platinum-xGnP.

229

 

— LJ force for Pt-Pt

 

 

F (eVInm)

-5-

 

 

 

0.3 0.4 0.5 0.6 0.7

ril (nm)

 

0.6

0.4 -

— LJ force for Pt-C

 

0.2 -1

 

0.0 ~

F (eVInm)

-0.2 A

-04 q

 

 

 

I j I

0.3 0.4 0.5 0.6 0.7

rIi (nm)

Figure 7.7 Force interaction in platinum-platinum (top) and platinum-graphite
(bottom) obtained by Lennard-Jones formuation.

230

7.2.4 Particle Growth of Metal Particles on xGnP
Cluster growth of metal atoms or very small particles deposited on the basal plane of

xGnP can be simplified as in Figure 7.8. Individual metal nanoparticles deposited on

xGnP undergo Brownian motion with diffusion coefficient (D = d 2 /4T , d is the

diameter of metal and r is the diffusion time) to form small metal islands composed of
only two atoms or particles (Figure 7.8 a and b). The small metal islands grow by
incorporation of diffusing atoms (or nanoparticle or another small metal island) (Figure
7.8 c and d). Finally, a 3-dimensional island is formed by the total coalescence (Figure
7.8 f). The shape of metal nanoparticles or 3D metal island can be half spheres or even
flatter shape due to their interactions with xGnP.

What is the mechanism for 2D and 3D growth of metal islands? One of the answers
on this question can be partially explained by the expected good mobility of Pt particles
on xGnP due to their higher attractive force. Molecular dynamic study provides an
additional important answer [24]. The metal particles can be locked on the surface of
xGnP when xGnP and metal particle lattice parameters are compatible. However, slight
deviations from this match result in a huge increase of the diffusivity of metal particles,
suggesting the significant dependence of the diffusion coefficient on the lattice parameter
mismatch of xGnP and metal nanoparticles [25]. When the mismatch is large, the
particles undergo Brownian-like diffusion with no influence of the substrate. If the
mismatch is small, the particles diffuse by hopping-like motion, jumping from site to site
on the lattice defined by the substrate. The diffusion decreases as the number of atoms in

metal nanoparticles increases and finally stop at the critical number of atoms.

231

/ (c) \
I __ mm
C (b) (d) O,”

.4

O (a)

 

Figure 7.8. Main successive processes representing the formation of metal
nanoparticles on the basal plane of xGnP.

7.2.5 Metal Particle Deposition on xGnP in the Presence of RTIL

The size and dispersion of metal particles on xGnP relies on when bmimCH3C02, as
an example of RTIL is added. Figure 7.9 briefly shows the schematic of RTIL location in
a solution containing platinum ions and TEM morphologies of the samples prepared by
changing the sequence of bmimCH3C02 addition to the solution. When bmimCH3C02 is
introduced in xGnP/ethylene glycol solutions before to the addition of a metal precursor
(Sequence-A, Figure 7.9 a), bmimCH3C02 adsorbs onto the surface of xGnP and thus
metal ions interact weakly with xGnP through the adsorbed layer of bmimCH3C02.
Hence, there is a possibility of the formation of some metal atoms and particles separate
from the xGnP surface, which may lead to the deposition of a small quantity of metal
nanoparticles on xGnP. If bmimCH3C02 is added in xGnP/ethylene glycol solutions after
the addition of the metal precursor (Sequence-B, Figure 7.9 c), metal ions directly
interact with the it sites on xGnP basal plane and some oxygen groups at the xGnP edge
area. bmimCH3C02 may adsorb on the surface of metal ions or the xGnP surface

unoccupied by metal ions. Therefore, more metal atoms or particles form on the surface

232

of xGnP. TEM results shown in Figure 7.9 (b) and (d) support such assumptions well.
The sample prepared from Sequence-B has more particles on xGnP than that from

Sequence-A at no expense of particle size.

 

Ethylene glycol

 

 

 

 

 

 

 

(C) ((0

Figure 7.9. Suggested RTIL locations and TEM morphology of Pt/xGnP-
bmimCH3C02 sample when RTIL is added (a, b) before and (c, d) after Pt
precursor.

233

F TIR data obtained from Pt/xGnP-bmimCH3COz nancomposites also support that
the assumptions mentioned above are reasonable. As seen in the plots of Figure 7.10, the
sample for Sequence-B has the same peak features as xGnP indicating there is no
specific interaction between bmimCH3C02 and xGnP and thus bmimCH3C02 can be
removed with easy after mild-washing with acetone. Meanwhile, in the case of
Sequence-A, the sample shows peaks similar to those of bmimCH3C02, suggesting that
bmimCH3C02 molecules mostly adsorb onto the surface of xGnP and considerable traces
of bmimCH3C02 still remains on xGnP even after mild washing. In this sequence, too
much RTIL may be detrimental to achieve the targeted metal loading because it prevents

metal ions from interacting with xGnP.

 

 

 

 

 

 

04 ,_... . .. .. ”,2--. . ....,.,--_,-. , 3.-.",H3,-\._,,,,-__2_____ ., ,AM_2“__.,-_,.M--,_“24-0“..- -.-_
(a)
0.3 w‘
Q)
2
g (c)
s 0.2
3 . W
< A' II 1‘
0.1 III: ,1 1
.I' I
In Mr 'Mlj' l
0 I “‘1“ r J
600 1 100 1600 2100 4100
Frequency (l/cm)

Figure 7.10 FTIR of (a) Pt/xGnP-bmimCH3C02 (the addition of bmimCH3C02
prior to Pt precursor), (b) xGnP, (c) Pt/xGnP-bmimCH3C02 (the addition of Pt
precursor prior to bmimCH3C02) (d) ethylene glycol, and (e) bmimCH3C02.

234

7.3 Summary

Nucleation and growth of metal particles in solvent without xGnP has been
explained with classical nucleation theory and collision theory. The difference in the
formation of metal particles in solvent in the absence and the presence of RTIL was
explained with a simplified hypothesis. The principle of metal precursor-xGnP and metal
precursor-metal oxide interactions was explained when xGnP or metal oxide is present in
the synthesis of metal nanoparticles. It was found from calculation of the Lennard-Jones
force that the interaction between platinum atoms is much stronger than that between
platinum and xGnP, suggesting high mobility of platinum atoms on the surface xGnP.
The effect of the sequence of the addition of RTIL and the metal precursor on the size
and dispersion of metal particles was explained, and was supported. by TEM and UV-vis

spectroscopy data.

235

10.

11.

7.4. Bibliography

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V.K. LaMer and RH. Dinegar, “Theory, Production and Mechanism of Formation of
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S. Libert, V. Gorshkov, V. Privman, D. Goia, and E. Matijevic, “Formation of
monodispersed cadmium sulfide particles by aggregation of nanosize precursors”,
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J. Park, V. Privman, E. Matijevic, “Model of F orrnation of Monodispersed Colloids”,
J. Phys. Chem. B, 105, 11630, 2001.

M.A. Watzky and R.G. Finke, “Transition Metal Nanocluster Formation Kinetic and
Mechanistic Studies. A New Mechanism When Hydrogen Is the Reductant: Slow,
Continuous Nucleation and Fast Autocatalytic Surface Growth”, J. Am. Chem. Soc.,
1 19, p10382-10400, 1997.

D.W. Oxtoby, “Nucleation of First Order Phase Transitions”, Acc. Chem. Res., 31,
p9l-97, 1998.

H. Furedi-Milhofer, “Spontaneous precipitation from electrolyte solutions”, Pure
Appl. Chem, 53, p204l-2055, 1981.

C. Ribeiro, E.J.H. Lee, T.R. Giraldi, E. Longo, J .A. Varela, and ER. Leite, “Study of
Synthesis Variables in the Nanocrystal Growth Behavior of Tin Oxide Processed by
Controlled Hydrolysis”, J. Phys. Chem. B, 108, p15612-15617, 2004.

G.H. Weiss, “Overview of theoretical models for reaction rates”, J. Stat. Phys, 42,
p3-36, 1986.

D. Mozyrsky and V. Privman, “Diffusional growth of colloids”, J. Chem. Phys., 110,
p9254-9258, 1999.

C. Ribeiro, E.J.H. Lee, E. Longo, and ER. Leite, “A kinetic model to describe
nanocrystal growth by the oriented attachment mechanisum”, ChemPhysChem, 6,
p690-696, 2005.

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14.

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18.

19.

20.

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F. Huang, H.Z. Zhang, and J .F . Banfield, “Two-Stage Crystal-Growth Kinetics

Observed during Hydrothermal Coarsening of Nanocrystalline ZnS”, Nano Lett., 3,
p373-378, 2003.

. E.J.H. Lee, C. Ribeiro, E. Longo, and ER. Leite, “Oriented Attachment: An Effective

Mechanism in the Formation of Anisotropic Nanocrystals”, J. Phys. Chem. B, 109,
p20842-20846, 2005.

X.G. Peng, J. Wickham, and AP. Alivisatos, “Kinetics of II-VI and III-V Colloidal
Semiconductor Nanocrystal Growth: "Focusing" of Size Distributions”, J. Am. Chem.
Soc., 120, p5343-5344, 1998.

SS. Barton, M.J.B. Evans, E. Halliop, J .A.F. MacDonald, “Acidic and basic sites on
the surface of porous carbon”, Carbon, 35, pl 361, 1997.

ML. Toebes, J .A. van Dillen, and KP. de J ong, “Synthesis of supported palladium
catalysts”, J. Mole. Catal. A: Chem, 173, p75-98, 2001.

J .P. Brunelle, “Preparation of catalysts by metallic complex adsorption on metal
oxides”, Pure Appl. Chem, 50, p1211-1229, 1978.

SP. Huang and PB. Balbuena, “Pt nanoclusters on graphite substrates: a molecular
dynamics study”, Mole. Phys., 100, p2165-2174.

P. Deltour, J .L. Barrat, and P. Jensen, “Fast diffusion of a Lennard-Jones cluster on a
crystalline surface”, Phys. Rev. Lett., 78, p4597-4600, 1997.

E.J. Lamas, and PB. Balbuena, “Adsorbate effects on structure and shape of
supported nanoclusters: a molecular dynamic study”, J. Phys. Chem. B, 107, p11682-
11689, 2003.

AP. Sutton and J. Chen, “Long-range finnis Sinclair potentials”, Philos. Mag. Lett.,
61, p139-146,1990.

P.M. Agrawal, B.M. Rice, and D.L. Thompson, “Predicting trends in rate parameters
for self-diffusion on FCC metal surfaces”, Surface Science, 515, p21-35, 2002.

SR. Calvo and PB. Balbuena, “Molecular dynamics studies of phonon spectra in
mono- and bimetallic nanoclusters”, Surface Science, 581, p213—224, 2005.

237

24. H. Reiss, “Rotation and Translation of Islands in the Growth of Heteroepitaxial
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p1763-1766,1991.

238

 

CHAPTER 8

CONCLUSIONS

AND

RECOMMENDED WORK

239

An intriguing research theme in modern nanotechnology is to synthesize nanoscaled
metal colloids or clusters because they have great impact on the development of new
technological applications. They possess unique optical, thermal, electronic, magnetic
properties and chemical reactivity, which comes from their quantum size effect and is not
observed in their bulky status. Most of their unique physical and chemical properties
strongly depend on the size and shape of the metal nanoparticles. Therefore, an
alternative and effective synthetic method is required to obtain uniform small metal
nanoparticles with a controlled size and a narrow size distribution. On the other hand,
xGnP is an interesting carbon with excellent mechanical, thermal and electrical features
but much cost-effective compared to other carbon nanostructures. Hence,
nanocomposites composed of xGnP and metal particles hold promises for their
application in industrial catalysts, automobiles, batteries, supercapacitors, fuel cells,
hydrogen storage and production as well as sensor. The objectives of this research were:
0 Develop new synthetic technique to produce nanosized metals in a simple, effective
and economic way.

0 Extend the technique in order to produce various metal nanoparticles uniformly
dispersed on the surface of carbon materials.

0 Understand the formation mechanism of metal particles synthesized via the new
technique.

0 Fabricate xGnP-supported metal catalysts having possible application as
electrocatalysts for fuel cell.

0 Evaluate xGnP as a promising support to replace a commercial carbon

240

 

The accomplishments from the present work can be summarized as follows:

1) A simple and effective microwave heating process assisted with RTIL (MPR) to
synthesize noble metal nanoparticles with a narrow size distribution has been developed.
The average size of Pt, Ru, and, Pd blacks was < 3nm and they are almost monodispersed.
It has been confirmed that RTIL plays triple roles in synthesizing nanoscale metal
particles, that is, a promoter to increase the reduction rate of metal salt, a protecting agent
to prevent metal particles from growing and agglomerating each other, and a reductant to
convert metal ions to metal particles in microwave heating process. Such triple roles of
RTIL are responsible for the production of extremely small and well-stabilized individual

metal particles.

2) It was tested if MPR could be applied to produce carbonaceous materials- or metal
oxides-supported noble metal nanoparticles free from agglomeration. It was proved that
MPR make it possible to disperse metal phase on any kind of carbon surface. MPR

seemed effective of the deposition of metal particles on metal oxides, too.

3) Corrosion resistance and graphitization of exfoliated graphite nanoplatelets (xGnP)
has been studied. It turned out that xGnP possesses the greatest corrosion and oxidation
resistance in acidic environment and the highest degree of graphitization among various
carbon materials. Combined with its excellent electrical conductivity, good mechanical
strength and less impurity, this results support the potential of xGnP as a good substrate

for metal catalysts.

241

4) xGnP-supported platinum catalysts was synthesized by MPR and their catalytic
activity for methanol electro-oxidation was evaluated. Such catalysts showed much
higher catalytic activity compared to a commercial platinum catalyst on the surface of
carbon black. A main reason of excellent activity of the xGnP-supported catalyst stems
from the geometric feature of xGnP which makes the reactant easily access to the surface
of the active phases, leading to improve the utilization of platinum particles. This result

proves the potential of xGnP as a support to replace the current carbon black.

5) xGnP-supported or unsupported PtRu bimetallic nanoparticles have also been
synthesized via MPR. It was proved that MPR is facile to produce smaller PtRu
bimetallic alloy particles with a narrow size distribution for unsupported samples and
high dispersion of the particles on the surface of xGnP. The methanol oxidation of PtRu
bimetallic particles dispersed on xGnP was evaluated and they displayed the higher active
surface area and the higher activity than a commercial PtRu catalyst deposited onto
carbon black surface, which is attributed to the improved utilization of the PtRu particles

on xGnP surface.

The following works are recommended to extend the accomplishments from the
present work for the next generation energy and power devices:
1) Hydrogen Production: The challenge is to find inexpensive and efficient routes to
generate hydrogen in sufficient quantities from non-fossil natural resources.
Hydrogen generated from water splitting process using solar energy is the most

promising candidate as a renewable and environmentally safe energy carrier for

242

2)

the future. Solar hydrogen technologies using solar energy for hydrogen
production mainly consist of photoelectrochemical cell and photocatalytic water-
gas shift reaction. Both technologies are relatively simple but the raw material for
the former is only water and the latter needs scavenger reagent. The development
of both solar hydrogen technologies depends on special materials serving as
photosensitive photoelectordes in photoelectrochemical cell and as
heterocatalysts in photoassisted water-gas shift. Among many candidate

materials for photo-electrode and photocatalyst, TiOz.x has unique properties that

 

make it the most promising. Deposition of transition metals on Ti02.x has a great
impact on the catalytic activity of catalysts which depend on the method applied

to synthesize them. Hence, it is worth investigating the effect of Ti02-supported

transition metal catalysts synthesized by the present process for the

photocatalytic hydrogen generation.

Hydrogen Storage: The development of safe, reliable, cost-effective, and
practical ways of storing hydrogen is the bottleneck of the breakthrough for the
success of hydrogen economy and firel-cell technologies. Hydrogen uptake of
carbon materials by hydrogen spillover mechanism has recently attracted much
attention to achieve the target department of energy set. To facilitate hydrogen
spillover on carbon materials, metallic nanoparticles should be deposited on the
surface of carbons. Thus, the investigation of hydrogen storage for metal-
decorated carbon materials, especially xGnP, must be an intriguing research

theme.

243

3) Lithium Batteries: In lithium batteries, the anode materials are more important
than the cathode materials because the cycling behavior of the anode determines
the cell life completely. There are two categories in anode materials: one is
lithium alloy and metal oxides and the other is carbon materials. The former has
higher theoretical capacity and better cycleability than carbon materials.
However, large first cycle capacity loss and a large specific volume change
during Li insertion and extraction reactions are major problems. Carbon materials
have been used either as the anode itself or as a conductive enhancer for the
active materials mainly due to their high electrical conductivity and good
corrosion resistance in many electrolytes. Among various carbon materials,
graphite is the most attractive due to its low cost, high specific capacity, and
desirable charge-discharge curves. However, poor high rate performance due to
the limited electrochemical reaction at the interface between graphite and
electrolyte because of Li+ migration taking place only at the edge plane of
graphite limits their use. The graphite coated with metal thin film or deposited
with metal or metal oxide nanoparticles can be a solution to enhance charge-
discharge performance, coulombic efficiency, and cycling life at the same time.
Hence, the surface and interface modification of the graphite with metal or metal
oxide nanoparticles will be technical challenges to improve their electrochemical

reaction.

4) Supercapacitors: Supercapacitors can be divided into two types on the basis of

. the charge storage mechanism: the double-layer capacitors and the redox

244

capacitors. The former stores charges within the interfacial double-layer of high
specific area electrode in an aqueous or an organic electrolyte. This is called the
electrical or electrochemical double-layer capacitor (EDLC). Activated carbons
with very high surfacearea of 1000~2000m2/ g have been widely used for the
electrodes. In the latter, Faradaic redox reactions take place at the electrode
surface consisting of electroactive materials, as in a battery. This is called as
pseudocapacitor which has been developed with transition metal oxide and
conducting polymer. Since pseudocapacitor has a higher specific capacitance
than EDLC, the former type is more attractive. The electrode materials of
pseudocapacitors are electroactive materials with redox state and structure such
as transition-metal oxides (oxides of Ru, Co, Ir, Ni, Mg, In, Sn, Fe, etc). Due to
high specific capacitance and high chemical stability, iridium and ruthenium
oxides are widely sued. However, the high cost of ruthenium and iridium
precursors has limited their practical usage. Hence, there have been efforts to
reduce the cost of materials. To do so, the recent research has focused on
developing of ruthenium oxides/carbon composites. Carbon nanotube, carbon
black, activated carbon, and mesoporous carbon have been studied but there is no
report with xGnP which is expected to enhance utilization of ruthenium oxide
and other active metal oxides, resulting in improved capacitance. Hence, metal
oxide/carbons, especially metal oxides/xGnP nanocomposites fabricated by the

present method may be a good research subject for this application.

245

5) Sensors: Sensors are devices converting subtle change of physical and chemical
quantities into an electrical signal. Sensors continue to impact everyday life with
a variety of applications such for biomedical, automotive, food, agriculture
industry, and even security. Especially, sensors based on carbon nanotubes have
the great potential of revolutionizing the sensor industry due to their
exceptionally excellent properties. The decoration of carbon nanotubes with
nanosized metal oxides or quantum dots is most interesting research theme for
next generation sensor development. Hence, the present work may be applied for
the deposition of such materials on carbon nanotube surface, which will be of

importance for creating new types of sensors.

246

 

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