ELECTROCATALYTIC WATER SPLITTING TO PRODUCE FUEL HYDROGEN
By
HAO YUAN

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Chemical Engineering – Doctor of Philosophy
2017

ABSTRACT
ELECTROCATALYTIC WATER SPLITTING TO PRODUCE FUEL HYDROGEN
By
HAO YUAN
Solar energy is regarded as a promising source for clean and sustainable energy. However, it is
not a continuous energy source, thus certain strategies have to be developed to effectively
convert and store it. Solar-driven electrocatalytic water splitting, which converts solar energy
into chemical energy for storage as fuel hydrogen, can effectively mitigate the intermittence of
solar radiation. Water splitting consists of two half reactions: water oxidation and hydrogen
evolution. Both reactions rely on highly effective electrocatalysts. This dissertation is an account
of four detailed studies on developing highly effective low-cost electrocatalysts for both
reactions, and includes a preliminary attempt at system integration to build a functional
photoanode for solar-driven water oxidation.

For the water oxidation reaction, we have developed an electrochemical method to immobilize a
cobalt-based (Co-OXO) water oxidation catalyst on a conductive surface to promote
recyclability and reusability without affecting functionality. We have also developed a method to
synthesize a manganese-based (MnOx) catalytic film in situ, generating a nanoscale fibrous
morphology that provides steady and excellent water oxidation performance. The new method
involves two series of cyclic voltammetry (CV) over different potential ranges, followed by
calcination to increase crystallinity. The research has the potential to open avenues for
synthesizing and optimizing other manganese-based water oxidation catalysts.

For the hydrogen evolution reaction, we have developed a new electrodeposition method to
synthesize Ni/Ni(OH)2 catalysts in situ on conductive surfaces. The new method involves only
two cycles of CV over a single potential range. The resulting catalytic film has a morphology of
packed walnut-shaped particles. It has superior catalytic activity and good stability over long
periods.

We have inestigated the feasibility of incorporating manganese-based water oxidation catalysts
into TiO2 to build a composite photoanode. Results show that the composite photoanode has
good activity under ultraviolet (UV) illumination, and that its catalytic performance can be
significantly improved by enabling light collection over a wider range of wavelengths.
We have also proposed a scheme for a solar-driven water splitting device that integrates both
electrocatalysts with solar energy collection and intensification, and is capable of minimizing
mass transfer resistance. The dissertation concludes with suggested future work to further
explore the MnOx and Ni/Ni(OH)2 electrocatalysts.

Copyright by
HAO YUAN
2017

To my parents, and all of my teachers and professors who have been outstanding mentors
throughout my entire academic career.

v

ACKNOWLEDGEMENTS

I would like to first thank my advisor, Dr. Robert Y. Ofoli, for being an excellent mentor and
encouraging my research at all times. His advice on both research and my career has been
invaluable.

I would also like to thank my tremendous committee members, Dr. Richard R. Lunt, Dr. Gary J.
Blanchard and Dr. Ilsoon Lee for serving on my committee and providing brilliant suggestions
and great help throughout my doctoral work at MSU. My thanks also to Dr. Yanzong Zhang,
who first led me into the world of science.

Next, I would like to thank everyone who has helped me with my research: Dr. Fleglers, Dr. Fan,
Amy, Dr. Staples, Dr. Askeland, and all the staff personnel in the CHEMS department.

Big thanks to my all my dear friends for all the good times we enjoyed together as well as the
times we wasted together. Special thanks to my roommate Weijie Ren for his great friendship
and help, and heartfelt thanks to my beloved girlfriend, Xiaolei, for her years-long support and
encouragement.

Finally, I would like to thank my family, especially my parents, for all the support and sacrifices
that they have made for me throughout my life.

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TABLE OF CONTENTS

LIST OF TABLES .......................................................................................................................... x
LIST OF FIGURES ....................................................................................................................... xi
KEY TO ABBREVIATIONS ..................................................................................................... xvii
CHAPTER 1. INTRODUCTION ................................................................................................... 1
1.1 Overview and significance of the problem ........................................................................... 1
1.2 Natural photosynthesis and artificial photosynthesis (AP) schemes ..................................... 5
1.3 Outline of this dissertation .................................................................................................... 7
CHAPTER 2. CHARACTERIZATION AND FUNCTIONAL ASSESSMENT OF A
COBALT(III)-OXO CUBANE CLUSTER WATER OXIDATION CATALYST
IMMOBILIZED ON A CONDUCTIVE SURFACE ..................................................................... 9
2.1 Abstract ................................................................................................................................. 9
2.2 Introduction ........................................................................................................................... 9
2.3 Materials and methods. ....................................................................................................... 12
2.3.1 Materials ....................................................................................................................... 12
2.3.2 Methods ........................................................................................................................ 12
2.3.2.1 Ex situ Synthesis of the Co-OXO cubane cluster ................................................. 12
2.3.2.2 Electrodeposition of Co-OXO catalyst synthesized ex situ .................................. 13
2.3.2.3 Catalytic functionality and stability assessments .................................................. 13
2.3.2.4 Characterization of catalysts by FTIR and SEM .................................................. 14
2.4 Results and Discussion ........................................................................................................ 14
2.4.1 Electrodeposition of Co-OXO ...................................................................................... 14
2.4.2 FTIR and surface FTIR characterization of the Co-OXO catalyst ............................... 16
2.4.3 Surface morphology and structure characterizations of immobilized catalyst ............. 17
2.4.4 Catalytic functionality, stability and recyclability assessments ................................... 19
2.5 Conclusions ......................................................................................................................... 24
CHAPTER 3. SYNTHESIS OF A MnOX CATALYTIC FILM ON FTO BY DUAL-SERIES
CYCLIC VOLTAMMETRY ........................................................................................................ 25
3.1 Abstract ............................................................................................................................... 25
3.2 Introduction ......................................................................................................................... 25
3.3 Materials and methods ........................................................................................................ 27
3.3.1 Materials ....................................................................................................................... 27
3.3.2 Electrodeposition of MnOx on FTO ............................................................................. 27
3.3.3 Surface morphology characterization ........................................................................... 28
3.3.4 Catalyst functionality assessments and comparisons ................................................... 28
3.3.5 Catalyst stability assessment ........................................................................................ 29
3.4 Results and discussions ....................................................................................................... 29
3.4.1 Surface morphology characterizations ......................................................................... 29
3.4.2 Catalytic functionality assessments .............................................................................. 32

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3.4.3 Proposed formation mechanism ................................................................................... 35
3.4.4 XRD characterizations .................................................................................................. 41
3.4.5 Stability tests................................................................................................................. 43
3.5 Conclusions ......................................................................................................................... 47
CHAPTER 4. FUNCTIONAL BIOMIMETIC COMPOSITE Mn/TiO2 PHOTOANODE FOR
WATER SPLITTING ................................................................................................................... 48
4.1 Abstract ............................................................................................................................... 48
4.2 Introduction ......................................................................................................................... 48
4.3 Materials and methods ........................................................................................................ 50
4.3.1 Materials ....................................................................................................................... 50
4.3.2 Methods ........................................................................................................................ 51
4.2.2.1 Spin coating of the TiO2 film ................................................................................ 51
4.3.2.2 In situ synthesis of the Mn WOC catalyst ............................................................ 51
4.3.2.3 Structural characterization by XRD and SEM ...................................................... 52
4.3.2.4 Functionality assessment by CV and EIS ............................................................. 52
4.4 Results and discussion......................................................................................................... 53
4.4.1 Structural and morphological characterization of the composite photoanode ............. 53
4.4.2 Electrochemical characterization of the Mn WOC....................................................... 58
4.4.3 Electrochemical characterization of the composite photoanode. ................................. 59
4.5 Conclusions ......................................................................................................................... 63
CHAPTER 5. ELECTRODEPOSITION OF Ni/Ni(OH)2 CATALYTIC FILMS BY CYCLIC
VOLTAMMETRY FOR THE HYDROGEN EVOLUTION REACTION ................................. 64
5.1 Abstract ............................................................................................................................... 64
5.2 Introduction ......................................................................................................................... 64
5.3 Experimental session ........................................................................................................... 65
5.3.1 Materials ....................................................................................................................... 65
5.3.2 Synthesis by electrodeposition ..................................................................................... 66
5.3.3 Materials characterization............................................................................................. 66
5.3.4 Electrochemical assessment of functionality ................................................................ 67
5.4 Results and discussions ....................................................................................................... 68
5.4.1 Structural characterization ............................................................................................ 68
5.4.2 Functionality assessments............................................................................................. 72
5.4.3 Stability assessments .................................................................................................... 76
5.4.4 Effects of electrolyte concentration on electrodeposition ............................................ 77
5.4.5 Hypothesis of the formation mechanism ...................................................................... 78
5.5 Conclusions ......................................................................................................................... 79
CHAPTER 6. SUGGESTIONS FOR FUTURE RESEARCH ..................................................... 80
6.1 Introduction ......................................................................................................................... 80
6.2 Further research on improving the MnOx WOC ................................................................. 82
6.3 Further research on improving the Ni/Ni(OH)2 HER catalyst ............................................ 82
6.4 Further research on system integration ............................................................................... 83
CHAPTER 7. CONCLUSIONS ................................................................................................... 86

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BIBLIOGRAPHY ......................................................................................................................... 88

ix

LIST OF TABLES

Table 5.1. Comparison of Tafel slopes and exchange current densities of cc-Ni, cv-Ni, and Ni foil
catalysts assessed in neutral media. The lower slope and higher exchange current density of cv-Ni
are indicators of much better catalytic performance. ……………………………………………..75
Table 5.2. Peak current density at -0.5 V vs. NHE for samples synthesized in different
concentrations of electrolyte……………..………………………..……………………………..77

x

LIST OF FIGURES

Figure 1.1. Diagram showing our current energy demand. Currently, we utilize 17.3TW of energy,
with more than 70% provided by fossil fuels. Solar energy only contributes ~10%..........3
Figure 1.2. Three schemes of typical artificial photosynthesis processes: a) direct hydrogen fuel
production through solar-driven water splitting; b) direct carbon-based fuel production using
hydrogen ions that react directly with captured CO2; c) indirect carbon-based fuel production with
captured CO2 reacting with reconstituted hydrogen gas….………………………………….4
Figure 1.3. Typical solar-driven electrocatalytic water splitting device to produce hydrogen fuel.
It usually consists of a composite photoanode and a counter electrode. The composite photoanode
should have a robust conductive substrate, a solar harvester which can convert solar energy into
the potential required to power the water splitting reaction, and a highly effective water oxidation
catalyst to lower the overpotential of the water oxidation reaction. The counter electrode should
also have a conductive substrate as well as a hydrogen evolution catalyst for hydrogen
reconstit uti on…………………… ……………………………………………………. . .6
Figure 2.1. Current density profile (red) of the Co-OXO WOC during electrodeposition on ITO.
The current density increased gradually to a maximum of 0.67 mA/cm2, which is considerably
higher than for the case where the electrode was placed in solution without the catalyst (blue
curve). Similar results were obtained on gold electrodes. The initial drop in current density is
likely due to mass transport effects.………………………..........................................................15
Figure 2.2. FTIR spectra of the catalyst powder prior to immobilization on conductive surfaces.
The stretching frequencies at 1556, 1407, and 669 cm-1 represent the pyridine ring group, the
acetate hydroxyl group, and the carbon backbone -CH- groups, respectively. These frequencies
confirm the existence of the stabilizing ligands used in synthesizing the catalyst complex. These
feature peaks are not present in the surface FTIR, indicating the loss of all organic ligands during
electrodeposition process…………………………………...……………………………………16
Figure 2.3. a) SEM image of the ITO surface following catalyst immobilization. It gives clear
evidence of attachment of the catalyst to the surface. By contrast, the SEM image of ITO prior to
catalyst deposition (Figure 2.3a inset) at an equivalent magnification has a featureless surface. b)
SEM image of a selected area at a higher magnification. c) EDS diagram of Co-OXO-coated ITO
sample, confirming that the catalyst core remained on the ITO substrate during the
electrodeposition process………………………………………………………………………...18
Figure 2.4. Cyclic voltammograms acquired at a scan rate of 10 mV/s on bare ITO (blue curve),
Co-OXO immobilized on gold (red), and Co-OXO immobilized on ITO (green) in 0.1M neutral
phosphate buffer solution at pH 7.0. This diagram shows that water oxidation occurs at
approximately 1 V for Co-OXO on both gold and ITO. For comparison, we have also included the
CV of the catalyst in a pseudo-homogeneous system (purple). The profiles of the three scans are

xi

essentially identical, providing strong evidence that Co-OXO maintains its catalytic function
whether in solution, or immobilized on gold or ITO.……………………………………………20
Figure 2.5. Voltammograms acquired at a scan rate of 10 mV/s over 14 cycles of catalyst reuse.
To acquire these data, the same catalyst was left in place for 14 consecutive scans. The data show
negligible variation in both current density and peak currents, providing strong evidence that the
immobilized catalyst is very stable, with a reproducible performance…………….. ......22
Figure 2.6. Assessment of catalyst recyclability and reusability. In this test, we acquired eight CV
scans on a single sample of Co-OXO catalyst coated on ITO. After each scan, the sample was
taken out of the solution, rinsed with DI water, dried in air and reinserted into the solution for the
next scan. As the data show, all the voltammograms are distributed over a small range of values,
confirming a high degree of catalyst reusability, as well as reproducibility in performance…...23
Figure 3.1. SEM images of the surface morphologies of the FTO substrate, and of MnOx after
different in-situ synthesis methods. Image a: Bare FTO, showing a significant degree of surface
roughness. Image b: MnOx after two series of electrodeposition by CV, followed by calcination;
the surface is a network of interconnected fibers in both the SEM and TEM (inset) images. Images
c and d: MnOx after electrodeposition by single CV series (0-0.6 V and 0-2.0 V, respectively);
image c has a similar morphology to image b, although a few small spherical clusters are visible;
image d shows a network of larger micron-sized fibers with random orientations.…………….…31
Figure 3.2. Catalytic functionality of the three MnOx films on FTO. The blue curve is the cyclic
voltammogram of the MnOx film synthesized by the dual-series CV method; the purple and red
profiles are the single-series synthesis from 0-0.6 V and 0-2.0 V, respectively; and the green curve
is for bare FTO under identical assessment conditions. The MnOx catalyst synthesized by dualseries CV is clearly superior to the single series species, because it benefits from the nanostructure
developed in the first series, and the elevated oxidation state acquired during the second series.
The importance of nanostructure is further reflected by the fact that the 0-0.6 V catalyst performs
slightly better than the 0-2.0 V sample, even though the latter is at a higher oxidation
state………………………………………………………………………………………………33
Figure 3.3. Comparison of the catalytic performances of dual series and single session samples
by cyclic voltammetry in 0.1M phosphate buffer (pH 7.0). All the single series samples were
synthesized under identical conditions as described in the experimental session, with the only
difference being the ranges of potential ramps. Bare FTO was used again as a control. All singleseries samples had similar performances, except the 0-0.4V sample which has a much lower
activity. The dual-series sample has a much better performance than all single-series samples,
which we believe to be the result of both an elevated oxidation state and nanoscale surface
morphology………………….…………………………………………………………………...34
Figure 3.4. Cartoon illustration of the proposed formation mechanism. We propose that Mn2+ ions
are first oxidized on the protrusions of the FTO surface during the first CV series, forming fibrous
MnO(OH) species. During the second CV series, the oxidation state of the catalytic film is further
elevated while the surface morphology remains largely intact……………………….........……..36

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Figure 3.5. Time-lapse photographs of MnOx samples at selected stages of deposition. During the
first CV series, a light yellowish film could be observed on the FTO surface. The film turned to
dark brown during the second CV series……………………………………………….……...36
Figure 3.6 a. SEM images at various magnifications of the sample after two (2) cycles in the first
CV series. Small clusters can be observed on the protrusions on the FTO surface ….………….37
Figure 3.6 b. SEM images at various magnifications of sample after four (4) cycles in the first
CV series. Small clusters in Figure 3.6 a have grown into a larger scale………………………..37
Figure 3.6 c. SEM images at various magnifications of the sample after six (6) cycles of the first
CV series. The clusters in Figure 3.6 b continue to grow as the catalytic film is forming ..…....38
Figure 3.6 d. SEM images at various magnifications of the sample after eight (8) cycles in the
first CV series. The catalytic film in Figure 3.6 c has now covered the whole surface ..……….38
Figure 3.6 e. SEM images at various magnifications of the sample after four (4) cycles in the
second CV series. The surface morphology of the catalytic film did not change much during the
second CV Series………………………………………………………………………………..39
Figure 3.7. Functionality assessment after selected stages of electrodeposition. There is a gradual
increase in peak current density during the first CV series. However, the peak current density
increases dramatically during the second CV series, which we speculate to be the result of
continued enhancement of the oxidation state of the catalyst. It is particularly worth noting that
the slope of the functionality curve during the second session is strongly positive, and nowhere
near an asymptote. Thus, there is clearly a possibility of much further enhanced oxidation state
with additional scans over this potential range (0.6 to 2.0 V) or a third series o f CV
electrodeposition over a different potential range, with the potential that this method can be used
to optimize catalyst function….…………………………………………………………………..40
Figure 3.8. X-ray diffraction spectra of the three catalysts: a) MnOx after two consecutive series
of CV electrodeposition (scans over 0-0.6 V and 0.6-2.0 V) followed by calcination; it has the
peaks of both Mn2O3 and MnO2, which indicates that the MnO(OH) formed in the first CV series
has been further oxidized. b) MnOx after one series of CV (0-2.0 V) and calcination, showing a
similar pattern to trace a); this is an indication that the single 0-2.0 V series produces a MnOx
catalyst with a similar oxidation state as the one synthesized by the dual-series. c) MnOx after one
CV series (0-0.6 V) and calcination; the scan shows that the major phase of MnO x is Mn2O3,
meaning that the MnOx before calcination is MnO(OH) as proposed. The symbols +, *, and # in
this figure designate the feature peaks of the FTO substrate, Mn2O3 and MnO2, respectively……42
Figure 3.9. Current density profile of the MnOx catalyst synthesized by the dual-series method
during long term catalytic assessment at 1.2 V (vs. SHE) in a 0.1 M phosphate buffer solution (pH
7.0). The steady profile over a long period of time gives an indication of a stable MnOx film.
………………………………………………………………………………………………….44

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Figure 3.10. Current density profiles showing the stability of the catalytic films: bare FTO (purple
curve), single session 0-0.6V sample (black), single session 0-2.0V sample (red), and dual session
sample (blue). All tests were carried out in a 0.1M phosphate buffer solution (pH 7.0) at 1.2V (vs.
SHE) for six hours. All single session and dual session samples had good stability during the six
hours of assessment. However, the dual session sample had a higher stable current density
compared to the single session samples, providing further evidence that the dual series method is
better than the traditional single session methods……………………………………………….45
Figure 3.11. Consecutive CV scans on one catalyst sample to assess stability. All 18 scans have
similar profiles, with the peak current density falling within a narrow range of values (1.96Âą0.11
mA/cm2)………………………………………………………………………………………….46
Figure 4.1. XRD spectra of a) pure P25 TiO2 powder, b) pure FTO glass, c) FTO glass coated
with TiO2, and d) the composite photoanode. In this figure, + designates the feature peaks of TiO2,
and * denotes those of FTO. Traces “a” and “b” show the typical patterns for TiO 2 and FTO,
respectively. Trace “c” shows all the peaks associated with both TiO2 and FTO, confirming that
the composite photoanode contains all the required elements. However, trace “d” is not
significantly different from trace “c,” which could mean that the method described above did not
produce the expected composite catalyst, or that XRD is not an appropriate technique for detection
of manganese in the complex………….………………………………………………………….54
Figure 4.2. SEM images of a) clean FTO substrate; b) FTO surface after coating with TiO 2 (at
x200 magnification); c) FTO coated with TiO2 (at x800 magnification); d) composite photoanode
(at x160 magnification); e) composite photoanode (at x3,300 magnification). Images b and c show
the morphology of the TiO2 coating to be essentially a uniform layer, with a porous structure that
contains cracks. Images d and e show the morphology of the composite photoanode. Following
Mn WOC deposition, the morphology of the surface does not change substantially; however, a
new layer could be observed underneath the porous TiO2 film..………………………………….56
Figure 4.3. EDS mapping of selected area. a) The SEM image provides evidence of both TiO2
and manganese on the surface. The triangular area at the top is presumed to be the TiO2 layer,
while the rest of the area is covered by the manganese catalyst. b) The distribution of titanium
within the selected area. As postulated, the highest accumulation of titanium is in the triangular
area. c) Distribution of manganese in the selected area, with the heaviest concentration outside the
triangular area.……………………………………………………………………………………57
Figure 4.4. Typical cyclic voltammograms during assessment of the manganese catalyst. The red
trace represents data for clean FTO glass. The blue trace represents data acquired on the manganese
catalyst deposited on FTO, and provides confirmation that the catalyst synthesized by
electrochemical deposition is active……………………………………………………………..58
Figure 4.5. First derivative of the current density profile with respect to applied potential, used
here as a reliable aid to determining the oxidation voltage. a) Data acquired under UV illumination,
showing an oxidation voltage of approximately 0.8 V vs. Ag/AgCl. b) The oxidation voltage for
the same sample evaluated without UV illuminationwas of approximately 1.1 V vs.
Ag/AgCl………………………………………………………………………………………..60

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Figure 4.6. Nyquist plot of EIS data acquired at 0.8 V for the composite photoanode under UV
illumination. The semicircle at high frequency denotes reaction, while the straight line at low
frequency denotes diffusion. Along with the Randles circuit (inset) used to simulate the data, this
result provides confirmation that the Mn WOC is covered by a porous layer of TiO2……...……..62
Figure 5.1. SEM images and EDS characterization of cc-Ni and cv-Ni. a) SEM image of a bare
FTO substrate, showing a featureless but rough surface. b) SEM image of the cc-Ni sample,
showing a featureless metallic surface. c) cv-Ni at an identical magnification, showing a film
consisting of a monolayer of evenly-distributed walnut-shaped particles with diameters of 0.1~0.2
m. The inset is the surface of the cv-Ni sample at a higher magnification, showing a detailed
morphology of spherical particles with patterns on the surface. d) EDS characterization of cc-Ni,
showing only the signal for Ni, which is an indication that it consists mainly of a Ni film. e) EDS
characterization result for cv-Ni showing both Ni and O, an indication that cv-Ni contains both Ni
and Ni(OH)2. The carbon signals in both EDS spectra are from supporting SEM slabs and
patches……………………………………………………………………………………………69
Figure 5.2. Cross-sectional SEM image of the cv-Ni sample. The thickness of the Ni/Ni(OH)2
catalytic film was estimated at ~0.3Îźm. This also confirms that the surface morphology of the
catalyst layer is a monolayer of firmly packed spherical particles………………………………70
Figure 5.3. XRD spectra of a) cv-Ni, b) cc-Ni and FTO films. The symbol + designates feature
peaks of FTO, * of Ni, and # of Ni(OH)2. The cc-Ni film has peaks of FTO and Ni, while the cvNi spectrum has peaks of FTO, Ni and Ni(OH)2. Due to the small amount of Ni(OH)2 in the cvNi samples compared with FTO and Ni, the Ni(OH)2 peaks are not prominent……………….….71
Figure 5.4. Comparison of the functionality of cv-Ni, cc-Ni, and platinum mesh by cyclic
voltammetry in neutral media. The black, red, and green curves are for cc-Ni, cv-Ni, and Pt mesh,
respectively. Both cc- and cv- Ni have higher onset potentials than Pt, but the cv-Ni catalyst has a
lower onset potential than cc-Ni……………...…………………………………………………..73
Figure 5.5. Comparison of the functionality of cv-Ni, cc-Ni, and platinum mesh by cyclic
voltammetry in alkaline media. The black, red, and green curves are for cc-Ni, cv-Ni, and Pt mesh,
respectively. Both cc- and cv- Ni have higher onset potentials compared to Pt, but the cv-Ni
catalyst has a lower onset potential than cc-Ni…………………………………………………..74
Figure 5.6. Current density profile of cv-Ni in a 0.1 M neutral phosphate buffer under an applied
potential of -1.0V vs. Ag/AgCl. After the first two hours, the current density was steady over the
remaining eight-hour testing period, demonstrating stable catalytic performance. The inset shows
the cyclic voltammograms of the first and 201st cycles of a functionality test on a single cv–Ni
catalytic sample. The similarity in the two profiles as well as peak current densities is an indication
of very stable catalytic performance……………………………………………………………76
Figure 6.1. Schematics of two water splitting devices. Each scheme has a photoanode complex
containing a solar harvester and a WOC, and a counter electrode for hydrogen ion reconstitution
to produce fuel. a. Two-chip protocol (face-to-face scheme), where the counter electrode does not

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share the conductive support with the photoanode; b. One-chip protocol, where the counter
electrode and photoanode share one conductive support…………………………………………81
Figure 6.2. Overview of the proposed scheme. All the components of the proposed solar-driven
water splitting device are supported by a robust conductive substrate. The squares are the AP
modules which can be engineered into various shapes and sizes. Water channels are formed by the
edges of the AP modules. The water oxidation catalyst is coated on the edges of the AP modules,
and the hydrogen evolution catalyst is coated on the bottom of the water channel to greatly reduce
the distance for ion migration, resulting in a much lower resistance to mass transfer. The detailed
structure of the AP module is shown in the figure below……… ………………….84
Figure 6.3. Cross-sectional view of the AP module in a microfluidic channel. Each AP module
features a) a solar concentrator to collect solar energy on the larger surface and transmit it to the
edge; b) a convertor to transform the concentrated solar energy into the potential required for the
water splitting reaction; c) a conductive layer for the electrodeposition of WOC; d) an optional p+
layer to promote electron-hole separation; and e) a WOC layer directly in contact with
water……………………………………………………………………………………………..84

xvi

KEY TO ABBREVIATIONS

AP

artificial photosynthesis

CV

cyclic voltammetry

DSSC

dye sensitized solar cell

EDS

dispersive x-ray spectroscopy

EIS

electrochemical impedance spectroscopy

EXAFS

extended x-ray absorption fine structure

FESEM

field emission scanning electron microscope

FTIR

Fourier transform infrared spectroscopy

FTO

fluorine-doped tin oxide

HER

hydrogen evolution reaction

HERC

hydrogen evolution reaction catalyst

ITO

indium tin oxide

NADPH

nicotinamide adenine dinucleotide phosphate

NHE

normal hydrogen electrode

OEC

oxygen evolution complex

PBS

phosphate buffer solution

PV

photovoltaic

SEM

scanning electron microscope

TEM

transmission electron microscope

UV

ultraviolet

WOC

water oxidation catalyst

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XANES

x-ray absorption near edge structure

XAS

x-ray absorption spectroscopy

XRD

x-ray powder diffraction

xviii

CHAPTER 1. INTRODUCTION

1.1 Overview and significance of the problem
Society relies on sufficient energy supplies to meet a variety of needs1. Currently, more than
70% of the global energy supply comes from fossil fuels, including crude oil, natural gas and
coal (Figure 1.1). However, the rate of consumption of fossil fuels is much higher than the
carbon fixation rate of natural photosynthesis2. Also, due to its uneven distribution among
countries, decreasing availability, and green-house gas emission issues, fossil fuels are no longer
an ideal energy source. This has created a critical global challenge for sustainable energy
production. The most effective way to do this is to produce energy from renewable and/or earthabundant materials.

Solar energy is the only renewable energy source that is capable of powering the entire planet. It
has a net output of 125,000 TJ/s, which is magnitudes higher than our current global energy
needs3,4. However, there are two major drawbacks that prevent solar energy from practical
utilization on a larger scale. The first drawback is that, except in very few situations, it cannot be
used directly5. It always needs to be converted into other forms such as heat6, electrical7-9 or
chemical energy10-13. Another drawback is that solar radiation is not a continuous energy source.
Sunlight is generally available during the day but not at night14. Thus, strategies have to be
developed to store the energy collected during the periods of sunshine. Traditionally, this
intermittency has been mitigated by photovoltaic (PV) technology, which stores solar energy in
batteries, or solar-thermal technologies which store solar energy in the form of heat. Artificial
photosynthesis (AP), a process that mimics natural photosynthesis, has been gaining attention

1

recently as a promising strategy to convert and store solar energy in the form of chemical
energy15. Typically, there are three artificial photosynthesis schemes: direct hydrogen fuel
production through water splitting, direct carbon-based fuel production, and indirect carbonbased fuel production. Figure 1.2a portrays the direct hydrogen production via water splitting,
while Figures 1.2b and 1.2c are schemes for direct and indirect carbon-based fuels production
that also require carbon dioxide (CO2) sequestration. In each of the three schemes, the only
remnants following consumption of the produced fuels are the water (H2O) and carbon dioxide
(CO2) used to produce them. Thus, all three schemes are self-sustainable and carbon-neutral.

This dissertation focuses on our group’s research on direct hydrogen fuel production via catalytic
solar-driven water splitting (Figure 1a).

2

Figure 1.1. Diagram showing our current energy demand. Currently, we utilize 17.3TW of
energy, with more than 70% provided by fossil fuels. Solar energy only contributes ~10%.

3

Figure 1.2. Three schemes of typical artificial photosynthesis processes: a) direct hydrogen fuel
production through solar-driven water splitting; b) direct carbon-based fuel production using
hydrogen ions that react directly with captured CO2; c) indirect carbon-based fuel production
with captured CO2 reacting with reconstituted hydrogen gas.

4

1.2 Natural photosynthesis and artificial photosynthesis (AP) schemes
There are two main catalytic reactions in the natural photosynthesis process16,17: the lightdependent reaction (light reaction); and the light-independent reaction (dark reaction). In the
light reaction, pigments in chlorophyll capture sunlight and use the converted solar energy to
oxidize water, producing oxygen gas and protons (in the form of NADPH)18,19. In the following
dark reaction, the protons (NADPH) are used to produce organic molecules with CO2 through
the Calvin cycle20. A biomembrane separates the two processes, and provides a means to
selectively transfer protons (NADPH).

Similar to natural photosynthesis, the solar-driven electrocatalytic water splitting process
involves two half reactions21: the water oxidation reaction generates oxygen gas and hydrogen
ions (Equation 1.1), and the hydrogen evolution reaction reconstitutes the hydrogen ions into
hydrogen gas (Equation 1.2).

1

𝐻2 𝑂  2 𝑂2 + 2𝐻 + + 2𝑒 −
2𝐻 + + 2𝑒 − 𝐻2

(1.1)
(1.2)

As a result, a typical water splitting device has two major components: a solar-powered
photoanode and a catalytic hydrogen evolution cathode. Shown in Figure 1.3, the photoanode
usually consists of a robust substrate, a layer or layers of semiconducting material for solar
energy capture, and a highly-effective water oxidation catalyst. The cathode side consists of a
conductive substrate and a hydrogen evolution catalyst.

5

Figure 1.3. Typical solar-driven electrocatalytic water splitting device to produce hydrogen fuel.
It usually consists of a composite photoanode and a counter electrode. The composite
photoanode should have a robust conductive substrate, a solar harvester which can convert solar
energy into the potential required to power the water splitting reaction, and a highly effective
water oxidation catalyst to lower the overpotential of the water oxidation reaction. The counter
electrode should also have a conductive substrate as well as a hydrogen evolution catalyst for
hydrogen reconstitution.

6

1.3 Outline of this dissertation
The goal of this dissertation is to present the details of our work on developing electrocatalysts
for both the water oxidation and hydrogen evolution reactions. We will also present a scheme for
system integration to make functional photoanodes for solar-driven water oxidation.

Chapter 2 will focus on our research on the synthesis, immobilization and assessment of a cobaltbased (Co-OXO) water oxidation catalyst (WOC). We developed an electrodeposition method to
successfully immobilize the Co-OXO WOC synthesized ex situ onto conductive surfaces. The
resulting film enabled enhanced recyclability and reusability, while maintaining catalyst
functionality in solution.

Chapter 3 describes our research on developing a dual-series CV electrodeposition method to
synthesize a MnOx WOC in situ on conductive surfaces. The resulting film provides excellent
catalytic performance over long periods of time.

Chapter 4 presents our work on a preliminary system integration of TiO2 and a Mn WOC to
build a functional water oxidation photoanode for solar-driven water oxidation. Assessment of
the photoanode showed that it is very effective towards water oxidation under UV illumination.

In Chapter 5, we describe our work on developing a Ni/Ni(OH)2 electrocatalyst for the hydrogen
evolution reaction. The new electrodeposition method involves only two cycles of CV in a single
potential range, and produces a film with excellent catalytic properties.

7

Chapter 6 presents some suggestions for future research. We propose a device capable of
minimizing mass transfer resistance, and also suggest future research activities on enhancing the
performance of MnOx and Ni/Ni(OH)2 electrocatalysts.

Chapter 7 presents the conclusions of the dissertation.

8

CHAPTER 2. CHARACTERIZATION AND FUNCTIONAL ASSESSMENT OF A
COBALT(III)-OXO CUBANE CLUSTER WATER OXIDATION CATALYST
IMMOBILIZED ON A CONDUCTIVE SURFACE

2.1 Abstract
We have developed an electrodeposition method that successfully immobilizes a cobalt(III)OXO (Co-OXO) water oxidation catalyst (WOC) synthesized ex situ onto conductive surfaces.
This Co-OXO WOC has a cubane-like catalytic core surrounded by stabilizing ligands, which is
a structural mimic of the natural oxygen evolving complex (OEC). This method promotes
recyclability and reusability of the Co-OXO catalyst. We took Fourier transform-infrared (FTIR)
and surface FTIR spectra before and after deposition, respectively, to assess the presence of
organic ligands around the core. We used scanning electron microscopy (SEM) to characterize
the surface morphology of the immobilized catalyst, and energy dispersive x-ray spectroscopy
(EDS) to determine the elemental composition. Finally, we assessed its functionality, stability,
and reusability using cyclic voltammetry (CV). The results show that this catalyst can be
successfully immobilized on conductive surfaces, achieving a functionality comparable to that of
the same catalyst dissolved in aqueous solutions. We believe the separation of the synthesis and
immobilization processes has great potential for optimizing catalytic perfomance.

2.2 Introduction
As discussed in Chapter 1, a complete water splitting process consists of two half reactions22,23:
water oxidation and hydrogen evolution. In this study, we focused mainly on the water oxidation
half reaction. The water oxidation reaction is important in the natural process of photosynthesis,
and the oxygen evolving complex (OEC) in photosynthetic organisms is the only known natural
system capable of catalyzing that reaction. It has been shown that the natural OEC has a structure

9

consisting of an “oxo-bridged heteronuclear cubane cluster of manganese and calcium”24. Much
progress has been made in the past two decades towards synthesizing efficient WOCs that are
structural mimics of the natural OEC. Since 2000, various catalysts with similar cubane
structures have been synthesized and assessed. Successful examples include those developed by
Kanady et al.25, Mukherjee et al.26, and Brimblecombe et al27.

Dismukes and co-workers are generally recognized as the first group to successfully synthesize a
WOC that structurally simulates the natural OEC28. Their catalyst had a manganese cubane
(Mn4O4) core surrounded by stabilizing ligands. A significant problem with this catalyst is that it
is insoluble in water and common organic solvents29. As a result, they used porous membranes
such as Nafion to immobilize it for assessment and utilization. Other manganese-based
compounds such as Mn3Ca and Mn3CaO4 have also been developed recently for the same
purpose. In 2007, Das and co-workers introduced a cobalt(III)-based WOC [Co4(3-O)4(O2CMe)4(py)4], which is a complex with a [Co4O4] cubane-like catalytic core surrounded by
pyridine and acetate anion stabilizing ligands30. This complex is soluble in water. Thus, its
electrochemical and catalytic properties can be easily assessed by a standard electrochemical
system, with dissolved catalyst as electrolyte. In this chapter, we refer to this as the “pseudohomogeneous” system because, even though the catalyst is dissolved in water, the reaction takes
place at the electrode-electrolyte interface. While the pseudo-homogeneous system works well
on a laboratory scale and enables highly effective catalyst performance, it does not enable
catalyst recovery and reuse on a larger scale. As a result, strategies have to be developed to
immobilize the catalysts onto substrates to promote recyclability and reusability. The successful
immobilization of these WOCs has scientific, technological and environmental merit.

10

In this study, we focused mainly on designs that promote catalyst recyclability and reusability.
We have achieved this by developing a new electrodeposition protocol which cam immobilize
the catalyst onto conductive surfaces. This method can transform the pseudo-homogeneous
reaction system to a heterogeneous system. We chose a Co-OXO WOC as the candidate catalyst,
with the goal of determining whether the OEC-mimicking catalytic core (with or without
stabilizing ligands) can maintain catalytic function after being anchored onto conductive
surfaces. We took Fourier transform-infrared (FTIR) and surface FTIR spectra before and after
deposition, respectively, to investigate the presence of organic ligands around the core. We used
scanning electron microscopy (SEM) to characterize the surface morphology of the immobilized
catalyst, and energy dispersive x-ray spectroscopy (EDS) to determine the elemental
composition. Finally, we assessed catalyst functionality, stability and reusability using cyclic
voltammetry (CV). The results show that this catalyst can be successfully immobilized on
conductive surfaces, with a functionality comparable to the same catalyst dissolved in aqueous
solutions. Although some WOCs can be synthesized in situ, we still believe that the separation of
catalyst synthesis and immobilization processes can enable better control of the catalyst and
provide an objective route to optimization of the resulting complex.

11

2.3 Materials and methods.
2.3.1 Materials
Cobalt nitrate hexahydrate (CoCl2•6H2O), sodium acetate trihydrate (CH3COONa•3H2O),
pyridine (C5H5N) and hydrogen peroxide (H2O2) were all purchased from the Sigma-Aldrich
Company, LLC (St. Louis, MO). All chemicals were used without further purification.
Polyethylene terephthalate (PET) films coated with indium tin oxide (ITO) were also purchased
from Sigma-Aldrich. Prior to using, ITO substrates were cut into 2cm by 1cm rectangular chips.

2.3.2 Methods
2.3.2.1 Ex situ Synthesis of the Co-OXO cubane cluster
We used the protocol reported by Chakrabarty et al. (2007) to synthesize the [Co4(3-O)4(O2CMe)4(py)4] catalyst. In the ex situ synthesis, 20 mmol (4.76 g) of CoCl2 was dissolved in
70ml methanol. The solution was kept at 70℃ in a water bath with constant reflux. Forty (40)
mmol (5.44 g) sodium acetate was added to the stirred solution, followed by addition of 16ml
(10mmol) of pyridine (py). Ten (10) ml H2O2 (30% v/v) was then added very slowly. A very
vigorous oxidation process could be observed, with the color of the mixture changing from
purple to black. The mixture was maintained at 80℃ for 4 hours. After three days of aging, a
dark green powder was observed.

12

2.3.2.2 Electrodeposition of Co-OXO catalyst synthesized ex situ
The electrochemical deposition was carried out on a CHI 660D electrochemical workstation (CH
Instruments Inc., Austin, Texas), using the traditional three-electrode setup. ITO chips were used
as both the working and counter electrodes, with Ag/AgCl as the reference electrode. We
dissolved 0.02 g of dry [Co4(3-O)4(-O2CMe)4(py)4] powder in an electrolyte containing 0.1 M
neutral phosphate buffer solution (NaH2PO4/Na2HPO4 buffer solution at pH 7.0). A potential of
+1.2 V was applied to the system for seven (7) hours. For comparison, an identical experiment
was run for three (3) hours, except without the catalyst in the solution.

2.3.2.3 Catalytic functionality and stability assessments
The electrodeposition process was tracked by CV, using a typical three-electrode setup on the
CHI 660D electrochemical workstation. We did a baseline test using a blank ITO chip as the
working electrode. We used two types of electrodes in the catalyst functionality tests: a CoOXO-coated ITO electrode and a Co-OXO-coated gold electrode. A platinum wire was used as
the counter electrode in all tests, with a Ag/AgCl electrode as reference, and the neutral 0.1M
phosphate buffer solution (pH 7.0) as the electrolyte. The scan rate was set at 10 mV/s.

The functionality tests were also done using CV in an identical setup as described above. We
tested the Co-OXO WOC coated on ITO and on gold substrates. Finally, we tested the bare ITO
and pseudo-homogeneous systems for comparison. Following these assessments, we tested the
stability and functionality of the catalyst. We obtained 14 CV scans on one sample of Co-OXO
immobilized on ITO. In this test, the Co-OXO-coated ITO was used as the working electrode,
and was left in the electrolyte solution for 14 consecutive CV scans. To test the recyclability of

13

the catalyst, we also took eight CV scans on one Co-OXO on ITO sample. Unlike the previous
tests, the working electrode was removed from solution after each CV scan, rinsed in DI water,
dried in air, and reinserted for the next scan.
2.3.2.4 Characterization of catalysts by FTIR and SEM
We used FTIR to evaluate the structure and morphology of the catalyst in powder form, and
following immobilization on ITO. The data for the immobilized catalyst were acquired on a
Nicolet 6700 surface FTIR (Thermo Scientific, Inc., West Palm Beach, FL). The data for the
catalyst in powder form were acquired on a Galaxy Series 3000 spectrophotometer (Mattson
Technology, Inc., Fremont, CA). SEM images were obtained at different magnifications using a
JSM 6610-LV electron microscope (JEOL Ltd, Tokyo, Japan). Finally, energy dispersive x-ray
spectroscopy (EDS) (Oxford Instruments, High Wycomb, Bucks, England) was used to evaluate
the elemental composition of the immobilized catalysts.

2.4 Results and Discussion
2.4.1 Electrodeposition of Co-OXO
A dark film could be clearly observed after the electrodeposition process. We believe this
provides direct evidence of the successful coating of the catalytic film on the conductive surface.
A control experiment was also performed under identical conditions without dissolved Co-OXO
in the electrolyte. The control sample did not produce any color changes on the surface. We
recorded the current density profile during the seven-hour electrodeposition process. The red
curve in Figure 2.1 shows that the current density of the catalyst sample increased steadily before
reaching a steady state value. The blue curve in Figure 2.1 shows the current density profile of

14

the control sample, which decreased gradually, then reached and maintained a steady state at a
current density level magnitudes lower than the electrodeposition sample. We should notice that
there is an initial decrease of current density during the first few minutes of deposition. Although
we do not have a clear explanation why this occurs, we speculate that it is due to mass transfer
resistances31.

Figure 2.1. Current density profile (red) of the Co-OXO WOC during electrodeposition on ITO.
The current density increased gradually to a maximum of 0.67 mA/cm2, which is considerably
higher than for the case where the electrode was placed in solution without the catalyst (blue
curve). Similar results were obtained on gold electrodes. The initial drop in current density is
likely due to mass transport effects.

15

2.4.2 FTIR and surface FTIR characterization of the Co-OXO catalyst

The FTIR spectrum of the catalyst powder sample synthesized ex situ has strong peaks at
frequencies of 1556, 1407 and 669 cm-1 (Figure 2.2), which were assigned to the pyridine ring
group, the acetate hydroxyl group, and the carbon backbone -CH- group, respectively. These
peaks confirmed the existence of the ligands used to stabilize the catalytic complex during the ex
situ synthesis process. However, those feature peaks disappeared on the surface FTIR spectrum
of the immobilized catalyst samples, which indicates the loss of all organic ligands during
electrodeposition, leaving only the catalytic cubane core on the surface.

1407
1556

669

Figure 2.2. FTIR spectra of the catalyst powder prior to immobilization on conductive surfaces.
The stretching frequencies at 1556, 1407, and 669 cm-1 represent the pyridine ring group, the
acetate hydroxyl group, and the carbon backbone -CH- groups, respectively. These frequencies
confirm the existence of the stabilizing ligands used in synthesizing the catalyst complex. These
feature peaks are not present in the surface FTIR, indicating the loss of all organic ligands during
electrodeposition process.

16

2.4.3 Surface morphology and structure characterizations of immobilized catalyst
As reported earlier, the FTIR results showed that the stabilizing ligands were lost during the
deposition process. To determine if the Co-OXO WOC was successfully immobilized, we
acquired SEM images at different magnifications, and EDS spectra of the Co-OXO-coated ITO
samples. Figure 2.3 shows that the surface morphology of Co-OXO coated on ITO is uniform,
although there is some degree of agglomeration. By contrast, the SEM image of bare ITO
(Figure 2.3) at the same magnification shows a featureless surface. Figure 2.3c shows the EDS
spectrum of the Co-OXO-coated ITO sample, which confirmed the existence of Co by the strong
Co peaks. When the data provided by the SEM images and EDS spectra are taken together, we
can confirm that the Co-OXO catalytic core was successfully immobilized on the ITO surface.

17

a

b

c

Figure 2.3. a) SEM image of the ITO surface following catalyst immobilization. It gives clear
evidence of attachment of the catalyst to the surface. By contrast, the SEM image of ITO prior to
catalyst deposition (Figure 2.3a inset) at an equivalent magnification has a featureless surface. b)
SEM image of a selected area at a higher magnification. c) EDS diagram of Co-OXO-coated ITO
sample, confirming that the catalyst core remained on the ITO substrate during the
electrodeposition process.

18

2.4.4 Catalytic functionality, stability and recyclability assessments
Although we have confirmed that the catalytic core of Co-OXO WOC can be successfully
immobilized via the electrodeposition method we developed, it was still necessary to determine
whether the catalytic function of the catalyst has been affected by the loss of the stabilizing
ligands. We did this by assessing and comparing the catalytic performance of both the
heterogeneous and pseudo-homogeneous systems under identical conditions using cyclic
voltammetry. As shown in Figure 2.4, we took CV scans of four systems: the catalyst coated on
the ITO electrode (green), catalyst coated on gold (red), Co-OXO in a pseudo-homogeneous
system (purple), and a bare ITO electrode (blue) to serve as baseline and control. The Co-OXOcoated ITO, Co-OXO-coated gold and pseudo-homogeneous systems have about the same onset
water oxidation potentials, approximately 1.0 V vs. Ag/AgCl. We did a more careful
examination of the data (Figure 2.4 inset), e to get more accurate estimates of onset potentials
0.93, 0.95 and 1.0 V, respectively, for pseudo-homogeneous, catalyst-coated ITO, and catalyst
coated gold. As expected, bare ITO has a flat profile with no significant water oxidation
functionality. Based on the analysis above, we can conclude that although Co-OXO WOC lost its
stabilizing ligands during the electrodeposition process, its catalytic function remained intact.

19

Figure 2.4. Cyclic voltammograms acquired at a scan rate of 10 mV/s on bare ITO (blue curve),
Co-OXO immobilized on gold (red), and Co-OXO immobilized on ITO (green) in 0.1M neutral
phosphate buffer solution at pH 7.0. This diagram shows that water oxidation occurs at
approximately 1 V for Co-OXO on both gold and ITO. For comparison, we have also included the
CV of the catalyst in a pseudo-homogeneous system (purple). The profiles of the three scans are
essentially identical, providing strong evidence that Co-OXO maintains its catalytic function
whether in solution, or immobilized on gold or ITO.

20

To test for catalytic stability, we acquired 14 consecutive CV scans on one catalyst sample
(Figure 2.5). The peak current densities of all 14 CV scans fall in a narrow range (1.54Âą0.04
mA/cm2). The profiles are also nearly identical for all scans. The total duration of the 14
consecutive scans was 74 min. These facts provide strong evidence that the immobilized catalyst
is stable and has reproducible performance over a relatively long period of time.

To test the reusability and recyclability of the catalyst, we obtained 8 CV scans using a single
immobilized catalyst sample. As opposed to the stability experiment we described above, this
sample was pulled out of the testing solution after each scan, cleaned with DI water and dried in
air before being returned to the solution for the next test. The results are shown in Figure 2.6.
The profiles lie in a narrow range (1.47 Âą 0.05 mA/cm2), providing strong evidence that the
immobilized catalyst has stable and reproducible performance upon recycle and reuse.

21

2.0

Current density / (mA/cm2)

1.5

1.0

0.5

0.0

-0.5

1

2

3

4

5

6

7

8

9

10

11 12 13 14

Run number
-1.0

Figure 2.5. Voltammograms acquired at a scan rate of 10 mV/s over 14 cycles of catalyst reuse.
To acquire these data, the same catalyst was left in place for 14 consecutive scans. The data show
negligible variation in both current density and peak currents, providing strong evidence that the
immobilized catalyst is very stable, with a reproducible performance.

22

1.6

Current density / (mA/cm2)

1.2

0.8

0.4

0.0

-0.4

1

2

3

4

5

6

7

8

Recycle number

Figure 2.6. Assessment of catalyst recyclability and reusability. In this test, we acquired eight CV
scans on a single sample of Co-OXO catalyst coated on ITO. After each scan, the sample was
taken out of the solution, rinsed with DI water, dried in air and reinserted into the solution for the
next scan. As the data show, all the voltammograms are distributed over a small range of values,
confirming a high degree of catalyst reusability, as well as reproducibility in performance.

23

2.5 Conclusions
In this study, we developed an electrodeposition approach that can successfully immobilize an
OEC mimic Co-OXO WOC onto conductive surfaces to promote catalyst recovery and
reusability. The results show that, although all the stabilizing ligands of the catalytic complex
were lost during the electrodeposition process, the functionality of the catalyst remained intact.
This study also shows that the immobilized catalyst has the same performance as one dissolved
in solution. We believe that the separation of catalyst synthesis and immobilization into separate
processes has the potential of enabling systematic design and optimization of catalysts. The CoOXO used in this study is only one example of a category of cubane-like WOCs which can be
synthesized ex situ and immobilized on different conductive surfaces via electrodeposition. This
study has provided a reliable pathway for effective recycling of this category of catalysts.

24

CHAPTER 3. SYNTHESIS OF A MnOX CATALYTIC FILM ON FTO BY DUALSERIES CYCLIC VOLTAMMETRY

3.1 Abstract
We have developed a new method to synthesize a manganese-based water oxidation catalyst in
situ in aqueous media. This new method involved multiple cyclic voltammetry (CV) series,
followed by programed calcination to increase crystallinity. The method provides powerful
tunability of the electrodeposition parameters, potentially enabling independent control of the
surface morphology and the oxidation state of the catalytic film. Our results show that the first
CV series produces a film with a nanoscale surface morphology, while the following CV series
improves catalytic performance by elevating the oxidation state of the film while largely
maintaining the existing surface morphology. Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) were used to characterize the morphology of the MnOx
film, cyclic voltammetry (CV) to assess catalyst functionality, and long term amperometry to
assess the stability.

3.2 Introduction
As we described in the previous chapters, the water oxidation reaction is the rate-limiting step in
the artificial photosynthesis (AP) process, and the major source of overpotential1,32,33. Thus, an
effective AP process requires highly effective water oxidation catalysts (WOCs)34. In the past
decade, various catalytic complexes capable of water oxidation have been synthesized. Most of
these complexes were based on Ru35-37, Pd38 or Ir39-41, all of which are earth-rare. These heavy
transition metal-based complexes are expensive and usually toxic, which greatly limits their use
on a large scale. As a result, using light first row transition metals to replace them has become a

25

research focus. The potential choices of transition metals are cobalt (Co) and manganese (Mn).
Many Co-based WOCs have been successfully developed and tested in the past decade, the most
successful example being the ‘Co-Pi’ catalyst developed by Nocera and coworkers42. Other
examples include the Co-OXO catalytic complexes we described in Chapter 243. In the last five
years, manganese has gained popularity as a new base metal for WOCs because it can form more
than 30 minerals, with most of them having water oxidation properties44-46. Also, manganesebased materials are usually very flexible and can be synthesized in situ26,28,47,48 or ex situ49-51.
More importantly, manganese is the key element in the reaction center of the natural oxygen
evolution center (OEC) in plants.

Various methods have been reported for synthesizing manganese-based WOCs, both in situ and
ex situ. In situ synthesis, especially by electrodeposition, usually benefits from a simple and
easily-controllable setup. Also, the resulting catalysts usually enable much better recyclability.
Typically, electrodeposition is carried out by using chronoamperometry52,53 or a single-range of
potentials48,54.

In this study, we have developed a new method to synthesize a manganese-based water oxidation
catalyst in situ in aqueous media. This new method involves two series of cyclic voltammetry
(CV), followed by calcination to increase crystallinity. This method provides great tunability of
the electrodeposition parameters, enabling independent control of the surface morphology and
the oxidation state of the catalytic film. The goal of the study was to achieve an enhanced
oxidation state and a nanoscale surface morphology55.

26

3.3 Materials and methods
3.3.1 Materials
Manganese acetate tetrahydrate (Mn(CH3COO)2 4H2O) was purchased from Sigma-Aldrich LLC
(St. Louis, MO). Sodium phosphate monobasic (NaH2PO4) and sodium phosphate dibasic
(Na2HPO4) were purchased from J.T. Baker (Phillipsburg, NJ). All chemicals were used without
further modification. Fluorine-doped tin oxide (FTO)-coated glass (surface resistance ~7 Ί/cm2)
was purchased from the Sigma-Aldrich Company LLC (St. Louis, MO). Prior to use, the FTO
glass was cut into 2cm × 1cm rectangular chips, cleaned with acetone, and rinsed in DI water.

3.3.2 Electrodeposition of MnOx on FTO
The electrodeposition process was performed using a typical three-electrode experimental setup
on a CHI 660D electrochemical workstation (CH instruments Inc., Austin, Texas). During the
deposition process, a blank FTO chip was used as the working electrode, indium tin oxide (ITO)
as a counter electrode, and a Ag/AgCl as a reference electrode. The electrolyte was 25 mL of a
50 mM manganese acetate solution. The deposition process was done by conducting two
consecutive CV series over different potential ranges. The first series was ten cycles of potential
ramps ranging from 0 to 0.6 V, at a scan rate of 10 mV/s. The second CV series was five cycles
ranging from 0.6 to 2.0 V at an identical scan rate. During the first CV series, a light yellowish
film could be observed forming on the FTO surface. The film turned dark brown during the
second CV series. After deposition, the chip was rinsed in DI water and dried in air. It was then
calcined at 450℃ for five hours following a temperature ramp at a rate of 10℃/min in a

27

Lindberg/Blue M tube furnace (Thermo Fisher Science Inc., Waltham, MA), and then cooled to
25℃.

3.3.3 Surface morphology characterization
The surface morphology of the samples was characterized on a JSM 6610 LV SEM (JEOL Ltd,
Tokyo, Japan) as well as a JSM 2500FM SEM from the same manufacturer. Tunneling electron
microscopy (TEM) images were taken on a JEM 2200FS TEM (JEOL Ltd, Tokyo, Japan). X-ray
diffraction (XRD) spectra were acquired on a Bruker Davinci D8 Advanced X-ray diffractometer
(Bruker Corporation, Madison, WI).

3.3.4 Catalyst functionality assessments and comparisons
The functionalities of the synthesized catalytic films were assessed by cyclic voltammetry on the
CHI 660D electrochemical workstation. In each test, the catalytic film was used as a working
electrode, a platinum mesh as counter electrode and a Ag/AgCl electrode as reference. The
electrolyte used for all the tests was 25 ml of a 0.1M neutral phosphate buffer solution (pH 7.0)
to simulate natural water. Potential ranges for all the CV tests were between -0.2V to 1.4V, at a
scan rate of 10 mV/s. In a control experiment, a bare FTO chip was also tested under identical
conditions.

28

3.3.5 Catalyst stability assessment
The stability of the catalytic film was assessed by amperometry at a constant potential of 1.2 V
for a period of six hours under identical conditions as described in the functionality test. A
control experiment was carried out on a bare FTO chip.

Another stability test was carried out by performing 18 consecutive CV scans on a single
catalytic film. The testing and CV conditions were identical to those used for the functionality
tests described above.

3.4 Results and discussions
3.4.1 Surface morphology characterizations
Figure 3.1 shows the SEM images of the surface morphology of: a) bare FTO, b) MnOx film
synthesized by dual series CV (S0), c) MnOx film synthesized by single series CV 0-0.6V (S1),
and d) MnOx film synthesized by single series CV 0-2.0V (S2), respectively. The bare FTO has a
featureless but rough surface morphology. We believe that the surface roughness plays an
important role in the formation of the MnOx films, as discussed in the latter part of this chapter.
Figure 3.1 b and the inset show that the morphology of the MnOx film synthesized by dual series
CV has a network of long intermingled nanofibers. The diameter of the fibers was estimated to
be 10~20 nm. Figure 3.1 c shows the morphology of samples S1 and S2, respectively. S1 has a
nanofibrous morphology, which is quite similar to sample S0. This similarity provides important
evidence that the morphology of the catalytic film synthesized by the dual-series method was
primarily developed during the first CV series, but remained largely intact during the second CV

29

series. On the contrary, the S2 catalytic film developed into a morphology of micron-scale
clusters, which is not optimal for mass transfer. Our analysis provides preliminary but important
evidence that the surface morphology could be controlled by tuning the electrodeposition
parameters. Since the dual- or multi-series deposition protocol provides more controllable
parameters than the traditional single-series protocol, it could become an invaluable tool to
enable predictable tuning of surface morphologies that promote catalytic performance.

30

Figure 3.1. SEM images of the surface morphologies of the FTO substrate, and of MnOx after
different in-situ synthesis methods. Image a: Bare FTO, showing a significant degree of surface
roughness. Image b: MnOx after two series of electrodeposition by CV, followed by calcination;
the surface is a network of interconnected fibers in both the SEM and TEM (inset) images.
Images c and d: MnOx after electrodeposition by single CV series (0-0.6 V and 0-2.0 V,
respectively); image c has a similar morphology to image b, although a few small spherical
clusters are visible; image d shows a network of larger micron-sized fibers with random
orientations.

31

3.4.2 Catalytic functionality assessments
As described above, the catalytic functionalities of the three samples were assessed by CV in a
0.1 M neutral phosphate buffer solution (pH 7.0). The S0 sample has an onset potential for water
oxidation of approximately 1.2 V vs. SHE (Figure 3.2). We should note that the current density
of S0 exceeds 1 mA/cm2 at 1.45 V vs. SHE, which is the typical standard required to rank a
WOC as highly active 54. For comparison, both the S1 and S2 samples were assessed under
identical conditions. The S1 sample, which was synthesized by the single-series (0-0.6 V)
method, produced the desired nanoscale surface morphology similar to the dual-series sample.
However, the catalytic performance was much lower. We believe this was due to inadequate
elevation of oxidation states, as discussed in the next section. The S2 sample has a better
catalytic performance than S1, which we believe is due to the more enhanced oxidation state.
However, the surface morphology of S2 consists of much larger micrometer-scale fibers that
likely retarded electron transfer, leading to a poorer catalytic performance.

To further assess if the dual-series method produces a catalyst that is superior to the catalytic
films synthesized by single-series protocols, we evaluated and compared three additional
samples synthesized by single-series over different potential ranges: 0-0.4 V, 0-0.8 V, and 0-1.5
V (Figure 3.3). All samples were assessed under the same conditions described above. As shown
in Figure 3.3, all the single-series samples have similar catalytic performances except the 0-0.4 V
sample, which is essentially non-catalytic. We believe the inadequate development of oxidation
state led to the poor performance of the 0-0.4 V sample. The catalytic performance of the dualseries sample is clearly superior in comparison to all the single-series samples. As expected, the
FTO chip had no measureable water oxidation effects in the potential range tested.

32

Figure 3.2. Catalytic functionality of the three MnOx films on FTO. The blue curve is the cyclic
voltammogram of the MnOx film synthesized by the dual-series CV method; the purple and red
profiles are the single-series synthesis from 0-0.6 V and 0-2.0 V, respectively; and the green
curve is for bare FTO under identical assessment conditions. The MnOx catalyst synthesized by
dual-series CV is clearly superior to the single series species, because it benefits from the
nanostructure developed in the first series, and the elevated oxidation state acquired during the
second series. The importance of nanostructure is further reflected by the fact that the 0-0.6 V
catalyst performs slightly better than the 0-2.0 V sample, even though the latter is at a higher
oxidation state.

33

Figure 3.3. Comparison of the catalytic performances of dual series and single session samples
by cyclic voltammetry in 0.1M phosphate buffer (pH 7.0). All the single series samples were
synthesized under identical conditions as described in the experimental session, with the only
difference being the ranges of potential ramps. Bare FTO was used again as a control. All singleseries samples had similar performances, except the 0-0.4V sample which has a much lower
activity. The dual-series sample has a much better performance than all single-series samples,
which we believe to be the result of both an elevated oxidation state and nanoscale surface
morphology.

34

3.4.3 Proposed formation mechanism
As we discussed before, the catalytic film synthesized by the dual-series method benefits from
both nanoscale surface morphology and enhanced oxidation state. The data suggest that the
surface morphology of the catalytic film was mainly formed during the first CV series, and the
oxidation state of the film was elevated during the second CV series.

Currently, we do not know the exact mechanism for the formation of this unique surface
morphology, and can only propose a possible hypothesis. We speculate that the protrusions on
the rough FTO surface serve as nucleation sites for the nanofibers. During the first CV series, the
Mn2+ ions would first be oxidized to Mn(III) hydrolysis species which form small nuclei on the
protrusions 56. The small nuclei then grow and extend across the whole surface. During the
second CV series, the existing hydrolysis species would be partly oxidized to higher Mn(IV)
species while the surface morphology remains largely intact. A schematic of the proposed
mechanism is shown in Figure 3.4.

To evaluate this possible mechanism, we took photographs and SEM images after selected stages
of deposition. We observed a yellowish film forming on the FTO surface during the first CV
series (Figure 3.5). The film then turned to dark brown during the second CV series. Figure 3.6
shows a series of SEM images at different magnifications of the samples shown in the previous
figure. Figure 3.6 a shows the sample after two cycles of the first series. Small clusters are
clearly observable on the protrusions on the FTO surface. The small clusters gradually developed
into a complete film across the conductive surface (Figure 3.6 b-d). During the second CV series,
the surface morphology of the catalytic film did not change much (Figure 3.6 e).

35

Figure 3.4. Cartoon illustration of the proposed formation mechanism. We propose that Mn2+
ions are first oxidized on the protrusions of the FTO surface during the first CV series, forming
fibrous MnO(OH) species. During the second CV series, the oxidation state of the catalytic film
is further elevated while the surface morphology remains largely intact.

2

4

6

8

10

First CV series

2

4

5

Second CV session

Figure 3.5. Time-lapse photographs of MnOx samples at selected stages of deposition. During
the first CV series, a light yellowish film could be observed on the FTO surface. The film turned
to dark brown during the second CV series.

36

Figure 3.6 a. SEM images at various magnifications of the sample after two (2) cycles in the
first CV series. Small clusters can be observed on the protrusions on the FTO surface.

Figure 3.6 b. SEM images at various magnifications of the sample after four (4) cycles of the
first CV series. The small clusters in Figure 3.6a have developed into larger clusters.

37

Figure 3.6 c. SEM images at various magnifications of the sample after six (6) cycles of the first
CV series. The clusters in Figure 3.6 b continue to grow as the catalytic film is forming.

Figure 3.6 d. SEM images at various magnifications of the sample after eight (8) cycles in the
first CV series. The catalytic film in Figure 3.6 c has now covered the whole surface.

38

Figure 3.6 e. SEM images at various magnifications of the sample after four (4) cycles in the
second CV series. The surface morphology of the catalytic film did not change much during the
second CV series.

39

We also did functionality tests after selected stages of deposition. As shown in Figure 3.7, there
is a gradual increase in peak current density during the first CV series. However, the current
density increased dramatically during the second CV series, which we believe is due to the
increase in the oxidation state of the MnOx film. It is particularly worth noting the slope of the
functionality curve during the second session. There is clearly the possibility of a much further
enhanced oxidation state with additional scans over this potential range, or with the use of a third
series of electrodeposition over a different potential range.

Figure 3.7. Functionality assessment after selected stages of electrodeposition. There is a
gradual increase in peak current density during the first CV series. However, the peak current
density increases dramatically during the second CV series, which we speculate to be the result
of continued enhancement of the oxidation state of the catalyst. It is particularly worth noting
that the slope of the functionality curve during the second session is strongly positive, and
nowhere near an asymptote. Thus, there is clearly a possibility of much further enhanced
oxidation state with additional scans over this potential range (0.6 to 2.0 V) or a third series of
CV electrodeposition over a different potential range, with the potential that this method can be
used to optimize catalyst function.

40

3.4.4 XRD characterizations
Our XRD results provide important evidence for this proposition. The black, green and red traces
in Figure 3.8 show the XRD patterns for samples S0, S1 and S2, respectively. The major
components for the dual-series sample were Mn2O3 and MnO2, which is similar to sample S2.
Sample S1, on the contrary, only has feature peaks for Mn2O3. We speculate that the major
transformation in the first CV series (0-0.6V) was the conversion of Mn(II) to Mn(III), with the
conversion of Mn(III) to Mn(IV) taking place during the second CV series (0.6-2.0V). Since
there was only a very small number of cycles (5 cycles) during the second series, some Mn(III)
species remained intact. We should also note that the XRD profile of sample S2 has the same
oxidation state as the dual-series catalyst, so its poorer performance could be attributed entirely
to transport resistances due to the larger microscale morphology.

41

Figure 3.8. X-ray diffraction spectra of the three catalysts: a) MnOx after two consecutive series
of CV electrodeposition (scans over 0-0.6 V and 0.6-2.0 V) followed by calcination; it has the
peaks of both Mn2O3 and MnO2, which indicates that the MnO(OH) formed in the first CV series
has been further oxidized. b) MnOx after one series of CV (0-2.0 V) and calcination, showing a
similar pattern to trace a); this is an indication that the single 0-2.0 V series produces a MnOx
catalyst with a similar oxidation state as the one synthesized by the dual-series. c) MnOx after
one CV series (0-0.6 V) and calcination; the scan shows that the major phase of MnOx is Mn2O3,
meaning that the MnOx before calcination is MnO(OH) as proposed. The symbols +, *, and # in
this figure designate the feature peaks of the FTO substrate, Mn2O3 and MnO2, respectively.

42

3.4.5 Stability tests
Long and stable catalytic performance is also a very important aspect of functional WOCs 57.
The catalytic film synthesized by dual-series methods has very stable catalytic performance
during consecutive CV tests and chronoamperometry tests over prolonged periods of time. All
the tests were performed using the identical setup and conditions described in the functionality
assessments session. Figure 3.9 shows the current densities when 1.2 V (vs. SHE) of potential
was applied to the system. After about 1.5 h of decrease in signal (which is typical58), the current
density was steady for the next 4.5 hours of the six-hour assessment period. As expected, FTO
has a flat profile throughout the assessment period, confirming that it is not catalytic.

We performed the same test on samples S1 and S2. The results of these tests show that they are
equally stable but at lower current densities than the dual-series sample (Figure 3.10). We also
did stability tests with 18 consecutive CV cycles on one S0 sample. Figure 3.11 shows that all
the resulting curves were of similar shape, with the peak current densities falling within a narrow
range (1.96Âą0.11 mA/cm2).

43

Figure 3.9. Current density profile of the MnOx catalyst synthesized by the dual-series method
during long term catalytic assessment at 1.2 V (vs. SHE) in a 0.1 M phosphate buffer solution
(pH 7.0). The steady profile over a long period of time gives an indication of a stable MnOx film.

44

Figure 3.10. Current density profiles showing the stability of the catalytic films: bare FTO
(purple curve), single session 0-0.6V sample (black), single session 0-2.0V sample (red), and
dual session sample (blue). All tests were carried out in a 0.1M phosphate buffer solution (pH
7.0) at 1.2V (vs. SHE) for six hours. All single session and dual session samples had good
stability during the six hours of assessment. However, the dual session sample had a higher
stable current density compared to the single session samples, providing further evidence that the
dual series method is better than the traditional single session methods.

45

Figure 3.11. Consecutive CV scans on one catalyst sample to assess stability. All 18 scans have
similar profiles, with the peak current density falling within a narrow range of values (1.96Âą0.11
mA/cm2)

46

3.5 Conclusions
We have developed and assessed a new electrodeposition method for synthesizing stable and
highly efficient manganese-based WOCs in situ on conductive surfaces. This new method
involves multiple consecutive series of CV scans over different potential ranges, followed by
calcination. The resulting MnOx films have well-dispersed porous surface morphologies
consisting of nanoscale fibers that promote catalytic performance. Our characterization results
support a hypothesis that the first CV series produces a fibrous nanoscale film, while the second
series elevates the oxidation state of the catalyst without significantly changing the surface
morphology acquired during the first series. The potential to enable independent control of
surface morphology and catalyst oxidation state(s) makes this a powerful tunable method for
rational catalyst synthesis and optimization.

47

CHAPTER 4. FUNCTIONAL BIOMIMETIC COMPOSITE Mn/TiO2 PHOTOANODE
FOR WATER SPLITTING

4.1 Abstract
We have designed, fabricated and assessed a composite photoanode for solar-driven water
splitting, using an easy approach and inexpensive materials. The photoanode consists of a solarharvesting layer and a catalytic layer. Scanning electron microscopy (SEM) and x-ray powder
diffraction (XRD) were used to characterize the surface morphology and structure, and cyclic
voltammetry (CV) was used to assess functionality with and without ultraviolet (UV)
illumination. The resulting photoanode provides stable and effective water oxidation under UV
illumination. Another potential benefit of this work is that, since TiO2 is used as the solarharvesting layer, various dyes can be integrated into the device to broaden its absorption
spectrum.

4.2 Introduction
In the previous chapters, we discussed work on developing cost-effective, highly active water
oxidation catalysts. In this chapter, we focus on designing, fabricating and assessing a composite
photoanode for solar-driven water splitting, using a simple approach and inexpensive materials.

As we described in Chapter 1, a functional artificial photosynthesis device requires an effective
solar-driven water oxidation photoanode. A functional photoanode should incorporate the
following features: i) a robust and conductive substrate upon which components like a solar
harvester and a WOC can both be mounted; ii) a solar harvester that can effectively capture,
intensify and convert sunlight from a target range of the solar spectrum; iii) an effective WOC

48

that is able to effectively catalyze the water oxidation reaction. A double-layer structure is the
best configuration for the photoanode, because it has the potential to enable independent design
and optimization of the individual components, which could enhance the overall performance of
the integrated system.

We had initially planned to integrate a dye-sensitized solar cell (DSSC) with a manganese-based
WOC to build the photoanode system. DSSC is a flexible solar converter system that has tunable
absorption spectra when different types of dyes are incorporated 59-61. A DSSC usually has a
TiO2 support as a substrate incorporated with dyes 62-65. TiO2 by itself has a large bandgap (~
3eV), which is sufficient for the water oxidation reaction 66-68. In fact, Fujishima and Honda used
TiO2 under intense ultraviolet (UV) light illumination in their pioneering work on using
photoelectrochemistry to split water 69. However, as shown by Equation 4.1, this bandgap limits
its absorption spectrum to no more than 413 nm, which falls in the ultra-violet region.

𝜆=

6.63×10−34 𝐽∙𝑠 ×3×108 𝑚/𝑠
3×1.60×10−19 𝐽

𝐸=

ℎ𝑐
𝜆

𝜆=

ℎ𝑐
𝐸

= 4.14×10−7 𝑚 = 414 𝑛𝑚

(Eq 4.1)

On the other hand, TiO2 has a low and negative conductive band, which is unlike many other
semiconducting materials, and can thus enable water splitting without an external electrical bias

49

70,71

. TiO2 also provides excellent mechanical, thermal and chemical stability under the harsh

conditions typically encountered in water oxidation. Finally, TiO2 can be easily engineered into
various morphologies and coated onto different surfaces via a variety of methods. All these
benefits make TiO2 an ideal substrate for the solar harvesting system. To overcome the limitation
of the limited absorption spectrum, various dyes can be integrated with TiO2 to tune and broaden
the spectrum and boost the efficiency of the solar harvesting system.

Our goal was to demonstrate the feasibility of building a photoanode using a manganese-based
WOC and TiO2 as a solar harvesting material. If the composite photoanode is capable of solardriven water splitting, then we can simply add various dyes to improve the absorption spectrum
and thus the performance.

We took advantage of the desirable properties of both TiO2 and manganese-based WOC to
develop the photoanode. We then performed electrochemical measurements to assess the
functionality of the photoanode, and used SEM and XRD to characterize the structure and
surface morphology.

4.3 Materials and methods
4.3.1 Materials
Manganese (II) acetate tetrahydrate (Mn(CH3COO)2 • 4H2O) and P25 titanium dioxide
nanoparticles (TiO2 with an anatase to rutile ratio of 4:1) were purchased from the SigmaAldrich Company, LLC (St. Louis, MO) and used without further modification. Fluorine-doped

50

tin oxide (FTO with a surface resistance of ~ 7 /cm2) was also purchased from Sigma-Aldrich.
Prior to use, the FTO glass was cut into 2cm by 1cm rectangular substrates, and cleaned in
acetone and DI water.

4.3.2 Methods
4.2.2.1 Spin coating of the TiO2 film
We added 0.15g of P25 TiO2 nanoparticles to 10 ml of methanol. The mixture was then
sonicated for 10 minutes to make a P25 colloid. The colloid was then spincoated onto FTO glass
for 20 minutes. The TiO2-coated FTO glass was then dried in air.

4.3.2.2 In situ synthesis of the Mn WOC catalyst
The Mn WOC was synthesized by the electrodeposition protocol reported by Zaharieva and
coworkers (2012) 54. The electrodeposition was performed using a typical three-electrode setup
on a CHI 660D electrochemical workstation (CH Instruments, Inc., Austin, TX). During the
deposition, TiO2-coated FTO was used as the working electrode, an indium tin oxide-coated
polyethylene (ITO) chip as the counter electrode, and a Ag/AgCl electrode as the reference
electrode. Twenty five milliliters (25 ml) of a 0.1 M Mn(CH3COO)2 aqueous solution was used
as electrolyte. For electrodeposition, 25 cycles of potential ramps ranging from -0.75 V to +2.15
V were applied at a scan rate of 0.1 V/s. A light brown film could be observed forming on the
electrode surface, with the film becoming darker during the deposition. After deposition, the
photoanode sample was rinsed with D.I. water and dried in air. For comparison, we also

51

synthesized a Mn WOC on a bare FTO electrode without the TiO2 coating, using the identical
approach described above.

4.3.2.3 Structural characterization by XRD and SEM
We took XRD spectra after each deposition step to track the color changes on the surface. The
XRD spectra were acquired on a Bruker Davinci D8 advanced x-ray diffractometer (Bruker
Corporation, Madison, WI). We also used a JSM 6610-LV scanning electron microscope (JEOL
Ltd, Peabody, MA) to observe the surface morphology, and EDS (Oxford Instruments, High
Wycomb, Bucks, England) to determine the surface elemental composition.

4.3.2.4 Functionality assessment by CV and EIS
We did CV and EIS characterizations using a typical three-electrode electrochemical setup. In all
the assessments, a platinum wire was used as a counter electrode, a Ag/AgCl electrode as
reference electrode and 25ml of 0.1 M neutral phosphate solution (pH 7.0) as electrolyte. We
applied a potential range of -0.2 V to 1.4 V at a scan rate of 10 mV/s.

We first tested the Mn WOC that was synthesized directly on clean FTO without TiO2 coatings,
to make sure that the electrodeposition process we described above can produce an active
catalyst. As comparison, we used a piece of clean FTO glass as control. We then tested the
composite photoanode samples in the dark and under UV illumination. The UV source was a 4
W UVGL-15 compact UV lamp (UVP LLC, Cambridge, UK). For the EIS tests, we collected the

52

impedance spectrum on a photoanode sample under UV illumination at 0.8 V, using a 10 mV
perturbation between 0.1 Hz and 10 kHz.

4.4 Results and discussion
4.4.1 Structural and morphological characterization of the composite photoanode
To track changes on the FTO surface during the photoanode fabrication process, we took XRD
spectra after each step. Shown in Figure 4.1 are the XRD patterns for a) pure P25 TiO2 powder,
b) pure FTO substrate, c) FTO substrate coated with TiO2, and d) the composite photoanode with
TiO2 coating and Mn WOC. Trace ‘a’ is a typical pattern of TiO2 72, and trace ‘b’ a typical
pattern of the FTO substrate. When we compare traces ‘a’, ‘b’, and ‘c,’ we can conclude that
TiO2 was successfully immobilized on FTO glass using the method described above. We
expected to see the feature peaks for manganese in trace ‘d’ to differentiate it from trace ‘c.’
However, there were no significant differences between the two profiles. There are two plausible
explanations for this result: i) it is difficult for XRD to detect the manganese-based catalyst on
the surface; and ii) the in situ synthesis did not deposit the manganese catalyst as had been
expected.

53

+

*
*
*
*+
*
*
**
+*

*+
*
**
**
**
*
+ *
*
+
*

*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*

*
*
*
*
*
*
*
*
*
*
*
*
*
**
*
*
*
*

+

+

+

*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*

d

c

b

a

Figure 4.1. XRD spectra of a) pure P25 TiO2 powder, b) pure FTO glass, c) FTO glass coated
with TiO2, and d) the composite photoanode. In this figure, + designates the feature peaks of TiO2,
and * denotes those of FTO. Traces “a” and “b” show the typical patterns for TiO2 and FTO,
respectively. Trace “c” shows all the peaks associated with both TiO2 and FTO, confirming that
the composite photoanode contains all the required elements. However, trace “d” is not
significantly different from trace “c,” which could mean that the method described above did not
produce the expected composite catalyst, or that XRD is not an appropriate technique for detection
of manganese in the complex.

54

To resolve this issue, we took EDS images of the photoanode to check the structure of the
surface. Figure 4.2 shows SEM images of: a) clean FTO surface, b) FTO coated with TiO2 at low
(x200) magnification, c) FTO coated with TiO2 at a higher (x800) magnification, d) the
composite photoanode at low (x160) magnification, and e) the composite photoanode at a higher
(x3300) magnification. Compared with bare FTO, which has a flat featureless surface, image b
shows the surface has been covered by a film of TiO2. However, instead of a uniformly smooth
coated layer, we observed many cracks. We speculate that these cracks were formed during the
evaporation of the solvent, likely similar to the mechanism for the formation of cracks on the
surface of the ground during a long drought. This morphology can be seen more clearly in image
c. Image d shows the surface morphology after electrodeposition of the WOC. The morphology
appears to be similar to the previous samples with only TiO2 coatings. However, at a higher
magnification (3e), we observed a different coating layer of distinct morphology beneath the
TiO2 substrate. We believe this layer beneath the TiO2 coating is the manganese WOC film.
Since this film was covered by the TiO2 coating layer, it could not be effectively detected by
XRD, which is primarily a surface characterization technique. To further investigate this, we
took EDS elemental mapping of a selected area where both layers could be clearly observed.

55

a

b

c
TiO2

Mn-WOC

d

e

Figure 4.2. SEM images of a) clean FTO substrate; b) FTO surface after coating with TiO2 (at
x200 magnification); c) FTO coated with TiO2 (at x800 magnification); d) composite photoanode
(at x160 magnification); e) composite photoanode (at x3,300 magnification). Images b and c show
the morphology of the TiO2 coating to be essentially a uniform layer, with a porous structure that
contains cracks. Images d and e show the morphology of the composite photoanode. Following
Mn WOC deposition, the morphology of the surface does not change substantially; however, a
new layer could be observed underneath the porous TiO2 film.

56

Figure 4.3 is the SEM image of the area we selected for EDS examination. The triangular area in
this image is the top TiO2 coating layer, while the rest of the area is the ‘crack’ in the TiO2 layer
that shows the film underneath. The elemental distribution of titanium and manganese are shown
in Figure 4.3 b and c, respectively. Figure 4.3 b shows a high concentration of titanium in the
triangular area, with very little manganese detectable. On the other hand, the white dots in Figure
4.3 c clearly portray the presence of manganese in the area that is not covered by TiO2. This is an
interesting observation because, since TiO2 was coated prior to the electrodeposition of the Mn
WOC, we should expect the Mn WOC to be deposited above the TiO2 layer instead of below.
We believe this anomaly occurred because the Mn(II) ions migrated through the porous TiO2
layer to reach the electrode surface during electrodeposition, thus forming the catalytic layer
underneath the layer of TiO2 coating.

a

b

c

Figure 4.3. EDS mapping of selected area. a) The SEM image provides evidence of both TiO2 and
manganese on the surface. The triangular area at the top is presumed to be the TiO2 layer, while
the rest of the area is covered by the manganese catalyst. b) The distribution of titanium within the
selected area. As postulated, the highest accumulation of titanium is in the triangular area. c)
Distribution of manganese in the selected area, with the heaviest concentration outside the
triangular area.

57

4.4.2 Electrochemical characterization of the Mn WOC
According to a previous report,54 the catalyst comes in active and inactive forms. To determine
which form is generated by the method described above, we acquired CV data on pure FTO, and
FTO samples coated with Mn WOC. The blue curve in Figure 4.4 represents the CV for the Mn
WOC, and the red curve the bare FTO substrate. These data lead to the conclusion that the Mn
WOC synthesized using this protocol is active, with an onset potential for water oxidation of
approximately 1.1 V vs. Ag/AgCl. We have tested the Mn WOC sample both with and without
UV illumination. Similar results were obtained under both conditions, confirming that the Mn
WOC is not light-sensitive.

Figure 4.4. Typical cyclic voltammograms during assessment of the manganese catalyst. The red
trace represents data for clean FTO glass. The blue trace represents data acquired on the manganese
catalyst deposited on FTO, and provides confirmation that the catalyst synthesized by
electrochemical deposition is active.

58

4.4.3 Electrochemical characterization of the composite photoanode.
As described above, TiO2 has a band gap of ~3 ev which corresponds to an absorption spectrum
in the UV range73. Therefore, we characterized the photoanode with and without UV
illumination. To help identify the inflection point in the curve, we took the first derivative of the
current density with respect to the applied potential (Figure 4.5). The point where the first
derivative goes to zero can be used as a reliable estimate of the onset potential for the water
oxidation reaction. Figure 4.5 a shows the profile of the photoanode sample under UV
illumination, while Figure 4.5 b shows the sample without UV illumination. Without UV
illumination, the onset potential was estimated at approximately 1.1 V vs. Ag/AgCl,
demonstrating that the functionality of the photoanode is similar to that of the pure Mn WOC.
However, under UV illumination, the onset potential is reduced by 0.3 V to about 0.8 V vs.
Ag/AgCl. This decrease of onset potential represents enhancement of the catalytic functionality
under radiation. We also tested the sample with only the TiO2 coating under identical conditions
but, as expected, we observed no measurable catalytic activity. As mentioned before, the Mn
WOC by itself is not light-sensitive, thus the decrease of the onset potential should only be the
result of the TiO2 layer.

59

First derivative of current
density

a

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1

1.2

1.4

First derivative of current density

Potential / V

b

0

0.2

0.4

0.6

0.8

Potential /V

Figure 4.5. First derivative of the current density profile with respect to applied potential, used
here as a reliable aid to determining the oxidation voltage. a) Data acquired under UV illumination,
showing an oxidation voltage of approximately 0.8 V vs. Ag/AgCl. b) The oxidation voltage for
the same sample evaluated without UV illuminationwas of approximately 1.1 V vs. Ag/AgCl.

60

As described above, the photoanode should have a “diffusion-reaction” mechanism, where the
reactants diffuse through the porous TiO2 layer to the surface of the catalytic film where they
react, and the products diffuse in the opposite direction. To investigate this scheme, we acquired
EIS data at 0.8V vs. Ag/AgCl under UV illumination. The resulting Nyquist plot is shown in
Figure 4.6. The plot of the photoanode under this condition has a semicircle in the high
frequency range, followed by a 45 degree straight line in the low frequency range. A simple
Randles Circuit (Figure 4.5 inset) can be used to regenerate the Nyquist plot. The equivalent
circuit contains a surface resistance Rs, a double-layer capacitance Cdl, a charge transfer
resistance Rct, and a Warburg element Zw which represents the diffusion process. A Randles
circuit is a typical model for an electrochemical reaction when it is governed by a semi-infinite
diffusion process on a flat electrode. This result further confirms our hypothesis that, during the
deposition of the Mn WOC, Mn2+ ions diffused through the TiO2 layer, and formed the WOC
catalytic film underneath. Thus, although the catalytic layer is covered by TiO2, water molecules
can still reach the catalytic surface to enable the reaction.

61

0.1 Hz

50 Hz
104 Hz

Figure 4.6. Nyquist plot of EIS data acquired at 0.8 V for the composite photoanode under UV
illumination. The semicircle at high frequency denotes reaction, while the straight line at low
frequency denotes diffusion. Along with the Randles circuit (inset) used to simulate the data, this
result provides confirmation that the Mn WOC is covered by a porous layer of TiO2.

62

4.5 Conclusions
We have successfully developed a composite photoanode using a Mn WOC and TiO2 as a
conductive substrate. This work has demonstrated an important and practical approach to using
earth-abundant, non-toxic, easy to handle catalysts to fabricate a composite photoanode. Another
potential application of this work that is that this catalytic complex can be integrated with CO2
sequestration to achieve the two carbon-based-fuel production schemes we introduced in Chapter
1. The current photoanode is only functional under UV illumination. However, there are methods
to dope various types of substrates with selected dyes to build a DSSC-type solar harvester to
broaden the absorption spectrum of the photoanode to obtain a much more efficient system.

63

CHAPTER 5. ELECTRODEPOSITION OF Ni/Ni(OH)2 CATALYTIC FILMS BY
CYCLIC VOLTAMMETRY FOR THE HYDROGEN EVOLUTION REACTION

5.1 Abstract
We have developed a novel electrodeposition method that synthesizes Ni/Ni(OH)2 catalytic films
in situ on conductive surfaces. The new method involves only two cycles of cyclic voltammetry,
and the resulting film has a surface morphology consisting of a monolayer of firmly packed
walnut-shaped particles. The film has superior catalytic activity towards the hydrogen evolution
reaction compared to traditional hydrogen evolution reaction catalysts (HERCs) produced by
electrodeposition under constant current or constant potential. The method is a simpler and more
cost-effective way to produce highly-effective electrocatalysts for the hydrogen evolution
reaction (HER). It creates large numbers of Ni(OH)2/Ni interfaces that significantly promote
catalyst performance towards that of platinum mesh.

5.2 Introduction
As we described in Chapter 1, active and low-cost electrocatalysts for both the water oxidation
and hydrogen evolution reactions are crucial for an effective AP process to produce hydrogen by
water splitting33,74-76. For a long time, the water oxidation half-reaction attracted most of the
attention because it is the major source of overpotential in the water splitting process77. Our
group has also worked extensively on the water oxidation reaction, as discussed in the previous
chapters. For the hydrogen evolution half reaction, platinum and its alloys are considered the
“state of art” catalysts, and are widely used at the laboratory scale78-80. However, the high cost
and extremely low availability of platinum and its alloys render them impractical for use on a

64

large scale81,82. To make affordable artificial photosynthesis devices at scale, highly effective,
robust catalysts based on earth-abundant metals have to be developed. Over the last decade, HER
catalysts based on earth-abundant transition metals, especially nickel, have been extensively
studied and assessed83-93. Most of these Ni-based HERCs are synthesized ex situ through bench
top wet chemistry methods. As previously discussed, electrodeposition has the benefit of
significantly simplified experimental setup as well as easily-controllable synthesis conditions. It
can also promote catalyst recovery and recyclability, which are usually significant problems for
traditional catalysts synthesized by benchtop methods.

In this chapter, we discuss the development of an electrodeposition method to produce
Ni/Ni(OH)2 catalytic films in situ for hydrogen evolution on conductive surfaces. The new
method involves only two cycles of cyclic voltammetry over a voltage range of 0 V to -1.2 V vs.
Ag/AgCl (immersed in 3M NaCl). We believe the method provides a simpler and more
inexpensive way to fabricate highly-effective hydrogen evolution electrocatalysts.

5.3 Experimental session
5.3.1 Materials
Nickle acetate tetrahydrate (Ni(COOCH3)2 ¡ 4H2O) was purchased from the Sigma Aldrich
Company, LLC (St. Louis, MO). Sodium phosphate monobasic (NaH2PO4) and sodium
phosphate dibasic (Na2HPO4) were purchased from J.T. Baker (Phillipsburg, NJ). All chemicals
were used without further modification. Fluorine-doped tin oxide (FTO)-coated glass (surface
resistance ~ 7 Ί/sq) was also purchased from the Sigma-Aldrich Company, LLC (St. Louis,

65

MO). Prior to use, the FTO electrode was cut into 2cm × 1cm rectangular chips, cleaned in
methanol and acetone, and rinsed in D.I. water.

5.3.2 Synthesis by electrodeposition
The electrodeposition process was performed using a typical three-electrode setup on a CHI
660D Electrochemical Workstation (CH Instruments Inc., Austin, Texas). A blank FTO chip was
used as the working electrode, with an indium tin oxide (ITO) chip as a counter electrode, and a
Ag/AgCl electrode as reference. The electrolyte used for electrodeposition was 25 ml of a 150
mM Ni(OCOCH3)2 solution. Two cycles of cyclic voltammetry (CV) scans were conducted
between 0.0 V and -1.2 V vs. Ag/AgCl at a scan rate of 0.01 V/s. The cumulative charge was
~1.0 C. During the electrodeposition process, a light grayish film could be observed on the FTO
working electrode. After deposition, the chip was rinsed in DI water and dried in air.

For comparison, we also synthesized a Ni-based HER catalyst using the traditional
electrodeposition method of constant current density of 8 mA/cm2 for 125 seconds94,95. A shiny
silvery film could be observed after the deposition. We also compared the performance of the
Ni/Ni(OH)2 catalytic film to that of platinum mesh. For brevity, we will designate the
Ni/Ni(OH)2 catalyst as cv-Ni, and the traditional catalyst as cc-Ni.

5.3.3 Materials characterization
The surface morphologies of both samples were characterized by a JSM 6610-LV and a JSM
7500F scanning electron microscope (SEM) (JEOL Ltd, Tokyo, Japan). The detailed

66

morphology was evaluated by a JEM 2200FS transmission electron microscope (JEOL Ltd,
Tokyo, Japan). X-ray diffraction spectra were acquired on a Bruker Davinci D8 Advance X-ray
diffractometer (Bruker Corporation, Madison, WI).

5.3.4 Electrochemical assessment of functionality
The functionality of the samples was assessed by cyclic voltammetry on a CHI660D
electrochemical workstation. FTO glass coated with cc- or cv-Ni was used as a working
electrode, a platinum wire as counter electrode, and a Ag/AgCl electrode as reference, with 25
ml of 1 M KOH solution as electrolyte. Potential scans were conducted between -0.4 V and -0.8
V vs. Ag/AgCl at a scan rate of 0.01 V/s. A Tafel plot was also obtained at a scan rate of 0.01
V/s. The catalytic functionalities of all samples were assessed in neutral media with 25 mL 0.1 M
phosphate buffer (pH 7.0) as electrolyte. For comparison, a Ni foil and a platinum mesh were
also assessed.

The catalyst stability test was also performed using the setup described above. The potential was
maintained at -1.0 V for 10 hours and the corresponding current density was recorded as a
function of time. We also did 201 cycles of CV scans on one cv-Ni sample, with the same setup
used for the functionality tests in neutral media.

67

5.4 Results and discussions
5.4.1 Structural characterization
Figure 5.1 shows the surface morphology of the FTO substrate and the two samples obtained by
FESEM. The bare FTO again has a rough but otherwise featureless surface (Figure 5.1 a). The
cc-Ni sample also has a featureless surface (Figure 5.1 b). On the contrary, the cv-Ni sample has
a surface morphology consisting of a monolayer of evenly distributed, firmly packed spherical
walnut-shaped particles (Figure 5.1 c). The inset in Figure 5.1c shows the surface morphology of
the cv-Ni sample at a higher magnification. From this image, we can estimate the diameter of the
spherical particles at approximately 0.1~0.2Îźm (100-200 nm). We believe the uniform surface
morphology promotes mass transfer and enhances the catalytic performance of the cv-Ni
catalyst. Based on recent reports, we believe the edges on the particles are most likely nanowalled Ni(OH)296,97.

To determine the elemental composition of both cc- and cv-Ni samples, energy dispersive x-ray
spectra (EDS) were obtained on the two catalytic films. To eliminate the influence of oxygen
from the underlying tin oxide substrate, we scratched off the catalytic films before EDS
measurements. The EDS spectrum of cc-Ni only shows the signal for Ni (Figure 5.1 d). This
indicates that the cc-Ni catalyst is a purely metallic nickel film. On the other hand, the cv-Ni
sample (Figure 5.1 e) has signals for both Ni and O, which provides evidence that the spherical
particles have a composite structure with both Ni and Ni(OH)2 present. Figure 5.2 shows the
cross-section SEM image of cv-Ni sample. The thickness of the catalyst layer was estimated at ~
0.3Îźm. It also confirms that the surface morphology of the cv-Ni sample is a monolayer of
spherical particles.

68

Figure 5.1. SEM images and EDS characterization of cc-Ni and cv-Ni. a) SEM image of a bare
FTO substrate, showing a featureless but rough surface. b) SEM image of the cc-Ni sample,
showing a featureless metallic surface. c) cv-Ni at an identical magnification, showing a film
consisting of a monolayer of evenly-distributed walnut-shaped particles with diameters of 0.1~0.2
m. The inset is the surface of the cv-Ni sample at a higher magnification, showing a detailed
morphology of spherical particles with patterns on the surface. d) EDS characterization of cc-Ni,
showing only the signal for Ni, which is an indication that it consists mainly of a Ni film. e) EDS
characterization result for cv-Ni showing both Ni and O, an indication that cv-Ni contains both Ni
and Ni(OH)2. The carbon signals in both EDS spectra are from supporting SEM slabs and patches.

69

Glass
FTO

Ni/Ni(OH)2 catalyst layer

Figure 5.2. Cross-sectional SEM image of the cv-Ni sample. The thickness of the Ni/Ni(OH)2
catalytic film was estimated at ~0.3Îźm. This also confirms that the surface morphology of the
catalyst layer is a monolayer of firmly packed spherical particles.

70

Figure 5.3 shows the XRD spectra of bare FTO and both Ni-based catalyst samples. In all
spectra, + designates the feature peaks for the conductive substrate FTO, * the peaks for Ni, and
# the peaks for Ni(OH)2. As expected, both the cc- and cv- Ni samples clearly show the feature
peaks for Ni. However, the cv-Ni sample also shows additional small peaks at 34, 38 and 44
degrees, which are indicators of the existence of Ni(OH)298-100. Due to the relatively small
amount of Ni(OH)2 compared to much larger quantities of FTO and Ni, those peaks are not very
prominent.

Figure 5.3. XRD spectra of a) cv-Ni, b) cc-Ni and FTO films. The symbol + designates feature
peaks of FTO, * of Ni, and # of Ni(OH)2. The cc-Ni film has peaks of FTO and Ni, while the cvNi spectrum has peaks of FTO, Ni and Ni(OH)2. Due to the small amount of Ni(OH)2 in the cv-Ni
samples compared with FTO and Ni, the Ni(OH)2 peaks are not prominent.

71

5.4.2 Functionality assessments
Cyclic voltammetry was used to assess and compare the functionalities of the cc- and cv-Ni
samples, and Pt mesh in neutral and alkaline media. Figure 5.4 shows the CV spectra acquired in
0.1M neutral phosphate buffer. The black curve shows the current density of cc-Ni in response to
applied potential. The red and green curves are for cv-Ni and platinum mesh, respectively.
Although both the cc- and cv-Ni samples have higher onset potentials compared with Pt mesh,
cv-Ni is much closer to platinum than cc-Ni. This result indicates that cv-Ni has a better catalytic
performance than cc-Ni. The peak current density of cv-Ni (0.806 mA/cm2) is much larger than
for cc-Ni (0.211 mA/cm2), based on geometric surface area at -0.3V vs. NHE. We also assessed
the samples in 1 M KOH alkaline media (Figure 5.5). Again, the peak current density of cv-Ni is
about twice that of cc-Ni, which demonstrates much better catalytic performance.

72

Figure 5.4. Comparison of the functionality of cv-Ni, cc-Ni, and platinum mesh by cyclic
voltammetry in neutral media. The black, red, and green curves are for cc-Ni, cv-Ni, and Pt
mesh, respectively. Both cc- and cv- Ni have higher onset potentials than Pt, but the cv-Ni
catalyst has a lower onset potential than cc-Ni.

73

Figure 5.5. Comparison of the functionality of cv-Ni, cc-Ni, and platinum mesh by cyclic
voltammetry in alkaline media. The black, red, and green curves are for cc-Ni, cv-Ni, and Pt
mesh, respectively. Both cc- and cv- Ni have higher onset potentials compared to Pt, but the cvNi catalyst has a lower onset potential than cc-Ni.

74

We compared the Tafel slopes and exchange current densities of the cc- and cv-Ni catalysts in
0.1 M neutral phosphate buffer. The cc-Ni has a slope of 0.34 V/dec, while cv-Ni has a much
lower slope of 0.25 V/dec (Table 5.1). As comparison, the Ni foil has a slope of 0.35 V/dec,
which is very close to the value for cc-Ni. We believe this offers further evidence that cc-Ni is a
simple metallic Ni film, while cv-Ni has a Ni/Ni(OH)2 composite structure. The cv-Ni also has a
higher exchange current density compared with cc-Ni, which is an indication of much better
catalytic performance.

Table 5.1. Comparison of Tafel slopes and exchange current densities of cc-Ni, cv-Ni, and Ni
foil catalysts assessed in neutral media. The lower slope and higher exchange current density of
cv-Ni are indicators of much better catalytic performance.

Sample

Slope (mV/dec)

Log (iex)

cv-Ni

0.25

-3.13

cc-Ni

0.34

-3.97

Commercial Ni foil

0.32

-3.72

75

5.4.3 Stability assessments
Stability tests were carried out by applying a constant potential (-1.0 V vs. Ag/AgCl) for a
prolonged period of time on both the cc- and cv-Ni samples. As shown in Figure 5.6, the current
density was very stable during the ten-hour testing period, demonstrating excellent stability. We
also did a stability test involving 201 consecutive CV cycles with one cv-Ni catalyst sample. As
shown in the inset of Figure 5.6, the first and 201st cycles are identical in shape as well as peak
current densities, a result that also demonstrates excellent stability over a long period of time.

Figure 5.6. Current density profile of cv-Ni in a 0.1 M neutral phosphate buffer under an applied
potential of -1.0V vs. Ag/AgCl. After the first two hours, the current density was steady over the
remaining eight-hour testing period, demonstrating stable catalytic performance. The inset shows
the cyclic voltammograms of the first and 201st cycles of a functionality test on a single cv–Ni
catalytic sample. The similarity in the two profiles as well as peak current densities is an
indication of very stable catalytic performance.

76

5.4.4 Effects of electrolyte concentration on electrodeposition
We synthesized catalysts using different concentrations of electrolyte. The number of CV cycles
was tuned to ensure an identical accumulated charge of 1 C during each deposition. All samples
had a similar onset potential of -0.1V vs. NHE for the hydrogen evolution reaction, but the
catalytic performance varied with electrolyte concentration. Table 5.2 is a summary of the
corresponding current densities at -1.2 V vs. Ag/AgCl. The results portray a typical volcano
behavior, with the sample synthesized in 200 mM solution giving the best performance. In
general, this profile suggests that electrolyte concentration offers another possible parameter for
optimization.

Table 5.2. Peak current density at -0.5 V vs. NHE for samples synthesized in different
concentrations of electrolyte.
Electrolyte Concentration / mM

Peak current density / (mA/cm2)

25

0.77Âą0.01

50

1.22Âą0.16

100

1.49Âą0.08

200

1.83Âą0.04

250

1.45Âą0.10

77

5.4.5 Hypothesis of the formation mechanism
Although we do not yet understand the exact mechanism for the formation of the structured film,
we propose a plausible hypothesis. In the traditional constant current deposition process, Ni2+
ions are continuously dragged to the electrode surface at a constant rate, and reduced to metallic
Ni. In the CV deposition case, the applied potentials are constantly changing, therefore Ni2+ ions
are dragged to the surface at various rates, then reduced. Also, over certain ranges of potential,
the Ni2+ ions are not reduced, resulting in the formation of only Ni(OH)2 hydrolysis species.
Under this scenario, it is possible for the walnut-shaped ‘core-shell’ type of structures to form
with metallic Ni accumulating in the center of the particles, surrounded by the nanowall-shaped
Ni(OH)2 hydrolysis species on the outer shell. Under such conditions, we would expect a higher
Ni to Ni(OH)2 ratio in the central area of the particles than on the edge. Our preliminary EDS
and ID analysis data support this hypothesis, showing that the ratio of Ni to Ni(OH)2 in the
center is 6.74 to 1, which is about twice the ratio on the edge.

Some studies have reported that Ni(OH)2/metal interfaces play an important role in the hydrogen
evolution reaction, with the Ni(OH)2/Ni interface being the most effective101,102. Usually, such
interfaces are created by complicated wet chemistry procedures. However, we have shown in this
work that such interfaces can also be created using simple CV electrodeposition methods. This
simpler more controllable synthesis method has successfully enabled a core-shell structure that
contains large numbers of Ni(OH)2/Ni interfaces, leading to excellent catalytic performance
towards the hydrogen evolution reaction.

78

5.5 Conclusions
We have developed a new method of synthesizing Ni/Ni(OH)2 HER catalysts in situ onto
conductive surfaces via a simple electrodeposition method using cyclic voltammetry. This simple
and inexpensive approach involves a single CV session of two potential ramps ranging from 0 to
-1.2V. The resulting catalytic film has a uniform surface morphology consisting of an evenly
distributed monolayer of spherical particles that promotes overall catalytic performance. The
resulting film has stable and much better catalytic performance than traditional cc-Ni HER
catalysts or Ni foil, with an effectiveness that shifts significantly towards that of platinum. We
believe this work provides an easier inexpensive way of synthesizing this HER catalyst, and
demonstrates the potential for using CV as a deposition method. This work also has the potential
to serve as a new way of creating large quantities of Ni(OH)2/metal interfaces to promote the
performance of HER catalysts, and provide options for catalyst synthesis and optimization
through electrochemistry.

79

CHAPTER 6. SUGGESTIONS FOR FUTURE RESEARCH

6.1 Introduction
In Chapters 3 and 5, we discussed our research activities on developing electrocatalysts for both
the water oxidation and hydrogen evolution reactions. Although both the MnOx WOC and
Ni/Ni(OH)2 HERC have excellent and stable catalytic functionalities, there are still some
parameters that can be further explored to enhance performance. Also, some characterization
devices used in this work are not ideal for the current system. Thus, more advanced
instrumentation could be used in the future to better characterize both catalytic films. Finally, it
would serve a valuable function to integrate more precise measurements and computational work
to guide future electrocatalyst synthesis.

For system integration, our eventual goal was to build an affordable but highly effective solardriven water splitting device. As discussed in Chapters 1 and 4, traditional AP devices have been
based on the schemes in Figure 6.1. Our solar-driven photoanode for water splitting is also based
on the two-chip face-to-face scheme. Although performances of such devices could be enhanced
by adopting highly effective solar harvesting systems and electrocatalysts, its overall
performance is still limited by mass transfer resistance. Another big issue is that the density of
solar radiation is quite low, with the result that large quantities of semiconducting materials and
catalysts would be required to achieve effective hydrogen production.

80

In this chapter, we suggest some future work for both electrocatalyst synthesis and system
integration to build solar-driven water splitting devices.

Figure 6.1. Schematics of two water splitting devices. Each scheme has a photoanode complex
containing a solar harvester and a WOC, and a counter electrode for hydrogen ion reconstitution
to produce fuel. a. Two-chip protocol (face-to-face scheme), where the counter electrode does
not share the conductive support with the photoanode; b. One-chip protocol, where the counter
electrode and photoanode share one conductive support.

81

6.2 Further research on improving the MnOx WOC
The dual series CV electrodeposition protocol described in Chapter 3 is a novel and excellent
method for synthesizing WOCs, in comparison to other methods. Multi-series CV has tunable
parameters such as number of series, potential ranges of each series, number of cycles in each
series, ionic strength of the electrolyte, and the concentration of doping elements. Each
parameter may have a specific effect on the resulting catalyst. It would be of scientific value to
assess the effects of these parameters on the resulting catalytic films. This could also lead to a
comprehensive understanding to enable rational design of catalysts.

In our current research, we used XRD to measure the oxidation state of the MnOx catalyst.
However, XRD is not a very accurate technique for measuring the oxidation state of manganese
compounds. We recommend advanced x-ray absorption techniques such as x-ray absorption near
edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) to measure the
oxidation states and the bounding patterns for MnOx WOCs. Also, since manganese is
notoriously hard for traditional transmission detectors, we would recommend the use of off-angle
fluorescence detectors. This would be valuable for developing a comprehensive understanding of
the mechanism for the formation of the catalytic films.

6.3 Further research on improving the Ni/Ni(OH)2 HER catalyst
The single series CV procedure described in Chapter 5 is a new method for synthesizing
Ni/Ni(OH)2 HER catalysts in situ. There are still other parameters that should be explored.
Some, such as doping elements, may have high possibilities to promote catalytic performance or
catalyst stability. We again suggest using x-ray absorption (XAS) techniques to study the details

82

of changes in oxidation states, and bonding patterns during electrodeposition and functional
assessments. We also suggest the use of environmental SEM to characterize changes in surface
morphology during the deposition. Combining these two characterization results listed above,
one can get a detailed knowledge of both surface morphology and changes in oxidation states
and develop a theory to guide the future development of electrocatalysts.

6.4 Further research on system integration
To address the problems in our current water splitting system, we recommend a new scheme
featuring multiple photoanode modules for further investigation (Figure 6.2). Each photoanode
module contains: a) a device that can collect solar energy over a large area (front surface) and
concentrate it in a much smaller area along the edge; b) one or more layers of semiconducting
material to convert the solar energy to the electrical potential required to power the water
splitting reaction; c) a layer of conductive material such as ITO to be used as the substrate for the
electrodeposition of the MnOx WOC (to enhance the electron-hole separation, a thin holeattraction layer could also be coated); and d) a layer of MnOx WOC synthesized using the
method described above. To minimize mass transfer resistance, we propose using the photoanode
modules to form microfluidic water channels that reduce the distance for ion migration.
Hydrogen evolution catalysts would be deposited on the bottom of the water channels via
electrodeposition.

83

Figure 6.2. Overview of the proposed scheme. All the components of the proposed solar-driven
water splitting device are supported by a robust conductive substrate. The squares are the AP
modules which can be engineered into various shapes and sizes. Water channels are formed by
the edges of the AP modules. The water oxidation catalyst is coated on the edges of the AP
modules, and the hydrogen evolution catalyst is coated on the bottom of the water channel to
greatly reduce the distance for ion migration, resulting in a much lower resistance to mass
transfer. The detailed structure of the AP module is shown in the figure below.

Figure 6.3. Cross-sectional view of the AP module in a microfluidic channel. Each AP module
features a) a solar concentrator to collect solar energy on the larger surface and transmit it to the
edge; b) a convertor to transform the concentrated solar energy into the potential required for the
water splitting reaction; c) a conductive layer for the electrodeposition of WOC; d) an optional
p+ layer to promote electron-hole separation; and e) a WOC layer directly in contact with water.

84

Since the solar concentrator is a flexible material, the resulting photoanode modules can be
fabricated in various sizes and shapes. As a result, the size and shape of the final device could be
tuned by changing the number and pattern of the photoanode modules. Also, the final device
could be attached to any surfaces if the substrate is not rigid. Successful development of a device
based on this new scheme would be of great scientific and technological value.

As also described in Chapter 4, Mn-based WOCs have the capability for integration with
semiconducting materials like TiO2. The major problem with TiO2 is that its absorption spectrum
is quite limited to the far UV range. Thus, if future researchers could find suitable dyes to
broaden the absorption spectrum of the solar harvester, the efficiency of the photoanode modules
could be greatly improved.

85

CHAPTER 7. CONCLUSIONS

This dissertation is a detailed account of our group’s research activities in developing highly
effective yet inexpensive electrocatalysts for the water splitting reaction. In Chapter 2, we
demonstrated that a Co-OXO water oxidation catalyst that is synthesized ex situ can be
effectively immobilized onto conductive surfaces to enhance the recyclability and reusability of
the resulting catalytic film without losing functionality.

Chapter 3 covered our research on developing a dual-series cyclic voltammetry (CV)
electrodeposition method to synthesize a MnOx in situ on conductive surfaces. The new method
involves two consecutive series of CV scans over different potential ranges, followed by
calcination. Our results showed that the MnOx WOC has stable and excellent catalytic
performance over long periods of time. We also proposed a nucleation-growth formation theory
that agrees with the experimental observations.

In Chapter 4, we report on our study to integrate Mn-based WOC synthesized in situ with the
semiconducting material TiO2 to build a functional photoanode. The photoanode has a doublelayer structure that promotes water splitting under ultraviolet (UV) illumination. Although the
current system is only functional under UV light, the addition of various dyes could broaden its
absorption spectrum and lead to a more effective system.

Chapter 5 covered our study on developing effective electrocatalysts for the hydrogen evolution
reaction (HER). We used only one series of CV to synthesize the Ni/Ni(OH)2 HER catalyst in

86

situ onto conductive surfaces. Our results show that the catalytic film has a surface morphology
of firmly packed monolayer of walnut-shaped spherical particles, with stable and excellent
performance. We believe this work has provided an inexpensive way to create large numbers of
Ni(OH)2/metal interfaces to promote the performance of HER catalysts.

We believe the results of the four studies listed in this dissertation are valuable in opening new
ways of synthesizing highly effective yet low cost electrocatalysts in situ on conductive surfaces.
These studies have the potential to help guide the development of highly effective solar-driven
water splitting devices in the future.

87

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