ELECTROCHEMICAL STUDIES OF REDOX REACTIONS IN ROOM TEMPERATURE IONIC LIQUIDS AND IONIC LIQUID/ORGANIC SOLVENT BINARY MIXTURES By Shashika Gunathilaka Sabaragamuwe Jayasundara Korale Mudiyanse Ralahamilage A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Chemistry – Doctor of Philosophy 2024 ABSTRACT Room temperature ionic liquids (RTILs) are promising electrolytes for energy storage and electroanalytical applications due to their unique properties, such as low volatility, wide operating window, and good ionic conductivity. However, high viscosity, the absence of a dielectric solvent, and complex ion formation result in complicated electrochemical behavior, particularly for electron transfer, mass transfer and electric double layer formation. Therefore, it is necessary to fully understand these processes to expand applications of RTILs. The main objective of this dissertation work is to enhance knowledge about electron transfer, mass transfer, and double layer formation in RTILs and RTIL/organic solvent binary mixtures, and to compare them with organic electrolytes. In the first part of this work, the material properties of tetrahedral amorphous carbon (ta-C) and nitrogen-incorporated tetrahedral amorphous carbon (ta- C:N) were studied using Raman microscopy and scanning electron microscopy. The electrochemical properties of ta-C and ta-C:N were investigated in acetonitrile and propionitrile using anthracene derivatives as redox probes employing cyclic voltammetry. Results indicated that while the background current and double layer capacitance increase with nitrogen incorporation, electron-transfer processes for anthracene derivatives remain unaffected by the nitrogen content in the ta-C films. The sluggish electron transfer observed for anthracene derivatives is attributed to the cation radical formation of these molecules during oxidation. Anthracene derivatives showed much slower electron transfer in RTILs due to higher viscosity and ion-ion interactions, resulting a different chemical environment around anthracene than in a traditional organic solvent/electrolyte system, which was manifested in more positive oxidation potentials in RTILs. Further, to understand the effect of the added of organic solvent on the electric double layer formation and electron and mass transfer processes, experiments were conducted in mixtures of methanol and acetonitrile in different RTILs. In this work, the electron-transfer kinetics and molecular diffusion of ferrocene in an imidazolium-based RTIL/organic solvent binary mixtures were examined using cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy at a gold disk working electrode. Ferrocene exhibited a quasi-reversible electron- transfer behavior with a 6× increase in the electron transfer rate constant with organic solvent addition as the mole fraction of the organic solvent reached 0.4. Chronoamperometry indicated the diffusion coefficient of ferrocene increased 3× to 4× with organic solvent addition. Interestingly, the organic solvent formed a heterogeneous mixture with the RTIL, creating isolated pockets that resulted in non-linear diffusion of ferrocene. At higher solvent mole fractions, these pockets interconnected, providing lower-viscosity diffusion pathways for ferrocene molecules. Overall, these studies enhance the understanding of electrochemical properties ta-C electrodes in RTILs, which can benefit real world applications, including energy storage and electroanalytical technologies. ACKNOWLEDGEMENTS I would like to thank my advisor Professor Greg Swain and committee members Professor Gary Blanchard, Professor James Jackson, and Professor Seokhyoung Kim for their advice and guidance navigating my dissertation journey. Thank you to my fellow lab mates, Skye Henderson and Jack Walton for their friendship during my time at Michigan State University. I would like to express my heartfelt gratitude to my parents and my sister for their unwavering support throughout my academic endeavors. Finally, a special thank you to my wonderful wife, Isuri, whose unwavering support has been the foundation of this journey, through both its challenges and successes. I am truly grateful for the sacrifices you have made, and my appreciation for all that you have done is beyond words. iv TABLE OF CONTENTS CHAPTER 1. INTRODUCTION ................................................................................................ 1 Carbon Electrodes ........................................................................................................... 1 1.1 1.2 Glassy Carbon Electrode................................................................................................. 2 Boron Doped Diamond ................................................................................................... 4 1.3 Tetrahedral Amorpous Carbon ........................................................................................ 5 1.4 Room Temperature Ionic Liquids ................................................................................... 8 1.5 1.6 Research Objectives and Specific Aims……………………………………………….13 REFERENCES ......................................................................................................................... 15 CHAPTER 2.EXPERIMENTAL METHODS…………………………………………………22 2.1 Materials ....................................................................................................................... 22 Electrode Preparation .................................................................................................... 23 2.2 Instrumentation ............................................................................................................. 25 2.3 2.4 Electrochemcial Characterization ................................................................................. 26 REFERENCES ......................................................................................................................... 31 INVESTIGATIONS OF ANTHRACENE CHAPTER 3. ELECTROCHEMICAL DERIVATIVES AT NOVEL CARBON ELECTRODES IN ORGANIC ELECTROLYTES SOLUTIONS AND ROOM TEMPERATURE IONIC LIQUIDS…………………………. 33 Introduction ................................................................................................................... 33 3.1 Experimental Methods .................................................................................................. 35 3.2 3.3 Results ........................................................................................................................... 40 3.4 Discussion ..................................................................................................................... 60 Conclusions ................................................................................................................... 65 3.5 REFERENCES ......................................................................................................................... 67 CHAPTER 4. THE EFFECT OF ADDED ORGANIC SOLVENTS ON ELECTRON IN ROOM TRANSFER KINETICS AND DIFFUSION OF FERROCENE TEMPERATURE IONIC LIQUIDS…………………………………………………………..72 4.1 Introduction…………………………………………………………………………....72 Experimental Methods………………………………………………………………...75 4.2 4.3 Results ............................................................................................................................78 4.4 Discussion ......................................................................................................................88 4.5 Conclusions ....................................................................................................................95 REFERENCES ..........................................................................................................................97 CHAPTER 5. CONCLUSIONS AND FUTURE WORKS ....................................................104 Conclusions ..................................................................................................................104 Future works ................................................................................................................105 5.1 5.2 v CHAPTER 1. INTRODUCTION 1.1. Carbon Electrodes Carbon is one of the most versatile elements on earth that can be bonded with itself to create various stable allotropes.1-3 Carbon materials are categorized into two groups based on the bonding hybridization. The first group consists of sp2 hybridized carbon, which includes allotropes such as graphene, glassy carbon (GC), and highly ordered pyrolytic graphite (HOPG).1,4-7 A B Figure 1.1. Allotropes of carbon (A) graphite (B) diamond.13 Diamond is the sole allotrope in the second group, which consists of sp3 hybridized carbon materials.3 Figure 1.1A shows the atomic arrangement of carbon atoms in graphite, where each carbon creates three σ bonds with three adjacent carbon atoms within the X-Y plane. The fourth valence electron in the 2pz orbital does not participate in σ bonds and remains free to form a π bond. In graphite, weak Van der Waals forces help to hold individual planes (graphene layers) of carbon atoms together, and the distance between each plane is approximately 0.35 nm in the Z- direction.1,8 The carbon atoms in the X-Y plane are known as “basal” plane carbon and are considered less active for electron transfer when compared to the carbon atoms on the edges possessing high electrochemical activity towards electron transfer. Furthermore, exposed empty 2pz orbitals on edges may undergo adsorption due to their tendency to react with atmospheric 1 oxygen to form functional groups such as hydroxyl, carbonyl, and carboxylic acid.9-12 Diamond comprises a tetrahedral configuration packed in a face-centered cubic structure (Figure 1.1B). Four σ bonds in the diamond lattice are the reason behind its fundamental properties.3 Despite its unique properties such as exceptional hardness, high thermal conductivity, and optical transparency, the low density of electronic states and a wide bandgap (5.47 eV at T = 300 K) make it an insulator. Doping with n-type or p-type doping material is a way to increase conductivity by lowering the band gap and increasing the number of charge carriers in diamonds. Nitrogen, boron, sulfur, and phosphorous are the most common dopants to prepare conductive diamonds.1 Among the electrode materials employed in electrochemistry, carbon materials are preferred over metal electrodes, namely platinum and gold, due to their low cost and wide potential range. In addition to that, the ability to use them in aqueous, nonaqueous, and ionic liquids, coupled with surface modification, makes carbon electrodes more attractive for various electrochemical applications. The first experiment using a carbon electrode was conducted by Humphrey Davy in the 19th century, in which he used a graphite electrode for the electrowinning of alkali metals, a process used to extract metals from their solution using the electric current. Ever since, various carbon materials have been employed in electrochemistry such as glassy carbon (GC), carbon fiber (CF), boron-doped diamond (BDD), carbon nanotubes (CNTs), and tetrahedral amorphous carbon (ta-C).1 1.2. Glassy Carbon One of the most used carbon electrodes in electroanalytical chemistry owing to its electrochemical stability, good electrical conductivity, and low cost, is glassy carbon (GC). The microstructure of GC consists of disordered carbon arranged in regions that resemble both edge 2 and basal planes. The combination of these planes influences properties such as conductivity and chemical reactivity. Although GC consists of sp2-bonded carbon, its structure is hard and brittle since it is an amorphous allotrope of sp2 carbon, unlike graphite.1,3,14 Hard structure of GC can be attributed to the preparation conditions. GC is prepared through controlled pyrolysis of polyacrylonitrile at 1500-3000 ºC in an inert environment. The pyrolysis temperature is crucial in producing more microstructurally ordered GC.1 During pyrolysis, only the carbon atoms in the polymer backbone remain intact. All the other heteroatoms evaporate, resulting in the formation of sp2-bonded carbon structure with randomly oriented ribbons resembling graphitic planes. These ribbons are typically 15-70 Å in the lateral dimension with an interplanar spacing of 3.5 nm, as characterized by the XRD.8,15-17 GC has been widely employed as an electrode material due to its excellent conductivity. The high conductivity of GC is associated with its high density of electronic states and the narrow gap between its valence and conduction bands. In comparison to metal electrodes, the lower background current and broader operating potential window of GC make it ideal for various applications. However, its empty π-orbitals of carbon and oxygen make it more susceptible to absorbing certain substances. Therefore, to gain optimal performance, the surface of the GC electrode must be thoroughly pretreated before any electrochemical application. The most effective way to clean and activate the GC surface is through mechanical polishing with alumina powder.9 It is important to note that each polishing session exposes a new and fresh GC surface, which improves the electrochemical behavior. Therefore, the performance of GC is linked to the nature of the surface pretreatment process.1,9 3 Figure 1.2. Microstructure of glassy carbon.13 1.3. Boron Doped Diamond Diamond is a well-known electrical insulator with a low density of electronic states and a wide band gap (5.5 eV at T = 300 K).3 The low conductivity of diamond can be attributed to a low number of free charge carriers, as these cause little thermal activation of electrons in the conduction band at room temperature. Diamonds can be doped using p-type or n-type dopants to increase conductivity.1 Boron is the most used dopant for diamond doping because the size of boron atoms is closer to that of carbon, readily incorporating into the dense crystal lattice of the diamond and forming a stable bond. Moreover, incorporating boron atoms into the diamond generates an impurity band 0.35 eV above the valence band.18-20 Therefore, boron-doped diamond (BDD) is the most used doped diamond material for electrochemical applications. The conductivity of a diamond can be modulated by varying the boron content of the diamond. Typically, the doping level for a conductive diamond is 1020-1021 cm-3.21,22 BDD is prepared using chemical vapor deposition (CVD) with hydrogen, hydrocarbon, and diborane (B2H6) as dopant sources.23,24 The most used substrates to grow BDD are Si, Mo, W, Ti, and diamond.8,23,25 BDD is a widely used electrode material, especially in its form that is highly infused with boron (~1021 cm-3).26-28 The substantial usage of BDD as an electrode can be attributed to its 4 excellent electrochemical properties, such as a wide working potential window and low background current when compared to GC.29-32 These unique properties are associated with its stable microstructure and weak adsorption due to the lack of π bonds.33,34 However, it is important to understand that these distinct properties highly depend on the sp3 content in the diamond film. The quality of the BDD film depends on the presence of non-diamond sp2 carbon.35,36 The non- diamond carbon increases the reactivity of the electrode towards both oxygen and water and can also promote adsorption, leading to surface fouling.37,38 The electrochemical performance of BDD is determined by numerous factors, including doping level, surface termination, and crystal size.38- 41 Figure 1.3. Microstructure of boron doped diamond.13 1.4. Tetrahedral Amorphous Carbon Tetrahedral amorphous carbon (ta-C) is a variant of “diamond-like” carbon.42 The atomic structure of ta-C has been described by Robertson et al. ta-C electrodes can feature a random arrangement of sp3,sp2, and even sp bonded carbon, potentially including hydrogen.43 Both sp2 and sp3 bonding significantly influence the properties of the ta-C electrodes.44,45 For example, sp2- bonded carbon within ta-C enhances the presence of π electronic states in the bandgap, thereby elevating the electrical conductivity of the film.46 Conversely, sp3 carbon introduces diamond-like properties to the films by forming four strong sigma bonds with the adjacent carbon atoms.47 5 Figure 1.4. Ternary phase diagram of carbon and hydrogen showing the relationship among sp2, sp3, and hydrogen. Figure 1.4 shows a ternary phase diagram depicting different compositions of amorphous carbon including tetrahedral amorphous carbon (ta-C). These microstructural-different are shown in relation to their sp2-sp3 bonding and hydrogen content. In general, ta-C exhibits a high sp3 content, often reaching up to 85% depending on the deposition technique.38 The fraction of sp3 bonded carbon also impacts the hardness and band gap of the resulting ta-C film.38 Notably, the electronic and mechanical characteristics of the films are finely adjusted by varying the sp3:sp2 carbon ratio as well as the hydrogen content in the film.48,49 However, ta-C film possesses relatively lower conductivity compared to the other sp2 carbon materials, making them difficult to employ in electrochemical applications.50 To overcome the issue, doping can be done using impurities like nitrogen, phosphorus, or fluorine. Doping alters both the electrochemical and mechanical properties of the material.51 Doped tetrahedral amorphous carbons find applications across various fields, including biomedicine, electroanalysis, and coatings.52,53 Among the doped 6 amorphous carbons, nitrogen incorporated amorphous carbon (ta-C:N) stands out for its exceptional conductivity and mechanical stability.54,55 Doping with nitrogen allows the control of the sp2:sp3 ratio, thereby altering the properties of the film.56,57 Figure 1.5. Microstructure of tetrahedral amorphous carbon.13 1313carbon.47 Nitrogen doping increases the sp2 carbon content in the film, thereby increasing the conductivity, and confers n-type semiconducting electronic characteristics to the ta-C film. This enhancement facilitates an improved outer-sphere redox reaction on ta-C:N electrodes.43An excess of nitrogen can lead to the formation of trigonally bonded sp2 carbon structures, such as pyridine, which can diminish film conductivity.58,59 According to our previous work, nitrogen doping increases the sp2 carbon content in the ta-C film, thereby lowering the electrical resistivity in the film.38 The lower electrical resistivity suggests that incorporating a heteroatom changes the functional groups on the surface, consequently, affecting the electrochemical behavior. Therefore, it is essential to thoroughly examine and understand the effect of surface terminology on the electrochemical behavior of amorphous carbon films before employing them in real-world applications. Recent studies have highlighted the significant impact of surface wettability on electron transfer dynamics of redox species not only in conventional electrolyte systems such as aqueous electrolyte but also in novel electrolytes like ionic liquids.60,61 Surface wettability depends 7 on the surface terminology which relies on the functional groups of the carbon and can be modified trough nitrogen doping. 1.5. Room Temperature Ionic Liquids Room-temperature ionic liquids (RTILs) are salts with melting points below room temperature.62,63 Unlike conventional salts, RTILs do not contain solvent molecules and consist solely of pure ions. RTILs typically feature asymmetrical organic cations such as 1-alkyl-3- methylimidazolium or 1-alkylpyridinium, paired with a small symmetric inorganic anion, such as tetrafluoroborate or hexafluorophosphate or a larger organic cation.64-65 The asymmetrical shape and polarizability of the cation contribute to the weakened lattice of the RTILs compared to the other ionic lattices.65 RTILs have been widely used in electrochemical applications due to their unique qualities such as wide working potential windows, moderate electrical conductivities, and high thermal and chemical stabilities.66,67 Typically, a purified ionic liquid offers a working potential window of about 3-5 V, significantly wider compared to non-aqueous and aqueous media.68,69 This wider window enables exploration of various redox-active molecules across a wider potential range with less obstruction from solvent or RTIL electrolysis. The greater stability of ionic liquids at extremely high positive and negative potentials is associated with the electrochemical stability of their cations and anions. Viscosity plays a crucial role in electrochemical measurements, particularly in the case of RTILs due to their high viscosity when compared to conventional solvents. The high viscosities arise from the strong Van der Waals interactions between cations and anions.65,68,69 Specifically, in the presence of fluorinated anions such as BF4 -and PF6 -, they can form robust H-F bonds with organic cations, leading to extremely high viscosities. The higher viscosities inhibit the movement of redox molecules and slow down electron transfer at the electrode interface.65 8 A C E B [CnMIM] : [Py1e] [PF6]- F [N(Tf)2] D [BF4] [Nabcd] Figure 1.6. Some cations and anions used in room temperature ionic liquids. (A) [CnMIM]: 1- alkyl-3-methylimidazolium, (B) [N(Tf)2]: bis(trifluoromethylsulfonyl) imide, (C) [Py1e]: N- -]: hexafluorophosphate, (F) methyl-N-alkyl pyrrolidinium, (D) [BF4 [Nabcd]: tetraalkylammonium.63 -]: tetrafluoroborate, (E) [PF6 These drawbacks can be overcome by mixing RTILs with organic solvents such as acetonitrile (CH3CN), methanol (CH3OH), and ethanol (CH3CH2OH) since these organic solvents effectively reduce the viscosity of RTILs while maintaining a wide operating potential window. 70,71 Despite the promise of the RTILs/organic solvent binary mixtures, their behavior remains incompletely understood. Although some studies have been conducted focusing on the aspects of RTILs including viscosity, conductivity, microstructural, and dynamic heterogeneity, there is still a substantial amount of work that needs to be done to fully understand these binary mixtures. By exploring these interactions more extensively, researchers can gain novel insights into innovative solutions ranging from electrochemistry to material science.65 9 Diffusional mass transport and electron transfer are also influenced by the presence of impurities. Water is the most common impurity found in RTILs, as they readily absorb moisture from the atmosphere.72-74 The presence of water has a significant influence on their electrochemical behavior. For instance, even a minute amount of water can significantly decrease the working potential window due to the oxygen evolution reaction.74,75 The presence of water may also decrease the viscosity of RTILs and increase conductivity, thereby altering the electrochemical data.75,76 Additionally, water in RTILs can affect the structure of the electrical double-layer (EDL) and increase the background current. Therefore, removing water in RTILs is crucial to obtain accurate electrochemical data. A B Figure 1.7. Double-layer models in (A) aqueous solution (B) room temperature ionic liquids.78 The Electrode-electrolyte interface in RTILs is different compared to that in aqueous or non-aqueous media.77 The EDL forms as counter ions and solvent dipoles organize at the electrode surface in response to changes in the excess surface charges.79,80 An EDL serves as a key component in understanding the interfacial behavior of materials. Additionally, the structure of EDL also impacts the electrochemical processes significantly. For instance, to optimize heterogeneous electrochemical reactions, it is crucial to comprehend the EDL as these reactions 10 occur at the electrode–electrolyte interface. While the Gouy-Chapman-Stern model is commonly employed to elucidate the EDL, it has failed to explain EDLs in RTILs.79 This model originally proposed for dilute aqueous and non-aqueous solutions, where solvent concentration greatly exceeds ion concentration, resulting in negligible ion interactions, the model does not hold in solvent-free electrolytes such as RTILs where the concentration is about 3.0 - 6.0 M.81,82 In RTILs, strong interionic interactions occur due to the absence of solvent molecules acting as dielectric separators. Moreover, the long-range organization in RTILs distinguishes them from aqueous and non-aqueous systems. Therefore, an understanding of the long-range organization and EDLs of RTILs is necessary before utilizing them in electrochemical applications. Numerous research studies have been conducted to explore the EDL of RTILs.79,80 However, most of these studies have focused on metal electrodes, leaving carbon electrodes understudied. Over the past decade, researchers have employed diverse theoretical and experimental approaches to examine the capacitance of carbon electrodes in RTILs.79 These studies have unveiled asymmetrical profiles in the capacitance- electrode relationship. Furthermore, the formation of the EDL in RTILs at positive potentials relative to the zero charge was significantly different compared to the negative potentials. This difference was attributed to the asymmetry in the size and shape of the RTIL ions.79,80 While these studies provide plenty of information about the nature of the EDL at carbon electrodes with sp2 type carbon such as glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG), there is minimum information available for carbon electrodes contain sp3 type carbon such as ta-C and ta-C:N electrodes. Hence, comprehensive studies explaining the nature of EDLs at ta-C and BDD electrodes in RTILs are needed. The environment surrounding a redox analyte in an RTILs differs from that in an aqueous or nonaqueous solvent- electrolyte system. There is no solvation layer around a redox-active analyte in RTILs, due to the 11 absence of dielectric solvent molecules. Consequently, there is a significant reorganizational barrier to overcome in an RTIL when a redox molecule undergoes electron transfer. As a result, of this it is intriguing to study the electron transfer of various redox couples in RTILs. In recent years, a series of electrochemically active materials have been studied in RTILs to obtain valuable information, including diffusion coefficients, electrochemical reactions, electron transfer kinetics, and thermodynamics.81-83 Among these materials, ferrocene and its derivatives are the most studied redox couples in RTILs.84-86 Ferrocene oxidation in RTILs has demonstrated well-defined voltametric waves. Furthermore, the peak currents varied linearly with the square root of the scan rate, indicating that electrochemical processes are diffusion controlled. Moreover, the Fc/Fc+ redox couple was electrochemically reversible or quasi reversible depending on the water content in the RTILs. Based on the previous findings, some other electrochemically active materials show chemical reversibility in RTILs such as cobaltocene, and tetrathiafulvalene.87 Anthracene derivatives were also commonly used as outer sphere redox probes and were thoroughly studied in both organic solvents and RTILs. Anthracenes are particularly important in RTILs because highly charged redox probes, such as ruthenium hexamine, cannot oxidize in this media due to their high viscosity.81 However, the redox process of anthracene derivatives in RTILs is chemically irreversible. This makes the oxidation peaks of anthracene measurable, but the reduction peaks difficult to detect. The irreversibility of anthracene is associated with the instability of their oxidized product (Figure 1.8).89 These reactions are categorized as EC mechanism, where electron transfer is followed by a chemical reaction/step. Although this mechanism can be complex, we have simplified it by focusing on the single electron transfer process known for these anthracene derivatives.88,89 Understanding the electrochemical characteristics of anthracene derivatives in RTILs is important because knowing their behavior in 12 these media is essential for applications like electrochemical sensors. However, their chemical irreversibility in RTILs presents challenges that require more research. One of the main goals of this project is to thoroughly understand the behavior and stability of these structures in RTILs and organic electrolyte media. 1.6. Research Objectives and Specific Aims Figure 1.8. Oxidation of anthracene forming a cation as an intermediate product.91 Although extensive knowledge regarding electron transfer processes for redox systems and the formation of electric double layers in aqueous and organic electrolyte solutions at carbon electrodes has been developed, information on electron transfer kinetics and interfacial organization in RTILs is more limited. Research on electron transfer in RTILs is still in its developmental stages, and this field is not as advanced as it is for conventional electrolyte systems. Moreover, studies focusing on electron transfer in mixtures of RTILs and organic solvents, known as RTIL/organic binary mixtures, are even more limited. This significant knowledge gap creates problems for further development and utilization of these mixtures in practical applications. The dissertation work, therefore, focuses on addressing this knowledge gap. The primary objective of this dissertation research was to advance and understanding of electron transfer reactions, mass transfer phenomena, and double layer formation in RTILs at hybrid sp3/sp2 (nitrogen-incorporated tetrahedral amorphous carbon, ta-C:N) carbon electrodes. Furthermore, this work aimed to investigate these properties in RTIL/organic binary mixtures, 13 thereby broadening the scope of knowledge regarding these complex systems for potential energy storage applications. The research was conducted around three specific aims. Specific Aim 1: Preparation, material characterization, and electrochemical evaluation of ta-C and ta-C:N electrodes in organic electrolytes/solvent systems and RTILs media. Specific Aim 2: Investigation of the electron-transfer and molecular diffusion of anthracene derivatives in non-aqueous electrolyte/solvent systems and RTILs at ta-C, ta-C:N electrodes. 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Lagrost, C., Preda, L., Volanschi, E. and Hapiot, P "Heterogeneous electron-transfer kinetics of nitro compounds in room-temperature ionic liquids." Journal of Electroanalytical Chemistry 585, no. 1 (2005): 1-7. 93. Mellah, M., Zeitouny, J., Gmouh, S., Vaultier, M. and Jouikov, V. "Oxidative self-coupling of aromatic compounds in ionic liquids." Electrochemistry Communications 7, no. 9 (2005): 869- 874. 21 CHAPTER 2. EXPERIMENTAL METHODS 2.1. Materials 2.1.1. Room Temperature Ionic Liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] (≥97.0%), 1-butyl-3- methylimidazolium tetrafluoroborate [BMIM][BF4] (≥ 97.0%), and 1-hexyl-3-methylimidazolium tetrafluoroborate [HMIM][BF4] (≥ 97.0%) RTILs were purchased from a commercial source Iolitec, [Germany]. As the initial step in the purification process, the RTILs were stored over activated carbon for three days. They were then removed by collection with a syringe fitted with a filter having a 0.22 μm pore size (Whatman) to remove any residual activated carbon. The RTILs were then stored in a sealed glass vial over activated 5 Å molecular sieves for 5 days. Lastly, an RTIL was added into the electrochemical cell at the time of measurement and heated to 70 °C for 50 min while purging with ultrapure Ar (99.99%). The Ar gas was passed through a desiccant system to remove moisture before being introduced into the N2-purged glove box.1,2 After the purification, the water content in selected purified RTIL samples was qualitatively measured using cyclic voltammetry by observing the background current and potential window thereby providing an indirect measure of the residual water in purified RTILs. Karl Fischer titration was also used to get quantitative information about the water content in selected purified RTILs and the water content was below 100 ppm. All glassware was cleaned before use with ultrapure water, followed by two sequential rinses with isopropanol and ultrapure water. Finally, the glassware was dried in an oven at 150 oC for 12 h and transferred to the N2 purged glove box. This helped to minimize water contamination. 22 2.1.2. Room Temperature Ionic Liquids/ Organic Solvent Binary Mixture Acetonitrile (99.9%), methanol (99.8%), and isopropyl alcohol (99.9%) were purchased from Sigma-Aldrich. All the organic solvents were distilled and dried over activated 5 Å molecular sieves before being stored inside an N2-purged glove box. The measured masses of the RTIL and the organic solvent (acetonitrile or methanol) to obtain the desired mole fraction of organic were mixed and stirred vigorously using a magnetic stir bar for 1 h to achieve full mixing. All solution preparation was performed in the N2-purged glove box. 2.1.3. Organic Electrolytes Chemical reagents used were tetrabutylammonium hexafluorophosphate (TBAPF6; Aldrich, electrochemical grade, ≥ 99%), acetonitrile (Fisher Scientific, ≥ 99%), and propionitrile (Fisher scientific ≥ 99%). All the organic solvents were distilled and dried over activated 5 Å molecular sieves (Sigma Aldrich) for several days before use. All solutions contained 0.10 mol L-1 TBAPF6 as the supporting electrolyte and all were degassed with N2 before an electrochemical measurement. All experiments were performed at a room temperature of 23oC. 2.2 Electrode Preparation 2.2.1. Glassy Carbon Electrodes The glassy carbon electrode (GC-20, Tokai Ltd.) was polished using Micro Polish II alumina powder (Buehler) with sequentially decreasing diameters of 1.0, 0.3, and 0.05 µm. The alumina powder was mixed with deionized water to form a slurry. Each polishing step lasted approximately 10 min on a micro cloth PSA (Buehler) pad. After each polishing step, the electrode was rinsed with ultrapure water to remove alumina powder residue and then ultrasonicated in isopropanol for 10 min to remove polishing debris. After polishing, the electrode was thoroughly rinsed with deionized water and ultrasonic cleaned for 20 min to eliminate any polishing debris. Finally, the 23 electrode was air-dried with N2 and then immediately used for electrochemical measurements. 2.2.2. Preparation of ta-C and ta-C:N Thin Film Electrodes The ta-C:N films were deposited on polished boron-doped Si (111) substrates (Virginia Semiconductor, Fredericksburg, VA; ~ 0.001 Ω-cm) using a Laser-Arc physical vapor deposition system at the Fraunhofer Center for Coatings and Diamond Technologies, Michigan State University (MSU). The deposition process utilizes a laser-controlled, high-current cathodic vacuum arc deposition.3-5 During the process, a pulsed laser beam scans across a rotating high- purity graphite target functioning as the cathode. The substrate was placed approximately 30 cm away from the target, maintaining the temperature below 100 oC during the deposition. A discharge is produced at the cathode (+) that consists of highly ionized carbon atoms and small carbon atom clusters that are accelerated toward the substrate (anode (-), grounded). The nitrogen incorporation was done by flowing N2 gas at flow rates of 10 and 30 standard cubic centimeters per minute (sccm) into the rector. The reactive plasma discharge is produced with a pulsed laser at a pulse rate of 350 Hz and a peak arc current of > 100 A during the film deposition. The film thickness was 200-300 nm, and the hardness ranged from 30-50 GPa. The ta-C and ta-C:N thin-film electrodes were soaked in ultrapure isopropanol for 20 min and fully dried under a N2 gas before the electrochemical experiments. 2.2.3. Gold Disk Electrode The gold (Au) disk electrode was pretreated by mechanical polishing with decreasing grades (1.0, 0.3, and 0.05 μm) of alumina powder (Buehler Limited, IL). The polishing slurry was prepared by mixing alumina powder with ultrapure water. After each polishing step, the Au electrode was ultrasonically cleaned in ultrapure water for 20 min to remove any polishing debris. As a final pretreatment, the polished electrode was immersed in ultrapure isopropyl alcohol 24 (distilled and stored over activated carbon) for 20 min. The electrodes were then immediately used for the electrochemical measurements after purging with N2. 2.3. Instrumentation 2.3.1. Visible Raman Spectroscopy Visible Raman spectra were obtained using a Renishaw inVia Raman microscope equipped with a Nd:YAG laser source. The excitation wavelength was 532 nm. All the specimens were exposed to 2.25 W of laser power (45 W max. power laser with 5% of this focused at the specimen surface) and the radius of the spot was ~ 3µm. The power density of the laser spots was calculated using the laser power and spot size, which is 787 kWmm-2. The integration time for the experiment was 9 s. Each spectrum was generated from an average of 3 spectral acquisitions at each point. 2.3.2. Scanning Electron Microscopy (SEM) The morphology of the ta-C, ta-C:N, and BDD electrodes were analyzed using JEOL 6610LV and 7500F field-emission scanning electron microscopes (JEOL USA Inc., Center for Advanced Microscopy, MSU). The secondary electrons were used to construct micrographs. All micrographs were collected using an accelerating voltage of 5 kV and at a working distance of 4.5 mm. 2.3.3. Optical Digital Microscopy Optical digital micrographs of glassy carbon electrodes were collected using a Keyence VHX-600 microscope at magnifications of 50 and 100×. The microscope uses a depth of defocus method to determine three-dimensional depth information by analyzing the defocusing level in two-dimensional images. 25 2.4. Electrochemical Characterization 2.4.1. Electrochemical Cell Setup All electrochemical experiments were conducted in a three-electrode, single-compartment glass cell (Figure 2.1). The working electrode (ta-C, and ta-C:N, etc.) was clamped to the bottom of the cell for mounting against an O-ring. Electrical contact was made to the backside of the electrode after scratching with SiC 800 grit paper and cleaning to ensure low resistance ohmic contact with a copper foil current collector. The scratched backside area was cleaned and coated with a layer of carbon using a pencil. The geometric area of the electrode exposed to the electrolyte solution was 0.2 cm2 defined by a Viton O-ring between the working electrode and cell. In both organic and RTIL electrolyte solutions, a Pt flag was used as the counter electrode and an Ag quasi-reference (Ag QRE) electrode was used. For RTILs, all electrochemical experiments were performed at room temperature (~23 oC) inside an N2-purged glove box (Coy Laboratory Products, MI). Electrodes and cells were exposed to the N2 environment for 3 h before any electrochemical testing. Throughout the experiments, the RTILs were blanketed with Ar gas (99.99% Praxair Inc. to prevent any moisture incorporation. Additionally, to remove any dissolved oxygen in the organic electrolyte media, the solutions were degassed with N2 gas for 20 min. Figure 2.1. The three-electrode, single-compartment glass cell. 26 2.4.2. Cyclic Voltammetry Cyclic voltammetry is a widely used electrochemical technique to obtain information about redox potentials, electron-transfer kinetics, and molecular diffusion.7-9 In this technique, a linearly scanned potential is applied to the working electrode and the resulting current is monitored.10 The applied potential is swept linearly with time in a triangular waveform (Figure 2.2) between E1 and E2 at a known scan rate (ν), while the current response is recorded as a function of potential. Upon reaching E2, the potential is swept in the reverse direction between E2 to E1, and the current is measured. To get accurate information from cyclic voltammetry, the working electrode needs to be properly prepared, and the electrolyte should be inert and possess high ionic strength. Figure 2.2. The waveform for a cyclic voltammetric measurement. The potential is scanned from E1 to E2 at a scan rate (V/s) of ν. All the cyclic voltammetric measurements were performed using a Model 660B electro- chemical workstation (CH Instruments, Austin, Texas). Cyclic voltammograms were recorded as a function of scan rate from 0.10 to 0.50 Vs-1 in organic electrolyte and 0.025 to 0.25 Vs-1 in RTIL media. The dimensionless parameter, Ψ, was plotted against the ΔEp for a given redox system to 27 produce a working curve. ko app was then determined by applying the slope of the graph, Ψ vs. ν - 1/2 to the equation below.11 𝛹 = 𝑘𝑜 [ 1 2 ] 𝜋𝐷𝑛𝑓𝑣 𝑅𝑇 (1) All the cyclic voltammetric ΔEp values were corrected for uncompensated iR using the equivalent series resistance as determined from high frequency electrochemical impedance spectroscopy measurements. Here, “D” is the diffusion coefficient of the redox species, “ν” is the scan rate (V/s), “n” is the number of electrons transferred per molecule, “R” is the ideal gas constant, “T” is the temperature in Kelvin, “F” is the Faraday constant and “ψ” is the Nicolson parameter described by Nicholson.11 2.4.3. Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) is employed to measure the impedance (Z) of a system to obtain information about the electrode-electrolyte interface.12 This technique is useful in research areas including corrosion studies, battery research, sensors, and coatings.13-15 In this technique, a small amplitude-altering current (AC) voltage signal is applied over a range of frequencies, and the resulting current response is measured. Then “Z” is calculated using the ratio of the applied voltage to the measured current. EIS data are commonly represented using equivalent circuit models. The equivalent circuit is the simplest model (Figure 2.3) that is widely used to interpret EIS data. Figure 2.3. The equivalent circuit was used to fit the impedance data.16 28 This model consists of three elements: Rs, which is the bulk electrolyte resistance and electrode ohmic resistance, Rp which represents the polarization resistance and CPE is the constant phase element. EIS was performed over a range of DC potentials ranging from -0.6 to 0.6 V vs. Ag QRE. The frequency was spanned 0.1 to 105 Hz with 20 data points collected per decade and an equilibration time of 200 s. EIS data were analyzed using Randle’s equivalent circuit shown in Figure 2.3. 2.4.4. Chronoamperometry Chronoamperometry is a technique used in electrochemistry to provide information about the electron-transfer kinetics and molecular diffusion of an electrochemical system.17-19 Chronoamperometry is performed by applying a constant potential to the working electrode and observing the resulting current flowing through the working electrode as a function of time (Figure 2.4). Figure 2.4. The potential is increased from E1 to E2. Figure 2.4. The potential is increased from E1 to E2. All Chronoamperometric experiments were conducted in a three-electrode, single- compartment glass cell (Figure 2.5) 29 Figure 2.5. The three-electrode, single-compartment glass cell. Chronoamperometric measurements were conducted to determine the diffusion coefficient of Fc. The potential was applied from 0.1 to 0.6 V vs. Ag QRE, a potential at which the Fc oxidation reaction is diffusion controlled. 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Electroanalytical chemistry research developments. Nova Publishers. 19. Heerman, L. and Tarallo, A "Theory of the chronoamperometric transient for electrochemical nucleation with diffusion-controlled growth." Journal of Electroanalytical Chemistry 470, no. 1 (1999): 70-76. 20. Wang, H., Sayed, S.Y., Luber, E.J., Olsen, B.C., Shirurkar, S.M., Venkatakrishnan, S., Tefashe, U.M., Farquhar, A.K., Smotkin, E.S., McCreery, R.L. and Buriak, J.M. "Redox flow batteries: how to determine electrochemical kinetic parameters." ACS nano 14, no. 3 (2020): 2575-2584. 21. Mendoza, S., Bustos, E., Manríquez, J. and Godínez, L.A. "Voltammetric techniques." Agricultural and Food Electroanalysis (2015): 23-48. 22. Raeisi-Kheirabadi, N., Nezamzadeh-Ejhieh, A. and Aghaei, H. "Cyclic and linear sweep voltammetric studies of a modified carbon paste electrode with nickel oxide nanoparticles toward tamoxifen: effects of surface modification on electrode response kinetics." ACS omega 7, no. 35 (2022): 31413-31423. 32 CHAPTER 3. ELECTROCHEMICAL INVESTIGATIONS OF ANTHRACENE DERIVATIVES AT NOVEL CARBON ELECTRODES IN ORGANIC ELECTROLYTES SOLUTIONS AND ROOM TEMPERATURE IONIC LIQUIDS 3.1. Introduction Carbon is a widely used as an electrode material in electroanalytical chemistry due to its low cost, high mechanical strength, wide working potential window, chemical inertness, and compatibility under various reaction conditions.1-5 Among the available carbon electrodes, tetrahedral amorphous carbon (ta-C) electrodes have been found to exhibit some interesting properties, making them attractive for electroanalytical applications.6-9 ta-C electrodes consist of a mixture of randomly arranged sp2, and sp3 bonded carbon that can be tailored to achieve desired properties for a particular application.10,11 The sp3 content in ta-C ranges from 40 to 85 % (relative to the total carbon content), depending on the deposition conditions. As a result of the high sp3 content, ta-C possesses diamond-like properties such as high chemical inertness, hardness, and high optical transparency. Additionally, these films can be deposited at or near room temperature on various substrate materials.12,13 The properties of ta-C electrodes are dependent on both sp2 and sp3 carbon bonding. For instance, sp2-bonded carbon in ta-C, increases the π electronic states within the bandgap, thereby enhancing the electrical conductivity of the film.14-16 In contrast, sp3 carbon provides diamond-like properties to the films, as it forms four strong sigma bonds with the adjacent carbon atoms. Typically, sp3 content is quite high in ta-C electrodes and reaches up to 85% depending on the deposition method.17-20 The deposition methods can also affect the hardness and band gap of the ta-C film. Interestingly, all the electronic and mechanical properties can be tailored by adjusting the sp3: sp2 carbon ratio as well as the hydrogen content of the films.14 Moreover, the amount and 33 type of bonding of carbon can be altered during film production, which determines the physical, optical, and electrochemical properties of these films.21-23 ta-C tends to possess low conductivity compared to the other sp2 containing carbon materials due to its high amount of sp3 content in the film results in more localized electrons and less electron mobility. The lower conductivity raises some challenges for electrochemical applications. The conductivity of ta-C can be increased by doping with impurities like nitrogen, phosphorus, and fluorine etc.24-26 Doping alters its electrochemical, optical and mechanical properties making doped tetrahedral amorphous carbons by introducing new elements or functional groups that modify its electronic structure and bonding, making it suitable for various applications, including biomedical uses, electroanalysis and coating.24-26 Among these doped amorphous carbon electrodes, nitrogen-incorporated tetrahedral amorphous carbon is well-known for its high conductivity and high mechanical stability. The sp2:sp3 ratio in the film can be modulated by varying the nitrogen content which typically provides the n-type semiconducting properties to the material.27,28 Incorporating nitrogen increases the amount of sp2 carbon in the film, adding more π electronic states and leading to improved conductivity that enhances outer- sphere redox reactions on ta-C:N electrodes.6,29 However, excessive amounts of nitrogen leads to the formation of trigonally bonded sp2-bonded carbon structure, such as pyridine and pyrrole-like structures, which decreases conductivity of the film.29 Findings from our group indicate that electrical resistivity decreases with increasing nitrogen content in the film, which can be attributed to increased formation of sp2 carbon bonding with increasing nitrogen content.29,30 More importantly, incorporating a heteroatom into the film during deposition can significantly impact its electrochemical properties such as the working potential window and electron transfer rate.30,31 Recent studies have reported that the electron transfer of redox species 34 in the aqueous media can be significantly altered by controlling the surface wettability of the carbon film, which is directly related to the surface chemistry and to the functional groups on the carbon film.16 Furthermore, Behan et al suggested that there is a drastically different electrochemical behavior between outer-sphere redox molecules and inner-sphere redox probes at ta-C electrodes. Their findings suggested that the reaction kinetics of outer- sphere redox molecules increased with increasing nitrogen content, while an opposite trend was observed for inner sphere redox probes.17 Most previous publications on ta-C electrodes focused in aqueous media, with few investigations dedicated to redox system in the non-aqueous media.10,27,28 It is necessary to understand electron transfer and double layer formation in organic electrolytes and ionic liquids at ta-C and ta-C:N electrodes to advance technology in energy storage, catalysis, corrosion protection and sensor technology. This work aims to understand the double layer formation and electron transfer of anthracene derivatives at ta-C and ta-C:N electrodes. The comparison provides insight into the influence of carbon microstructure on the electron transfer kinetics of out sphere redox probes in non-aqueous media and will contribute to a comprehensive understanding and development of suitable carbon film electrode materials for electroanalysis in non-aqueous environments. 3.2. Experimental Methods 3.2.1. Reagents Chemical reagents used were 9-phenylanthracene (9-PA) (Sigma Aldrich, 98%), 9- choloroantracene (9-CA) (Sigma Aldrich, 98%), 9-nitroantrcene (9-NA) (Aldrich,98%), anthracene (AN) (Sigma Aldrich,98%), supporting electrolyte tetrabutylammonium hexafluorophosphate (TBAPF6; Aldrich, electrochemical grade, >99%), acetonitrile (Fisher 35 Scientific, ≥ 99%), and propionitrile (Fisher Scientific ≥ 99%). All organic solvents were distilled and dried over activated molecular sieves (Linde 5 Å, Aldrich) for several days before use. All the solutions contained 0.10 mol L-1 TBAPF6 as the supporting electrolyte and were purged with nitrogen to remove dissolved oxygen prior to measurement. All experiments were performed at room temperature, 23-25 oC. 3.2.2 Room Temperature Ionic Liquids 1-Ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] (≥ 97.0%), 1-butyl-3- methylimidazolium tetrafluoroborate [BMIM][BF4] (≥97.0%), and 1-hexyl-3-methylimidazolium tetrafluoroborate [HMIM][BF4] (≥ 97.0%) were purchased from a commercial source, Iolitec (Germany). As the initial step in the purification process, RTILs were stored over activated carbon for three days. The RTIL was then removed by passing through a syringe filter with a pore size of 0.22 μm (Whatman) to remove residual activated carbon. Then, the RTILs were stored in a sealed glass vial over activated 5 Å molecular sieves for 5 days. Lastly, the RTILs were added into the electrochemical cell and then heated to 70 °C for 50 min while purging with ultrapure argon (99.99%). The argon was passed through a desiccant system to remove moisture before being introduced into the nitrogen purged-glove box. To maintain a low moisture content, the nitrogen making up the environment in the nitrogen-purged glove box was also passed through a desiccant system. After the purification, the water content in the RTILs were assessed qualitatively using cyclic voltammetry by observing the background current and potential window. All glassware was cleaned with ultrapure water, followed by two sequential rinses with isopropanol and ultrapure water. Finally, the glassware was dried in an oven at 150 oC for 12 h and transferred to the nitrogen purged glove box. 36 3.2.3. The Preparation of ta-C and ta-C:N Electrodes The ta-C:N films were deposited on a Si (111) substrates (Virginia Semiconductor, Fredericksburg, VA; ~ 0.001 Ω-cm) using a Laser-Arc physical vapor deposition system at the Fraunhofer Center for Coatings and Diamond Technologies, Michigan State University (MSU). The deposition process utilizes laser-controlled, high-current cathodic vacuum arc deposition.1,2 During the process, a pulsed laser beam scans across a rotating high-purity graphite target functioning as the cathode. The substrate was placed approximately 30 cm away from the target at a system temperature below 100 oC. A discharge is generated with each laser pulse that is comprised of highly ionized carbon atoms and small carbon atom clusters that are accelerated toward the substrate (grounded). Nitrogen incorporation was achieved by flowing nitrogen gas through the reaction chamber at flow rates of 10 and 30 standard cubic centimeters per minute (sccm) with a laser pulse rate of 350 Hz and a peak arc current >100 A during the film deposition. The resulting ta-C:N film thickness was approximately 200-400 nm with a hardness ranging from 20-60 GPa. The ta-C:N electrode was soaked in ultrapure isopropanol for 20 minutes and fully dried under a stream of nitrogen gas before an electrochemical experiment. 3.2.4. Electrochemical Measurements 3.2.4.1. Electrode Preparation ta-C:N and ta-C electrodes were soaked in ultrapure (distilled and soaked over activated carbon) isopropanol for 20 min prior to an experiment. All measurements were performed at room temperature (23-25 oC). To remove dissolved oxygen from the organic solvents purged with nitrogen gas for 20 minutes. 37 3.2.4.2. Cyclic Voltammetry All the cyclic voltammetric measurements were performed using a Model 660B electro- chemical workstation (CH Instruments, Austin, Texas). A three-electrode setup was used, with a graphite rod as the counter electrode and an Ag QRE as the quasi reference. The ta-C or ta-C:N thin film served as the working electrode. A single-compartment, three-electrode glass cell was used for all the measurements. The working electrode was pressed against a Viton O-ring and clamped to the bottom of the glass cell. Ohmic contact was made by pressing a copper plate against the backside of the scratched and cleaned ta-C and ta-C:N film/Si substrate. For the electrodes, a layer of graphite from a pencil was applied to the backside of the Si substrate to ensure good ohmic contact with the copper current collector. Cyclic voltammograms (CV) were recorded as a function of the scan rate from 0.1 to 0.5 Vs-1, in different organic solvents; acetonitrile and propionitrile containing 0.1 mol L-1 TBAPF6 and three different ionic liquid; [EMIM][BF4], [BMIM][BF4], and [HMIM][BF4]. The capacitances were calculated from the slope of the background current vs scan rate plots ranging from -0.8 to 0.8 V using equation 1.32 j = Cdl v (1) In which j (Acm-2) is the average background current density for the positive-going and negative-going sweeps at a specific potential, ‘Cdl’ (F) is the double-layer capacitance, and v (Vs- 1) is the scan rate. These calculations were repeated for a series of potential scans starting from - 0.8 to 0.8 V. The potential dependent capacitance for ta-C and ta-C:N electrodes was measured in acetonitrile and propionitrile. The electron-transfer kinetics of the anthracene redox systems was studied at ta-C and ta- C:N electrodes in acetonitrile and propionitrile containing 0.1 mol L-1 TBAPF6 as the electrolyte. The anthracene redox systems were used, 9-phenylanthracene, 9-chloroanthracene, 9- 38 nitroanthracene, and anthracene. CVs were recorded as a function of scan rate at 0.1, 0.2, 0.3, 0.4, and 0.5 Vs-1. Peak current (ip), peak potential (Ep) and half wave potential (E1/2) were recorded at each scan rate. The effect of ohmic resistance was determined by measuring the oxidation peak potential, Ep ox for the different redox analytes at 0.1 Vs-1. The diffusion coefficients were calculated based on the slope of the plot of the current vs (scan rate)1/2 and using the equation is given below (Equation 2).32,33 ip = 2.99 × 105 n A C*D1/2 ν1/2 (2) ‘ip’ is the oxidation peak current ‘n’ is the number of electrons, “D” is the diffusion coefficient, “A” is the area of the electrode, “C” is the concentration of redox analyte and “v” is the scan rate.32 3.2.5. Scanning Electron Microscopy (SEM) The morphology of the ta-C and ta-C:N electrodes was studied using a JEOL 7500F field- emission scanning electron microscope (JEOL USA Inc., Centre for Advanced Microscopy, MSU). Secondary electrons were used to construct the micrographs. All micrographs were collected at 60,000 × magnification using an accelerating voltage of 5.0 kV and a working distance of 4.5 mm. SEM micrographs were collected at multiple sites on a ta-C:N or ta-C film to assess the uniformity of the morphology. 3.2.6. Raman Spectroscopy The microstructure of the ta-C and ta-C:N films was investigated using Raman spectroscopy. Raman spectra were collected using a Renishaw inVia Raman microscope equipped with a Nd:YAG laser (532 nm). Spectral data were acquired with commercial software (Wire Interface). All specimens were exposed to 0.4 W of laser power and the spot radius was ~ 3 µm. The integration time for each spectral acquisition was 9 s. Each spectrum was generated from an average of three spectral acquisitions at each point. 39 3.3. Results 3.3.1. Surface Characterization of ta-C and ta-C:N Electrodes 3.3.1.1. Raman Spectroscopy A B 1600 cm-1 “G” Band 1600 cm -1 “G” Band 1300 cm-1 “D” Band Figure 3.1. Raman spectra of (A) ta-C and (B) ta-C:N (30) films. Figure 3.1 presents the Raman spectra of ta-C and ta-C:N (30) films obtained using visible excitation. The (30) indicates the flow rate of nitrogen gas (sccm) during the film deposition. A prominent Raman band observed at around 1600 cm-1 corresponds to the E2g vibration mode and is known as the G-band. A weak Raman band observed around 1300 cm-1 exclusively for ta-C:N (30), is commonly referred as the D-band, corresponds to the A1g vibrational mode of disordered carbon atoms. Published work shows that increased nitrogen content reduces the width of the G- band and shifts in its position towards lower wavenumbers.23 These trends are attributed to the enhanced sp2 carbon content.24 While the G -band position remains unchanged, Figure 3.1 clearly indicates that the width of G-band reduces with nitrogen addition, which consists with previously published work. 40 3.3.1.2. Scanning Electron Microscopy A B Figure 3.2. SEM micrographs of (A) ta-C and (B) ta-C:N (30) films grown on Si. Figure 3.2. shows SEM micrographs of ta-C and ta-C:N (30) thin-film electrode. The films exhibit a nodular morphology, with carbon clusters visible on the surface. These clusters are 50- 150 nm in size and are formed during the Laser Arc pulses used for film deposition.34 Due to the absence of mass filtering in the Laser Arc system, so these carbon clusters are not removed and are accelerated toward the substrate surface and become part of the growing film.35 In contrast, these films deposited with mass filtering tends to be much smoother surface. The EDS analysis was conducted to determine the composition of these clusters. The EDS spectra (not reported here) confirmed that these clusters are composed of carbon. 3.3.2. Electrochemical Studies of ta-C and ta-C:N Electrodes in Organic Solvents 3.3.2.1. Double-Layer Capacitance of ta-C and ta-C:N Electrodes in Organic Solvents Figure 3.3 presents the background cyclic voltammetry curves for ta-C and ta-C:N films in acetonitrile, illustrating the dependence of background current increased on scan rate. Comparison of the background currents for the three ta-C electrodes demonstrated a clear increase 41 in background current as nitrogen content was increased. One explanation is that the increasing nitrogen level leads to higher background current due to an increase in sp2 carbon content in the film.36-38 A higher sp2 carbon percentage within films, results in a greater density of states (DOS), which enhanced background current. An additional contributing factor is that background current is directly proportional to the surface area of the electrode. Nitrogen incorporation induces surface roughness in the films, thereby enhances the background current.37,38 A B Forward Scan Forward Scan Figure 3.3. (A) Background cyclic voltammetric i−E curves as a function of scan rates from 0.1 to 0.5 Vs-1 for ta-C:N (30) (B) Comparison of background cyclic voltametric i−E curves for ta- C:N (30), ta-C:N (10), and ta-C in acetonitrile with 0.10 mol L-1 TBAPF6 at scan rate of 0.1 Vs- 1. T = 298 K. Figure 3.4 presents the variation in background cyclic voltammetry curves of ta-C and ta- C:N (30) films in acetonitrile and propionitrile. This comparison clearly highlights the influence of nitrogen doping levels within the film and dielectric constants of the solvent on the background voltammetric currents. The data indicate that the effect of nitrogen content in the films on background current is more pronounced than the effect of dielectric constant of organic solvent. The observed similarity in background currents for both solvents is attributed to their comparable dielectric constants. 42 Forward Scan Figure 3.4. Comparison of background cyclic voltammetric i−E curves for ta-C:N (30), and ta- C in acetonitrile and propionitrile with 0.10 mol L-1 TBAPF6 at a scan rate of 0.1 Vs-1.T = 298 K. Figure 3.5. A plot of corresponding current at 0 V vs. Ag QRE against scan rate for ta-C, ta- C:N (10), and ta-C:N (30) thin-film electrodes in acetonitrile with 0.10 mol L-1 TBAPF6. Current values are presented as mean ± std. dev. for n = 3. 43 The double-layer capacitances of the electrodes were calculated from the slope of the current vs. scan rate plots (Figure 3.5). For the three electrode materials, the average double-layer capacitance ranges from 17 to 26 μFcm-2 in both acetonitrile and propionitrile. The double-layer capacitance in non-aqueous solvents is lower than in aqueous solutions, primarily due to the higher dielectric constant of water and lower supporting electrolyte concentration (0.10 mol L-1) used in this experiment.28 Furthermore, this difference confirms the direct impact of dielectric constant on the formation of the electric double layer. A B Figure 3.6. Comparison of capacitance vs. potential (C-E) trends recorded for ta-C, ta-C:N (10), and ta-C:N (30) thin-film electrodes in (A) propionitrile and (B) acetonitrile with 0.10 mol L-1 TBAPF6. Data are presented as mean ± std. dev. for n = 3. The double layer capacitance values are higher at negative potential and lower at positive potentials suggesting potential dependent nature in the capacitance of ta-C films. This trend is consistent in both acetonitrile and propionitriles. Although nitrogen doping does not alter the overall trend, it largely affects the capacitance values, particularly at higher doping levels. While Figure 3.6 does not clearly indicate variation of capacitances with nitrogen doping, Figure 3.7 clearly demonstrates that the capacitance of ta-C films in organic electrolytes is strongly 44 influenced by the nitrogen doping. This difference is attributed to higher density of state (DOS) and increased surface area of ta-C films with nitrogen incorporation. Further, the higher double layer capacitance of ta-C:N (30) films suggests that their greater conductivity in organic electrolyte/ solvent media compared to ta-C electrodes. A B Figure 3.7. Comparison of capacitance vs. potential (C-E) trends recorded for ta-C and ta-C:N (30) thin-film electrodes in (A) propionitrile (B) acetonitrile with 0.10 mol L-1 TBAPF6. Data are presented as mean ± std. dev. for n = 3. 3.3.2.2 Electrochemical Studies of Anthracene Derivatives at ta-C and ta-C:N Electrodes in Organic Solvent Systems Substituted anthracenes were selected for this work due to their well-characterized-outer- sphere heterogeneous electron-transfer kinetics in non-aqueous media. Figure 3.8 illustrates the typical cyclic voltametric response of each anthracene derivative in acetonitrile containing the 0.1 mol L-1 TBAPF6 as the supporting electrolyte. All four-anthracene derivatives displayed clear oxidation peaks when scanning towards positive potentials at all scan rates. However, the corresponding reduction peaks in the reverse scans were less pronounced, consistent with an EC mechanism, in which the initial electron transfer is followed by a subsequent chemical reaction 45 step.38-42 This chemically irreversible nature of the anthracene derivatives is linked to the instability of cation radical formed during the forward sweep. (scheme 1). Scheme 1. Oxidation of anthracene forming a cation as an intermediate product. 37,38 The oxidation potential (Ep) of the anthracene derivatives increases in the following order, Ep ox of 9-PA (9-phenylantracene) < Ep ox of AN (anthracene) < Ep ox of 9-CA (9-choloroanthracene) < Ep ox of 9-NA (9-nitroanthracene) The stability of the cation radical generated during the initial oxidation was strongly influenced by the nature of the functional group attached to the anthracene ring. While electron donating groups stabilize the cation radical, electron withdrawing groups destabilize it. Greater cation stability corresponds to less positive oxidation potential.43 The stability of the cation radical increases in the following order, 9-PA > AN > 9-CA > 9-NA The high stability of the intermediate radical of 9-PA imparts some chemical reversibility, as reflected in the appearance of reduction peaks in the CV. In contrast, the other anthracenes demonstrate chemical irreversibility, attributed to the formation of unstable cation during oxidation. Furthermore, the oxidized products, electrochemically inactive species that do not reduce back to original anthracenes under these experimental conditions.43 This suggests that it is important to understand how formed cation radical interact with other substances in various experimental conditions is essential. 46 A B Forward Scan Forward Scan C D Forward Scan Forward Scan Forward Scan Figure 3.8. Cyclic voltammetric i−E curves for 0.1 mmol L-1 (A) 9-chloroanthracene (B) 9- nitroanthracene, (C) anthracene and (D) 9-phenylanthracene in 0.1 mol L -1 TBAPF6 at a ta-C:N (30) electrode in acetonitrile with 0.10 mol L-1 TBAPF6. Curves were recorded at scan rates from 0.1 to 0.5 Vs-1. Figure 3.9 presents a comparison of the CV data of 0.10 mmol L-1 of anthracene in acetonitrile at three distinct ta-C electrodes; ta-C, ta-C:N (10) and ta-C:N (30) respectively. The data suggest that the oxidation peak potential of anthracene is minimally affected by the nitrogen content in the ta-C film, indicating that the electrochemical oxidation of anthracenes does not depend on microstructure of the ta-C electrodes. 47 Forward Scan Figure 3.9. Comparison of cyclic voltametric i-E curves for 0.10 mmol L-1 anthracene in acetonitrile with 0.10 mol L-1 TBAPF6 as the supporting electrolyte at ta-C, ta-C:N (10) , and ta- C:N (30) at a scan rate of 0.1 Vs-1. T = 298 K. Table 3.1 summarizes the peak potential and corrected oxidation peak potential for the anthracene derivatives in acetonitrile. Corrections were applied for uncompensated ohmic resistance in the system, which can affect the peak positions and peak splitting (ΔEp). Uncompensated, or ohmic, resistance can alter volumetry curves in two different ways (i) by shifting oxidation peaks to appear more positive and reduction potential of a curve reduction peaks appear more negative, and (ii) by decreasing the peak current density. After obtaining the ohmic resistance (R) for the cell, assumed to be mainly the solution resistance, the correction for peak potentials were made using equation (3). These corrected EP values are used for further analysis.21 Ep, corr = Ep - (ip × R) (3) After calculating the EP ox (corrected) values for 9-PA, 9-CA, 9-NA, and AN at ta-C and ta-C:N (10 and 30) are given in the table below. 48 Table 3.1. Experimental cyclic voltammetric data for 0.10 mmol L-1 anthracene (AN), 9-phenyl anthracene (PA), 9-chloroanthracene (CA), and 9-nitroantracene (NA) at ta-C:N (30), ta-C:N (10), and ta-C electrodes in acetonitrile. Data are presented as mean ± std. dev. for n=3 electrodes. Anthracene Derivatives Electrodes EP ox (V vs. ox EP (V vs. Ag QRE) Ag QRE) (iR corrected) ta-C 1.31 ± 0.03 1.30 ± 0.03 AN ta-C:N (10) 1.29 ± 0.03 1.29 ± 0.03 ta-C:N (30) 1.27 ± 0.03 1.27 ± 0.03 ta-C:N 1.27 ± 0.02 1.26 ± 0.02 9-PA ta-C:N (10) 1.25 ± 0.03 1.24 ± 0.03 ta-C:N (30) 1.25 ± 0.04 1.24 ± 0.04 ta-C 1.55 ± 0.03 1.54 ± 0.03 9-CA ta-C:N (10) 1.52 ± 0.03 1.50 ± 0.03 ta-C:N (30) 1.47 ± 0.03 1.46 ± 0.03 ta-C 1.75 ± 0.03 1.73 ± 0.03 9-NA ta-C:N (10) 1.74 ± 0.04 1.72 ± 0.04 ta-C:N (30) 1.73 ± 0.03 1.71 ± 0.03 Table 3.1 indicates that the feasibility of the electrochemical oxidations of anthracene on ta-C films remains largely unaffected by increasing sp2 content in the films. This means that any surface nitrogen sites are not exclusively involved in the redox reaction and that all films are sufficiently electrically conducting such ohmic resistance effects within films are not influencing the voltammetric peak position or shape. The EP ox value increases as the stability of the oxidized 49 product decreases, providing a good indication of the stability of the anthracene derivatives during the oxidation process. EP ox is the potential required to oxidize a molecule, which indicates the tendency of the molecule to undergo oxidation, and this directly related to the activation energy barrier for electron transfer process. A larger energy barrier corresponds to a higher oxidation potential, resulting in sluggish electron transfer kinetics and vice versa. To extract more information from the cyclic voltammograms, plots of the oxidation peak currents as a function of the scan rate,1/2 were generated following equation (2). The equation applies to electrochemical reactions where the rate is diffusion-controlled, and that the diffusion is linear (time dependent) in one direction. In such a situation, the equation predicts that the peak current will increase linearly with the scan rate1/2.32 Slope of the plots (current vs. ν1/2) = 2.99 × 105 n A C*D1/2 (4) Table 3.2 presents the calculated diffusion coefficients for all anthracene derivatives. The radius of each anthracene derivative was then determined using the calculated diffusion coefficient values and the Stokes-Einstein equation given below.31 𝐷red = 𝑘𝐵𝑇/ 6𝜋𝑎η (5) “Dred” is the diffusion coefficient of molecules, “kB” is the Boltzmann constant, “T” is the temperature (K), “a” is radius of the molecule and “η” is the viscosity of the medium. Radius of anthracenes were calculated using the Stokes-Einstein equation. 50 Table 3.2. Calculated diffusion coefficients and radii of anthracene derivatives in acetonitrile with 0.10 mol L-1 TBAPF6. Anthracene Derivatives Dred (cm2/s) × 10-5 44 Reported Dred (cm2/s) × 10-5 Radius of Anthracenes (nm) AN 1.11 ± 0.13 2.50 ± 0.20 1.18 ± 0.15 Reported Radius of Anthracenes (nm)44 0.38 ± 0.03 9-PA 0.91 ± 0.02 1.72 ± 0.14 1.43 ± 0.03 0.54 ± 0.04 9-CA 1.03 ± 0.01 2.30 ± 0.25 1.27 ± 0.02 0.41 ± 0.03 9-NA 0.99 ± 0.07 2.39 ± 0.15 1.31 ± 0.10 0.39 ± 0.03 Data are presented as mean ± std. dev. for n = 3. Although the calculated diffusion coefficients are slightly lower than those reported in literature, they fall within the expected range and follow the anticipated trend.44 This minor difference in diffusion coefficients is attributed to the deviation in the Randles-Sevcik equations for reaction undergoing an EC mechanism. Since these reactions involve chemical steps, as this step influences the concentration of reactive of reactive species, thereby complicating the current potential (i-E) curve response. Specifically, if the chemical step is rate-limiting, the reaction may deviate from diffusion controlled. As a result of that the radii of anthracenes were calculated using the diffusion coefficients, show a clear deviation from those reported in the previous work. The diffusion coefficient (D) for anthracene in acetonitrile derivatives increases in the following order: diffusion of AN > 9-CA > 9-NA > 9-PA. The radii of these anthracene derivatives follow the opposite trend as radius of AN < 9-CA < 9-NA < 9-PA. This consists with theoretical expectations, as the diffusion coefficient decreases with the size of the molecule.19 51 3.3.2.3. Electrochemical Behavior of Anthracene Derivatives at ta-C and ta-C:N Electrodes in Various Organic Solvents A B Forward Scan Forward Scan Forward Scan Forward Scan Figure 3.10. Cyclic voltammetric i−E curves for 0.10 mmol L-1 anthracene in (A) acetonitrile (B) propionitrile with 0.10 mol L-1 TBAPF6 at a ta-C:N (30) electrode. The next goal of this work is to examine the solvent effect on these electrochemical reactions using the same electrode material. Figure 3.10. presents the cyclic voltammetric data for the different anthracene derivatives at ta-C ta-C:N (10) and ta-C:N (30) electrodes in propionitrile. When comparing this with the data for acetonitrile, it is evident that the EP ox values are similar in both solvents. This trend was consistent across all the anthracene derivatives. Considering that both solvents have similar dielectric constants and viscosities, the EP ox values and peak current values in propionitrile closely resemble those in acetonitrile. Figure 3.11 shows that the half-peak and the peak potential are similar and follow the same trend in both organic solvents. Further studies can be performed to investigate changes in the electrochemical behavior of anthracenes by using the arenes possess significantly different polarity and viscosity. 52 A A B ox of anthracene derivatives at ta-C, ta-C:N Figure 3.11. Summary of oxidation peak potentials, EP (10), and ta-C:N (30) electrodes in (A) acetonitrile and (B) propionitrile with 0.10 mol L-1 TBAPF6. Data are presented as mean ± std. dev. for n = 3. Table 3.3. Calculated diffusion coefficients of anthracene derivatives in acetonitrile and propionitrile with 0.10 mol L-1 TBAPF6. Redox Probes Dred in CH3CN Dred in CH3CH2CN AN 9-PA 9-CA 9-NA (cm2/s) × 10-5 (cm2/s) × 10-5 1.11 ± 0.10 1.07 ± 0.06 0.91 ± 0.02 0.89 ± 0.05 1.03 ± 0.01 1.02 ± 0.07 0.99 ± 0.07 0.95 ± 0.04 Data are presented as mean ± std. dev. for n = 3. Table 3.3 presents the diffusion coefficients of anthracene derivatives in the two organic solvents, acetonitrile and propionitrile. Although, the diffusion coefficients follow a similar a trend in both solvents, molecular diffusion is higher in acetonitrile compared to propionitrile due to 53 lower viscosity of acetonitrile, which is attributed to the difference in alkyl chain length between the solvents and a similar trend was observed by Compton an et al. in their work.44 3.3.3. Electrochemical Studies of ta-C:N (30) Electrodes in RTILs 3.3.3.3.1. Double-Layer Capacitance of ta-C:N (30) Electrode in RTILs A B Forward Scan Forward Scan Forward Scan Forward Scan Figure 3.12. (A) Background cyclic voltammetric i−E curve in [EMIM][BF4] at ta-C:N (30), (B) Comparison of background cyclic voltametric i−E curves at a ta-C:N (30) in [EMIM][BF4], [BMIM][BF4], and [HMIM][BF4]. Figure 3.12 illustrates cyclic voltammograms for a ta-C:N (30) electrode in [EMIM][BF4], demonstrating a linear increase in current with respect to the scan rates across the potentials range suggesting the current arises from charging of the electric double layer. The stable potential window indicates the absence of impurities, which could undergo oxidation or reduction within this potential window. The voltammetric curves are symmetrical and display a rectangular shape, consistent with a capacitive process. According to Figure 3.12 B, all three ionic liquids give rise to lower background currents as compared to those observed in organic solvent/electrolyte systems. Among the three ionic liquids, [HMIM][BF4] shows the lowest background current, whereas [EMIM][BF4] shows the highest current indicating the influence of properties such as size of cation and the dielectric constant of the RTILs on the double layer formation. 54 A C B D Figure 3.13. (A) A plot of the corresponding current at 0 V vs. Ag QRE against scan rate for ta-C:N (30) in [EMIM][BF4],[BMIM][BF4], and [HMIM][BF4]. Capacitance vs. potential (C- E) trends calculated for ta-C:N (30) in (B) [EMIM][BF4], (C) [BMIM][BF4], and (D) [HMIM][BF4]. Data are presented as mean ± std. dev. for n = 3. Figure 3.13 shows the potential dependence of the capacitance of ta-C:N (30) film in [EMIM][BF4],[BMIM][BF4], and [HMIM][BF4] respectively. The trends of potential-dependent capacitance reveal that the ta-C:N (30) films exhibit p-type semi-conductor characteristics. As the potential increases, the capacitance initially decreases, reaching minimum when the potential reaches zero, and then begins to slightly increase at positive potentials. Even though the magnitude of the capacitance depends on the size of the anion, the trend remains consistent across all three RTILs, regardless of their differences in cation sizes. These C-E profiles 55 are derived from the data obtained for the background current of these RTILs. 3.3.3.2. Assessment of the Electrochemical Behavior of Anthracene Derivatives at ta-C:N (30) in RTILs A B Forward Scan Forward Scan C D Forward Scan Forward Scan Figure 3.14. Cyclic voltammetric i−E curves for 0.5 mmol L-1 (A) 9-chloroanthracene, (B) anthracene, (C) 9-nitroanthracene, and (D) 9-phenylanthracene in [BMIM][BF4] at a ta-C:N (30) electrode at varying scan rates from 0.1 to 0.5 Vs-1. T = 298 K. 56 Figure 3.14. show the electrochemical behavior of 4 different anthracene derivatives at ta- C:N (30) in [BMIM][BF4]. The oxidation peak potential and current for all derivatives are higher compared to those in arenes. However, all the anthracenes are irreversible in all three ionic liquids: [EMIM][BF4], [BMIM][BF4]and [HMIM][BF4] similar to their behavior in non-aqueous media. A B Forward Scan Forward Scan Figure 3.15. Cyclic voltametric i−E curves for 0.50 mmol L-1 of 9-nitroanthracene at a ta-C:N (30) electrode in (A) [BMIM][BF4] and (B) [HMIM][BF4] at varying scan rates from 0.1 to 0.5 Vs-1. T = 298 K. The data suggests that anthracenes exhibit irreversible oxidation peaks when scanning toward positive potentials at all scan rates, following the same trend as in non-aqueous media. The oxidation peak potentials are relatively higher in RTILs compared to the non-aqueous media suggesting that more sluggish electron transfer in RTILs. The lower oxidation peak currents and higher overpotentials are attributed to the higher viscosity of RTILs relative to the non-aqueous media. Table 3.4 presents the summary of peak potentials and currents for the four different anthracene derivatives in the RTILs. The ohmic resistance of the RTILs was determined using electrochemical impendence spectroscopy (EIS), with values ranging from 820 to 2500 Ω. However, the ohmic effects on voltametric peak splitting is small and similar to non-aqueous media. 57 Table 3.4. Experimental cyclic voltametric data for 0.50 mmol L-1 anthracene, 9-phenyl anthracene, 9-chloroanthracene and 9-nitroantracene at in [EMIM][BF4], [BMIM][BF4], and [HMIM][BF4] at a scan rate of 0.1 Vs-1. Table 1. Experimental cyclic voltametric data for 0.5 mmol L-1 Anthracene,9-Phenyl ta-C:N (30) electrodes Anthracene, 9-Choro Anthracene and 9-NitroAntracene at ta-C:N (30) electrodes in (V vs. Ag QRE) EP (V vs. Ag QRE) (iR Anthracene RTILs ox EP ox [EMIM][BF4], [HMIM][BF4] and [EMIM][BF4].Scan Rate 0.1 V/s. Data are presented as Derivatives corrected) mean std. dev. for n=3 [EMIM][BF4] 1.53 ± 0.03 1.53 ± 0.02 AN [BMIM][BF4] 1.60 ± 0.03 1.57 ± 0.04 [HMIM][BF4] 1.65 ± 0.03 1.63 ± 0.04 [EMIM][BF4] 1.30 ± 0.03 1.28 ± 0.02 9-PA [BMIM][BF4] 1.34 ± 0.03 1.31 ± 0.03 [HMIM][BF4] 1.39 ± 0.04 1.35 ± 0.04 [EMIM][BF4] 1.61 ± 0.03 1.57 ± 0.03 9-CA [BMIM][BF4] 1.73 ± 0.03 1.70 ± 0.03 [HMIM][BF4] 1.78 ± 0.03 1.75 ± 0.03 [EMIM][BF4] 1.74 ± 0.03 1.71 ± 0.03 9-NA [BMIM][BF4] 1.90 ± 0.04 1.87 ± 0.04 [HMIM][BF4] 1.95 ± 0.03 1.93 ± 0.03 Data are presented as mean ± std. dev. for n = 3. There is a notable difference in the data as the Ep values for the all the derivatives are more positive versus the same Ag QRE in the RTILs than they are in the organic electrolyte systems. Some of this shift maybe be due to differences in the potential of the Ag QRE in the two different media. However, this shift is also attributed to slower kinetics or greater positive potentials needed for oxidizing the redox systems. This was the case in organic electrolytes/solvents, as the 58 stability of the oxidized product decreases, more positive potentials are required to oxidize the anthracene derivative. The stability of the oxidized products depends on the nature of the substituted groups attached to anthracene (scheme 1). According to the Randles-Sevcik equation, the peak current is linearly proportional to the scan rate1/2 for diffusion-controlled process. All the graphs have good linearity, indicating that these derivatives behave as diffusion-controlled reactions in RTILs as well. The Diffusion coefficients of the anthracenes were calculated from the slope of the current vs. (scan rate)1/2 plots (Figure 3.16 A). The calculated diffusion coefficients values range between 10-7 to 10-8 cm2/s. These diffusion coefficients increase as the radius of the radius of the anthracene molecules decreases, following the same trend observed in organic solvents. A B Figure 3.16. (A) Corresponding Randles- Sevick plots of ip vs. (scan rate)1/2 for each of the three electrodes. R2 > 0.99 (B) Diffusion coefficient vs. viscosity for anthracene derivatives in [EMIM][BF4], [BMIM] [BF4], and [HMIM][BF4] at ta-C:N (30) electrode. Data are presented as mean ± std. dev. for n = 3. sccm) electrodes. The diffusion of anthracenes in ionic liquids is significantly influenced by viscosity and molecular structure. Higher viscosity reduces the diffusion rates, consistent with the Stokes- Einstein relationship. Diffusion coefficients for anthracene are consistently lower in all RTILs 59 compared to organic solvents, primarily due to significantly higher viscosity of RTILs. Additionally, the complex structures of ionic liquids create specific interactions, such as stacking between anthracene and aromatic cations or hydrogen bonding which further modulate the diffusion in RTILs.46,47 Table 3.5. Comparison of calculated diffusion coefficients of anthracene derivatives in [BMIM][BF4]. Redox Probes Dred Reported Dred 40.48 (cm2/s) × 10-7 (cm2/s) × 10-7 AN 9-PA 0.98 ± 0.12 2.8 ± 0.80 0.48 ± 0.05 0.90 ± 0.10 Table 3.5 compares the diffusion coefficients values of anthracene derivatives (AN and 9- PA) calculated in this study with the values reported in the published works.40.48 All the calculated diffusion coefficient values are lower than those reported in the published work suggesting that higher viscosity of the of the RTILs used in this work. This increased viscosity can be attributed to the lower water content in our purified RTIL samples.48 Although the calculated diffusion coefficient values are lower, they are in the range of expected values and follow the expected trend. The diffusion coefficient of anthracene derivatives in all three RTILs increases in the following order: diffusion coefficient of AN > 9-CA > 9-NA > 9-PA. Conversely, the radii of these anthracene derivatives follow the opposite trend as radius AN < 9-CA < 9-NA < 9-PA. 3.4. Discussion This study offers new insights into the electrochemical behavior of ta-C:N and ta-C electrodes in organic electrolytes and room temperature ionic liquids. Specifically, it aims to 60 elucidate the relationship between the electrochemical properties of these electrodes in various electrolyte systems, including organic electrolyte and RTILs. The Raman spectra of ta-C and ta-C:N electrode reveal valuable insight into their microstructure. A broad asymmetric peak around 1530 cm-1 was observed, becoming more asymmetric and broader with increasing nitrogen content in the film.4 These trends are due to the restructuring of the carbon matrix, leading to an increase in the disordered sp2 carbon phase as a result of nitrogen doping.23,24 SEM micrographs of ta-C:N (30) and ta-C thin-film electrodes exhibit a distinct nodular morphology with carbon clusters (white) on the surface. These clusters are 50-150 nm in size and are formed due to the Laser Arc pulses used during film deposition.6 Previous findings from our group indicate that the overall roughness of the ta-C:N films increase with increasing nitrogen content.16 Published work indicates that the ta-C thin-film electrodes are advantageous for electroanalytical applications, lower background current and wider potential window compared to GC.7 However, it is crucial to investigate the electrochemical behavior of these anthracenes at other nano structured carbon electrodes, as this will enhance our understanding of how electrode microstructure influences the electron transfer process of these anthracene derivatives in non- aqueous media. The current density versus potential (j-E) response shows a characteristic rectangular shape, which is associated with the non-faradaic charging currents. For All three RTILs, the background current increases linearly with the scan rate. Among the three ta-C electrodes, ta-C:N (30) exhibited the highest capacitance across all potentials, due to the increased surface area and the higher sp2 carbon content resulting from nitrogen doping. The double layer capacitance is 61 higher in acetonitrile than those in propionitrile due to higher dielectric constant of acetonitrile. The average double-layer capacitance for three electrode materials ranges between 16 - 22 μFcm- 2 in both acetonitrile and propionitrile. Previous studies have demonstrated that these values are lower compared to the capacitance values obtained in aqueous solutions. All four-anthracene derivatives undergo oxidation via the EC mechanism and the reason for this mechanism is the instability of the oxidized product which resulting in an unstable cation upon oxidation (scheme 1).27,28 These monosubstituted anthracenes are also known to be reactive and tend to undergo oxidative self-dimerization. In certain cases, this dimerized product can undergo further oxidation, leading to the formation of a tetramer.28 Scheme 3.2. Oxidation of anthracene to dimer and tetramer products.28 These reactions are complex, involving multiple electron transfer and subsequent chemical steps. In this study, we focused on the simplest EC mechanism, as these anthracene derivatives are known for their single-electron transfer process.14,20 In electrochemistry, the half-wave potential obtained from a cyclic voltammogram is equal to the formal potential E0’ , even to partially irreversible reactions such as 9-PA and fully irreversible reactions, including anthracene.46 While E1/2 can be determined for 9-PA, it is not possible to obtain E1/2 for the other anthracene derivatives due to their chemical irreversibility. In such cases, the inflection potentials Ei (this can be determined by using the second derivative of the voltammogram) and the half-peak potentials provide the best estimates for E0, for lower scan 62 rates, e.g., 0.1-0.5 Vs-1. Usually Ei and E1/2 are very similar in magnitude.46,47 Based on the E1/2 value obtained from 9-PA; it can be assumed that the E0’ is approximately equal to the E1/2 value for these anthracene derivatives. The diffusion coefficients in acetonitrile increase in the following order: diffusion coefficient of AN > 9-CA > 9-NA > 9-PA while their molecular radius follow the opposite trend as radius of AN < 9-CA < 9-NA < 9-PA. This trend is expected, as the diffusion coefficient decreases increasing molecular size.31 Moreover, the diffusion coefficient of anthracene derivatives are independent from the microstructure of the ta-C films. Overall, the data suggests that electron transfer of anthracene and its derivatives is sluggish in organic solvents at amorphous electrodes, compared to the metal electrodes. This notable difference can be attributed to the difference of electronic state between amorphous carbon and metal electrodes. 3.4.1. Understanding the Electrochemical Behavior of Anthracene Derivatives at ta-C:N (30) in RTILs During electrochemical oxidation, the solvation environment surrounding anthracenes undergoes substantial changes due to the formation of an intermediate cation radical.40 In organic electrolyte solutions, the organic solvent functions as a dielectric medium to both separate opposite ions and forms solvation sheath around anthracenes, thus stabilizing the environment around the anthracenes and enhances the electron transfer process.49 Additionally, the lower viscosity of organic electrolytes enhances the electron transfer kinetics, thereby lowering the oxidation peak potentials of anthracene derivatives.49 The investigation of the anthracene oxidation in RTILs is particularly interesting due to the unique electrochemical environment created by the ion clustering, and the higher viscosity of RTILs, which substantially differ from the conventional 63 electrolytes solvent system such as organic electrolyte/solvent system.48,49 Further understanding the differences in oxidation process between these media crucial to broadening our knowledge in RTILs media. Therefore, this part of the work aims to understand how the electrochemical behavior of anthracene in RTILs differs from organic electrolytes.48,49 Despite the change in magnitude of the capacitance the trend remains similar for all three RTILs regardless of their variation of cation size. Also, the trend is consistent with the background current observed for these ionic liquids. The capacitance values for ta-C are higher than that for BDD but lower than that for GC reported in the literature.11 This variation can be attributed to the higher sp2 content in GC compared to BDD and ta-C. The amount of sp2 content in carbon electrodes increases in the following order: GC > ta-C > BDD. As demonstrated in published work, the solution environment surrounding a redox analyte in RTILs differs significantly from that in conventional electrolyte systems.47-49 Investigating the oxidation of anthracene derivatives offers further insight into how solution environment in RTILs can affect the electron transfer processes.41,42 The reaction rates of substituted anthracenes are slower in RTILs than acetonitrile or propionitrile owing to large solvent reorganization energies in RTILs than in conventional electrolyte media.49 Further, these monosubstituted anthracenes are known to be reactive and tend to undergo oxidative self-dimerization in RTILs similar to their behavior in organic electrolyte systems.50,51 Viscosity is an important property in electrochemistry because high viscosities slow down the rate of diffusion-controlled chemical reaction. According to the Stokes-Einstein equation (equation 5) the diffusion coefficient is inversely proportional to viscosity.52 The viscosity of RTILs is significantly higher than that of non-aqueous and aqueous media and dependent on the nature of both the cations and anions.53 As the alkyl chain length of the cation increases, viscosity 64 increases due to a corresponding increase in van der Waals forces. When considering the anion effect, [BF4 -] anions form more viscous ionic liquids because of strong H ··· F. Since fluorinated anions were used during this experiment, the viscosity of these media is very high.54,55 This high viscosity results in smaller currents and lower diffusion coefficients of anthracene derivatives.47 In addition to their higher viscosity, the complex structures of RTILs give rise to distinct interactions that significantly influence molecular diffusion.48 For instance, stacking interactions between anthracene molecules and cations, as well as hydrogen bonding formation create localized environments that alter the diffusion of anthracenes, depending on the nature of the RTILs and the type of anthracene molecules.49 The lower calculated diffusion coefficients of anthracenes suggest that localized environments in RTILs hinder the anthracene diffusion in RTILs.56 Overall, the lower the diffusion rate of the anthracene molecules, corresponds to a higher viscosity, complex microenvironment of RTILs and larger the hydrodynamic radius of the anthracene molecules. 3.5. Conclusions The background current of ta-C electrodes is lower compared to the ta-C:N (10) and ta- C:N (30), and has the lowest double layer capacitance in both acetonitrile and propionitrile while ta-C:N (30) has the highest. There is no considerable difference in double layer capacitance between acetonitrile and propionitrile with values ranging from 17-26 μFcm-2. The double layer capacitance of ta-C:N (30) in RTTLs depends on the size of the cation of the of RTILs. The oxidation of anthracene and its derivatives shows unstable radical-cation with slow electron transfer at ta-C and ta-C:N in acetonitrile and propionitrile at lower scan rates (0.1 - 0.5 Vs-1) under steady-state conditions. This suggests that anthracene derivatives exhibit sluggish electron transfer kinetics at ta-C:N films. 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Journal of The Electrochemical Society 166, no. 5 (2019): H3175-H3187. 44. Zh ao, Z., Li, H. and Gao, X. "Microwave encounters ionic liquid: synergistic mechanism, synthesis and emerging applications." Chemical Reviews 124, no. 5 (2023): 2651-2698. 45. Zhao, D., Xiong, Y., Wang, Y., Lu, B. and Zhang, H "Separation of anthracene and carbazole from crude anthracene via imidazolium-based ionic liquids." Fuel 331 (2023): 125704. 46. Belding, S.R., Rees, N.V., Aldous, L., Hardacre, C. and Compton, R.G "Behavior of the heterogeneous electron-transfer rate constants of arenes and substituted anthracenes in room- temperature ionic liquids." The Journal of Physical Chemistry C 112, no. 5 (2008): 1650-1657. 47. Zigah, Dodzi, Jalal Ghilane, Corinne Lagrost, and Philippe Hapiot. "Variations of diffusion coefficients of redox active molecules in room temperature ionic liquids upon electron transfer." The Journal of Physical Chemistry B 112, no. 47 (2008): 14952-14958. 48. Zigah, D., Ghilane, J., Lagrost, C. and Hapiot, P. "Electrochemical analysis in charge-transfer science: The devil in the details." Current Opinion in Electrochemistry 31 (2022): 100862. 49. Wang, W., Fan, X., Liu, J., Yan, C. and Zeng, C. "Temperature-related reaction kinetics of the vanadium (IV)/(V) redox couple in acidic solutions." RSC Advances 4, no. 61 (2014): 32405- 32411. 70 50. Miller, C.C. "The Stokes-Einstein law for diffusion in solution." Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 106, no. 740 (1924): 724-749. 51. Galiński, M., Lewandowski, A. and Stępniak, I. "Ionic liquids as electrolytes." Electrochimica Acta 51, no. 26 (2006): 5567-5580. 52. Anthony, J.L., Brennecke, J.F., Holbrey, J.D., Maginn, E., Mantz, R.A., Rogers, R.D., Trulove, P.C., Visser, A.E. and Welton, T. "Physicochemical properties of ionic liquids." Ionic liquids in synthesis (2006): 41. 53. Chiappe, C. and Pieraccini, D "Ionic liquids: solvent properties and organic reactivity." Journal of Physical Organic Chemistry 18, no. 4 (2005): 275-297. 54. Castner Jr, E.W., Margulis, C.J., Maroncelli, M. and Wishart, J.F "Ionic liquids: structure and photochemical reactions." Annual Review of Physical Chemistry 62, no. 1 (2011): 85-105. 71 CHAPTER 4. THE EFFECT OF ADDED ORGANIC SOLVENTS ON ELECTRON TRANSFER KINETICS AND DIFFUSION OF FERROCENE IN ROOM TEMPERATURE IONIC LIQUIDS 4.1. Introduction Room temperature ionic liquids (RTILs) have gained significant attention owing to their physical and chemical properties, including low volatility, wide electrochemical window or breakdown voltage, and the ability to solubilize polar and nonpolar solvents.1-4 As a result, their application has increased across a diverse range of fields, including batteries, capacitors, fuel cells, and electrochemical sensors.5-10 There is an extensive body of published research available on the electrochemical properties of RTILs, such as the electron and mass transfer process at metal electrodes, namely Pt,11,12 Au13 etc., and also at various carbon electrodes such as glassy carbon14, boron-doped diamond (BDD),15,16 carbon nanotube (CNT)17 and tetrahedral amorphous carbon (ta-C:N).18 Literature suggests that electron and mass transfer processes in RTILs are notably slower compared to conventional electrolyte systems (aqueous, non-aqueous), mainly due to their high viscosity and the absence of dielectric solvents. The high viscosity suppresses ion movement, resulting in lower conductivity than aqueous or organic electrolyte solutions in RTILs.20,21 To enhance mass transport, organic solvents such as acetonitrile (CH3CN), methanol (CH3OH), and ethanol (CH3CH2OH), are often added to RTILs, effectively decreasing the overall viscosity of the RTIL/organic binary system. Significantly, mixing RTILs with organic solvents elevates conductivity while simultaneously preserving a wide operating potential window.22-24 Despite the ability to tune the properties of RTILs using many possible combinations, RTIL/organic binary mixtures are yet to be fully understood. While certain studies focus on physical properties such as 72 viscosity and conductivity, as well as microstructural and dynamic heterogeneity of the mixtures, there is still much to explore in elucidating the comprehensive behavior of these binary systems.25- 28 Microstructural and dynamical heterogeneities are unique characteristics that impact the attributes of RTILs. Initially, RTILs are viewed as homogenous systems similar to conventional electrolyte systems. In recent years, researchers have demonstrated structural and dynamic heterogeneity not only in neat RTILs but also in RTILs combined with organic compounds. This has been achieved through various experimental techniques such as fluorescence anisotropy,29 infrared (IR),30 Raman,31 dielectric and terahertz spectroscopies 32,33 and employing the theoretical studies such as density functional theory (DFT) calculations,34 and atomistic simulations.35 These results indicate that the interactions form short-range ordering structures, including ion pairs and clusters, thus resulting in noticeable mesoscopic agglomeration in RTIL/organic binary mixtures. Moreover, the formation of heterogeneous microstructures contributes to the dynamic properties of RTIL/binary mixtures.28 Despite having a great deal of applicability in RTIL/organic binary mixtures, very little research work is available on the electrochemical behavior of RTIL/organic mixtures.36,37 Thus far, no work is available on understanding the electron and mass transfer process in RTIL/organic binary mixtures. To address this knowledge gap, it is crucial to understand how the molecular level structuring and organization can affect the electron transfer and mass transfer processes in binary mixtures. Figure 4.1 shows the diffusion and electron transfer mechanisms of ferrocene (Fc) within homogeneous and heterogeneous binary mixtures of room-teparature ionic liquids (RTILs) and organic solvents. When organic solvents and RTILs mix homogeneously, as dipicted in Figure 4.1A, Fc diffusion and electron transfer should occurr in a uniform environment. However, when 73 RTILs and organic solvents mix heterogeneously, organic solvents may stay as nanodomains rather than making a well-mixed single phase with the RTILs. Therefore, electron transfer and diffusion processes can vary considerably with the molecular organization of the binary mixtures. Therefore, understanding the electrochemistry and obtaining insight into the molecular level structuring of RTIL/organic solvent binary mixtures is essential before employing these mixtures in real-world electrochemical storage devices such as capacitors and batteries.38,39 A B Figure 4.1. A simplified illustration of diffusion pathways and electron transfer of Fc in (A) RTILs/organic binary homogeneous mixture, and (B) RTILs/organic binary heterogeneous. mixture. We report herein how electron transfer and mass transfer kinetics of Fc are affected by the addition of two organic solvents, namely acetonitrile and methanol, to an ionic liquid. The RTIL was 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) since it has been extensively studied. Fc was used as the redox probe due to its outer sphere electron transfer mechanism, leading to relatively rapid electron transfer in both [BMIM][BF4] and organic solvents at Au electrode.40,41 CH3CN and CH3OH were selected as the organic solvents due to their extensive use in binary 74 mixtures, enabling an investigation into the effect of hydrogen bond formation contributes to the electrochemical properties of these mixtures. This work aimed to elucidate how the diffusion coefficient, diffusion pathway, and electron transfer kinetic of Fc are influenced by the volume fraction and the type of organic solvent incorporated within the imidazolium ionic liquid. 4.2. Experimental Methods 4.2.1. Chemicals and Reagents 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] (≥ 97.0%, CAS No. 91508) was purchased from a commercial supplier (Iolitec, Germany). Ferrocene (Fc) (98%, CAS No. 102–54-5) was purchased commercially (Sigma Aldrich) and used as received. Figure 4.2. The molecular structures of the RTIL cation and anion, and the redox system ferrocene. Figure 4.2 presents the molecular structures of the RTIL cation-anion, and the redox system Fc. Acetonitrile (anhydrous, 99.9%), methanol (anhydrous, 99.8%), and isopropyl alcohol (anhydrous, 99.9%) were purchased commercially (Sigma-Aldrich). All the organic solvents were distilled and dried over activated 5 Å molecular sieves for purification and stored inside N2-purged glove box prior to use. [BMIM][BF4] was purified using a process previously reported by our group.43 As received [BMIM][BF4] was initially stored over activated carbon for at least three days. The ionic liquid was then filtered through a syringe filter with a pore size of 0.22 μm to remove any residual activated carbon. The transferred [BMIM][BF4] was stored over activated 5 Å molecular sieves in a sealed glass vial for 3 days. After that, purified [BMIM][BF4] was added to the electrochemical cell at the time of measurements and heated to 70 °C for 50 min while 75 purging with ultrapure Ar (99.99%, Linde). After this purification, the water impurity content in a selected [BMIM][BF4] sample was measured using a Karl-Fischer titration. The water impurity level was determined to be below 100 ppm. All glassware was dried in an oven at 100 °C for at least 24 h before use to minimize water contamination. 4.2.2. Ferrocene Solution Preparation The Fc working solution in [BMIM][BF4] (1×10−4 mol L−1) was prepared according to the following procedure. First, a stock solution of 1×10−3 mol L−1 Fc was prepared quantitatively by dissolving the appropriate analyte mass in ultrapure (distilled and stored over activated carbon) isopropyl alcohol. 0.1 mL of this stock solution was then quantitatively transferred to a 1 mL volumetric flask and evaporated to dryness in an oven for 1h at 100 °C. The volumetric flask was then filled to the mark with the purified [BMIM][BF4] or the organic solvent/ionic liquid (CH3CN or CH3OH)/[BMIM][BF4] binary mixture to yield a 1×10−4 mol L−1 solution concentration. The solution was then stirred for 24 h to ensure complete dissolution and mixing of the solute. Measured masses of the [BMIM][BF4] and the organic solvent were used to calculate the mole fraction. All solution preparation was performed in an N2-purged glove box in which humidity levels were below 0.1%, as measured using a standard hygrometer. House N2 (< 25 ppb H2O) was purged through a desiccant system, which was part of the glove box to maintain a low H2O level. 4.2.3. Electrochemical Measurements The gold (Au) disk electrode was pretreated prior to use by mechanical polishing with decreasing grades (1.0, 0.3, and 0.05 μm) of alumina powder (Buehler Limited, IL). The polishing slurry was prepared by adding ultrapure water to alumina powder. Polishing was performed on separate and cleaned felt polishing pads. After each polishing step, the Au electrode was ultrasonically cleaned for 20 min in ultrapure water to remove polishing debris. As a final 76 pretreatment, the polished electrode was immersed in ultrapure (distilled and stored over activated carbon) in distilled isopropyl alcohol for at least 10 min. All the cyclic voltammetric and chronoamperometric measurements were performed using a Model 660B electrochemical workstation (CH Instruments, TX). A three-electrode arrangement was used with the gold disk as the working electrode (diam =1mm), a platinum (Pt) wire counter electrode, and a silver (Ag) wire as a quasi-reference electrode. A single-compartment, three-electrode glass cell was used for the measurements. Cyclic voltammograms (CV) were recorded as a function of the scan rate from 0.025 to 0.250 Vs-1. The ohmic equivalent series resistance was determined from electrochemical impedance spectroscopy (EIS) experiments in all the solutions by fitting the experimental data using a modified Randle's equivalent circuit. Constituting of a resistance term, Rs, reflecting the electrolyte and electrode ohmic resistance, in parallel combination of a constant phase element (CPE) explaining the double-layer capacitance, Cdl, and the resistance or polarization resistance of a Faradic reaction. The EIS measurements were made over a frequency range from 105-10-1 Hz. The high-frequency data was used to determine the equivalent series resistances (ESR). It was assumed that the solution resistance dominated the ESR. The apparent heterogeneous electron-transfer rate constant (ko app) for Fc in neat [BMIM][BF4] and the [BMIM][BF4]/organic binary mixtures was calculated from cyclic voltammetric peak-to-peak separation (ΔEp) values obtained at different scan rates (0.100 - 0.500 Vs-1). The dimensionless parameter, Ψ, was plotted against the ν-1/2 for a given redox system to produce a working curve. ko was then determined from the slope of the, Ψ vs. ν -1/2 plot according to the equation below. 44,45 𝛹 = 𝑘𝑜 [ 𝜋𝐷𝑛𝑓𝑣 𝑅𝑇 2 (1) ] 1 77 All the cyclic voltametric ΔEp values were corrected for uncompensated resistance (iR) using the ESR value determined from EIS measurements. It was assumed that this value was dominated by the solution’s ohmic resistance. Chronoamperometric measurements were conducted in 0.1 mmol L-1 Fc in [BMIM][BF4] and the [BMIM][BF4]/organic solvents binary mixtures to determine the diffusion coefficient. The potential was stepped from 0.1 to 0.6 V vs. Ag QRE, a potential at which the Fc oxidation reaction is diffusion controlled. The diffusion coefficient was determined from a plot of i(t) vs. t1/2 according to the Cottrell equation.46 ⅈ(𝑡) = [ 1 2⁄ 𝑛𝐹𝐴𝐶𝐷 ̅̅̅̅̅̅̅ 1 2⁄ 𝜋 𝑡 ] (2) All cyclic voltammetric and chronoamperometric measurements were made at room temperature. 4.3. Results 4.3.1. Cyclic Voltammetry Studies A B Forward Scan Forward Scan Forward Scan Figure 4.3. Cyclic voltammetric i-E curves for 0.1 mmol L-1 Fc in (A) in neat [BMIM][BF4] and (B) a [BMIM][BF4] / CH3CN binary mixture (0.4 mole fraction of CH3CN) at Au disk electrode. Scan rates were used from 0.025 to 0.250 Vs-1. 78 Table 4.1. Summary of cyclic voltammetric data for Fc at a Au disk electrode in [BMIM][BF4] and different [BMIM][BF4]/CH3CN mixtures. Electrolyte/Solvent Neat [BMIM][BF4] 0.1 mole fraction of CH3CN in [BMIM][BF4] 0.2 mole fraction of CH3CN in [BMIM][BF4] 0.3 mole fraction of CH3CN in [BMIM][BF4] 0.4 mole fraction of CH3CN in [BMIM][BF4] Pure CH3CN ox Ep (V) 0.419 ± 0.004 0.401 ± 0.003 0.400 ± 0.010 0.399 ± 0.010 0.401 ± 0.003 0.385 ± 0.002 Ep red (V) 0.319 ± 0.004 0.311 ± 0.004 0.316 ± 0.012 0.322 ± 0.011 0.329 ± 0.006 0.322 ± 0.003 ∆Ep (V) ( iR Corrected) 0.099 ± 0.003 0.089 ± 0.003 0.082 ± 0.004 0.076 ± 0.003 0.071 ± 0.002 0.062 ± 0.002 ox Ip (µA) Ip red (µA) 0.113 ± 0.005 0.130 ± 0.010 0.147 ± 0.012 0.167 ± 0.021 0.200 ± 0.017 1.020 ± 0.014 0.103 ± 0.001 0.121 ± 0.006 0.140 ± 0.011 0.159 ± 0.020 0.194 ± 0.015 1.010 ± 0.022 The Fc concentration was 0.1 mmol L-1. The scan rate was 0.100 Vs-1. Data are presented in mean ± std dev. for n = 3 measurements in each solution. Figures 4.3A and B show a series of cyclic voltammetric i-E curves for 0.1mmol L-1 Fc in neat [BMIM][BF4] and a [BMIM][BF4] / CH3CN (0.4 mole fraction) binary mixture as a function of scan rate, most notable are the larger currents in the binary mixture. Both ∆Ep and the peak currents increase with increasing scan rate in both solutions. ∆Ep and ip ox in neat [BMIM][BF4] increased from 100 to 110 mV and from 0.115 to 0.567 µA, respectively, when the scan rate was increased from 0.25 to 0.250 Vs-1. Upon addition of CH3CN (0.4 mole fraction), ∆Ep decreased from 100 to 89 mV, and ip ox increased from 0.113 to 0.120 µA at 0.100 Vs-1, as compared to values at the same scan rate in neat [BMIM][BF4]. Further addition of organic solvent (CH3CN or CH3OH) to [BMIM][BF4] caused significant changes in ip ox and ∆Ep similarly for both organic solvents. Increasing the organic solvent mole fraction to 0.4 increased ip ox to 0.200 µA and decreased ∆Ep to 71 mV at 0.100 Vs-1 as compared to the values in neat [BMIM][BF4]. The representative curves are presented in Figure 4.3B. A similar trend was observed when [BMIM][BF4] was diluted with CH3OH. The increase in ip ox and ip red with increasing mole fraction 79 of organic solvent is caused by reduced solution viscosity, leading to a large diffusion coefficient. The decreasing ∆Ep reflects an increase in ko, presumably due to a change in the solvent environment around the Fc molecule (i.e. less solvent reorganization energy).32,33 Figure 4.4 show background cyclic voltammetry curves in neat [BMIM][BF4] and [BMIM][BF4]/CH3CN (0.1 - 0.4 mole fraction) binary mixture at a scan rate 0.1 Vs-1. When comparing the 5 different electrolyte solutions, the currents increase slightly with organic solvent content in the mixture. A possible explanation is that the increasing organic mole fraction in the media leads to an increasing average dielectric constant of the electrolyte system. This is because properties such as double layer capacitance and dielectric constant are considered macroscopic properties, as they reflect the bulk behavior of the system rather than the localized solvation interactions. In contrast, properties such as electron transfer kinetics and solvation dynamic are more microscopic properties which depend more on the local environment.25 Forward Scan cyclic voltammetric Figure 4.4. Background [BMIM][BF4]/CH3CN (0.1 - 0.4 mole fraction) binary systems at Au disk electrode. curves i-E in [BMIM][BF4] and 80 The shapes of cyclic voltammetric i-E curves as a function of scan rate can be affected by both sluggish electron-transfer kinetics and ohmic resistance within the electrode and/or solution. If cyclic voltammetric data are to be used to extract information about ko app, the effects of solution resistance must be corrected for. In other words, ΔEp data can be utilized to extract ko app values from the voltammetric data if the i-E curves are corrected for any ohmic resistance effects arising from a combination of the solution, electrode, or electrode contact. Previous studies have reported Rs values ranging from 800 to 3000 Ω, depending on the type of RTIL.34 Since the ESR is dominated by solution resistance, it is a useful measure of Rs. Figure 4.5. Equivalent series resistance (ESR) vs. added mole fraction of CH3CN in [BMIM][BF4]. Data are presented as mean ± std. dev for n = 3 measurements. The ESR was determined from the high frequency impedance in the EIS measurements. Figure 4.5 shows the measured equivalent series resistance for the Au disk electrode in contact with neat [BMIM][BF4] and the CH3CN or CH3OH binary mixtures. The Rs values range from 1998 ± 45 to 202 ± 12 Ω and decrease with increasing mole fraction of organic solvent, similarly for both. The mean Rs values were used to correct the cyclic voltammetric ∆Ep values in each solution according to: Ep, corr. (V) = Ep, mean (V) ± ip (A) × ESR (Ω) (4) 81 Where “Ep, corr.” is the corrected peak potential, “Ep, mean” is the measured peak potential before ohmic correction, “ip” is the oxidation or reduction peak current, and “ESR” is the equivalent series resistance was used for Rs. Figure 4.6. Cyclic voltametric peak potential separation (ΔEp) vs mole fraction of added organic solvent (CH3CN/CH3OH) in [BMIM][BF4]. The measurements were made using a Au disk electrode. Data are presented as mean ± std dev. for n = 3 measurements in each solution. Although the ESR values are high in [BMIM][BF4], the iR products are small (a few mV) due to the low oxidation and reduction peak currents. The corrected ΔEp values for different binary mixtures in both CH3CN and CH3OH are presented in Figure 4.6. ΔEp for Fc in neat [BMIM][BF4] is significantly larger than the values in the binary mixtures, decreasing for 99 to 61 mV with added CH3CN (0.1 Vs-1). The ΔEp values for Fc in the CH3CN and CH3OH binary mixtures exhibit a similar trend with increasing mole fraction of solvent. In other words, both solvents similarly affected ΔEp and electron transfer, and therefore ko app, for Fc. 82 Figure 4.7. Cyclic voltametric i-E curves for 0.1 mmol L-1 of Fc in [BMIM][BF4], neat CH3CN and [BMIM][BF4] /CH3CN (0.4 mole fraction) binary mixtures at a Au electrode. The scan rate was 0.100 Vs-1. . Figure 4.7 compares cyclic voltametric i-E curves for Fc in a neat [BMIM][BF4], CH3CN, and in the [BMIM][BF4]/CH3CN binary mixture, (0.4 mole fraction). It can be seen that the ∆EP values decrease, and ip ox and ip red increase with the added organic solvent. The same trend was observed in CH3OH, suggesting that both solvents form similar mixtures with [BMIM][BF4]. Further, the largest peak currents and smallest ΔEp values are seen in the neat CH3CN due to the lowest viscosity and the highest dielectric constant of CH3CN. The larger peak currents result from the larger diffusion coefficient for Fc and the smaller ΔEp results from an increased nuclear frequency factor and a lower solvent reorganization energy barrier for Fc oxidation in organic solvents.16,18 83 Forward Scan 4.3.2. Chronoamperometric Studies Chronoamperometric i-t measurements were made to determine the diffusion coefficient of Fc in neat [BMIM][BF4] and the [BMIM][BF4]/organic (CH3CN, CH3OH) binary mixtures. Figure 4.8A shows i-t curves at the Au disk for 0.1 mmol L-1 Fc in neat [BMIM][BF4] and in the [BMIM][BF4]/ CH3CN binary mixtures with mole fractions ranging from 0.1 to 0.4. The potential was stepped from 0.10 to 0.60 V vs. Ag QRE and held for 20 s. As shown in Figure 4.8, 0.6 V is well past the oxidation peak potential at which the oxidation current is controlled by diffusion. Figure 4.8B presents the i vs. t -1/2 plots for the different solutions. All data presented have been background corrected. A B = Figure 4.8. (A) Chronoamperometric i-t curves were recorded with a Au disk working electrode in 0.1 mmol L-1 of Fc in neat [BMIM][BF4] and different [BMIM][BF4]/CH3CN (0.1 - 0.4 mole fraction) binary mixture. The potential was stepped from 0.10 to 0.60 V vs. Ag QRE and held for 20 s. (B) Cottrell plots for the oxidation current data. The current is limited by semi-infinite linear diffusion evaluated by the linearity of i vs t- 1/2 Cottrell plots. Although the curves exhibit linearity (R2 > 0.99), none of the extrapolated curves has a zero-y-axis intercept. Notably, the intercepts increased as the mole fraction of the organic solvent increased. Similar i-t behavior observed in both CH3CN and CH3OH indicates that this phenomenon is independent of the molecular structure of the solvents. 84 A B Figure 4.9. Plots of the it1/2 product vs. t for the oxidation of 0.1 mmol L-1 Fc in (A) [BMIM][BF4] and the [BMIM][BF4]/CH3CN binary mixtures (0.1 - 0.4 mole fraction) (B) [BMIM][BF4] and the [BMIM][BF4]/CH3OH binary mixtures (0.1 - 0.4 mole fraction). Figure 4.9A and B show plots of the it1/2 product vs. t for Fc in different solutions. If the faradic current is limited by semi-infinite linear diffusion, then the it1/2 product should remain constant over time in both CH3CN and CH3OH. The data in Figure 4.9 reveal that for the neat [BMIM][BF4] and the 0.1 mole fraction, the it1/2 product is constant over time in both CH3OH and CH3CN. However, as the mole fraction of the organic solvent increases to 0.4, the it1/2 product is no longer constant over time but trends in an increasing manner indicating non-Cottrollian diffusion. This behavior is attributed to the heterogeneous nature of the organic solvent when mixed with the RTIL. The diffusion coefficients for Fc in neat [BMIM][BF4] and the binary solvent mixtures were calculated from the linear region of the plots in Figure 4.8 using the Cottrell equation. The values increase with increasing the mole fraction of the organic solvents. For neat [BMIM][BF4], the Fc diffusion coefficient is 1.43×10-7 cm2 s-1 in agreement with published work.16,18 This value is lower compared to the values in all [BMIM][BF4]/organic binary mixtures. Further, the diffusion coefficient increased by approximately 4× when the organic mole fraction reached 0.4 in 85 [BMIM][BF4]/ organic binary mixtures. The similarity of diffusion coefficients in the CH3CN and CH3OH/RTIL binary mixtures is attributed to the similar viscosities (0.394,0.596 cP) and dielectric constants (37.5,32.4) of CH3CN and CH3OH rather than their interaction with [BMIM][BF4]. A B Figure 4.10. (A) Calculated DX/D[BMIM][BF4] as a function of CH3CN mole fraction (B) logg (DX/D[BMIM][BF4]) as a function of CH3CN mole fraction (R2 = 0.98). To further understand the trend of diffusion coefficients, the ratio of DX/D[BMIM][BF4] vs. the mole fraction of CH3CN was plotted (Figure 4.10A). Interestingly, the diffusion coefficient of Fc varies exponentially with the amount of CH3CN present in binary mixtures. This exponential trend indicates that the Fc diffusion in an [BMIM][BF4] / CH3CN mixture tends to behave according to the classical Stokes-Einstein equation given below (5), where “k” is the Boltzmann constant, “T” is the absolute temperature, “η” is the viscosity, and “d” is the hydrodynamic radius. 𝑑 = (𝑘𝑇)/(3𝜋ηD) (5) A similar observation was reported by Happiot et al. for Fc in [BMIM][BF4]/ dimethylformamide (DMF).14 Furthermore, the same trend was also confirmed with molecular dynamics simulations for Fc in [BMIM][BF4]/ naphthalene system by Lee et al.5 86 4.3.3. Heterogeneous Electron-Transfer Rate Constants The Nicholson method was used to calculate the ko app values from ∆Ep-ν trends. Since the electron transfer process of Fc in all the electrolyte/solvent mixtures, except CH3CN, exhibits a quasi-reversible nature, the ko app was determined from the slope of the Ψ vs. scan rate-1/2 plots. As presented in Figure 4.11, all the plots show good linearity (R2 > 0.98). The Ψ values increase with increasing mole fraction of the CH3CN or CH3OH. A B Figure 4.11. Ψ vs. scan rate -1/2 plots in (A) [BMIM][BF4] and [BMIM][BF4]/CH3CN binary mixtures (B) [BMIM][BF4] and [BMIM][BF4] / CH3OH binary mixtures at Au electrode from 0.025 to 0.250 Vs-1. Table 4.2 present ko app value for Fc in neat [BMIM][BF4] and in [BMIM][BF4]/CH3CN and CH3OH binary mixtures. ko app were determined from the slope of the curves to equation (1). The values increase with mole fraction of organic solvent ranging from 10-4 cm s-1 in neat RTILs to 5×10-3 cm s-1 in the binary mixtures. Clearly, adding the organic solvent lowers the activation barrier and increases the nuclear frequency factor for electron transfer and increases the heterogeneous electron transfer rate constant. ko app of Fc in neat [BMIM][BF4] is 8.70×10-3 cm s−1, and it increases with the amount of organic mole fraction present in the mixtures. Previously reported ko app values for Fc in RTILs range from 10−1 to 10−3 cm s−1, depending on factors such as 87 type and viscosity of RTIL, along with the electrode material.56-58 This is the first time that ko app has been reported for RTILs/organic binary systems, and ko app is increased by about 6× when the mole fraction of organic reaches 0.4. At mole fractions such as 0.1 and 0.2, ko app remains close to the that of Fc in neat [BMIM][BF4]. However, when the mole fraction of organic solvents is 0.4, ko app begins to change drastically. This observation is consistent with the changes observed for ∆Ep. Altogether both ko app and ∆Ep confirm that increasing organic solvent volume in [BMIM][BF4] significantly enhance the electron transfer kinetics in RTILs. Table 4.2. Apparent electron transfer rate constant, ko binary mixtures with CH3OH and CH3CN. app, for 0.1 mmol L-1 Fc in neat RTIL and Electrolyte/Solvent Neat [BMIM][BF4] [BMIM][BF4] + 0.1 mole fraction of organic solvents [BMIM][BF4] + 0.2 mole fraction of organic solvents [BMIM][BF4] + 0.3 mole fraction of organic solvents [BMIM][BF4] + 0.4 mole fraction of organic solvents ko app (cm s-1 × 10-3) for CH3CN mixtures 0.87 ± 0.11 ko app (cm s-1 × 10-3 ) for CH3OH mixtures 0.87 ± 0.11 2.00 ± 0.19 2.12 ± 0.27 2.36 ± 0.13 2.40 ± 0.19 3.37 ± 0.26 3.45 ± 0.23 5.03 ± 0.31 5.31 ± 0.29 Data are presented in mean ± std dev. for n = 3 measurements in each solution. 4.4. Discussion The result indicates that ko app and the diffusion coefficient for Fc both increase in the RTIL, [BMIM][BF4] with the addition of organic solvent. However, the effects on ko app are observed at lower mole fractions of the organic solvent before any macroscopic effects on diffusion coefficients are seen. The findings of this study provide new insights into how the addition of organic solvent impacts molecular diffusion and the heterogenous electron transfer of a neutral redox molecule, Fc. CH3CN and CH3OH were utilized to elucidate the effect of hydrogen bond 88 formation between organic solvents and [BMIM][BF4] on the electrochemical characteristics of [BMIM][BF4]. Voltammetric i-E curves for Fc in neat [BMIM][BF4] and [BMIM][BF4]/CH3CN (0.1 - 0.4 mole fraction) binary mixture showed that ∆Ep and the peak currents increase with increasing scan rate, indicating that redox process of Fc in both neat [BMIM][BF4] and [BMIM][BF4] / CH3CN (0.1 - 0.4 mole fraction) binary mixtures proceeds in a as quasi-reversible manner. The increasing current with the mole fraction of organic solvent in the mixture is linked to the enhanced conductivity of the medium as [BMIM][BF4] is diluted. Furthermore, the decreasing ∆Ep implies alterations in the surrounding solvent environment of Fc molecules with the dilution of [BMIM][BF4]. A similar trend was observed upon dilution of [BMIM][BF4] with MeOH. The primary objective of including CH3CN (aprotic solvents) and CH3OH (protic solvents) was to gain insights into the influence of hydrogen bond formation between solvents and RTIL on electrochemical properties of Fc, such as redox current and electron transfer process. Since there was no clear difference in peak currents and peak separations of Fc in [BMIM][BF4] / CH3CN and [BMIM][BF4]/ CH3OH media, it can be inferred that hydrogen bond formation does not directly correlate with the electrochemical properties in the RTIL/organic binary mixtures. The measured solution resistance ranged from 202 ± 12 to 1998 ± 45 Ω across the electrolyte system. Notably, the lowest resistance was recorded for the pure CH3CN system, while the highest solution resistance was observed for neat [BMIM][BF4]. As the amount of solvent in the mixture increased, the solution resistance exhibited a corresponding decrease. This can be attributed to the lower viscosity and higher free ion concentration of RTIL/organic binary mixtures in comparison to neat [BMIM][BF4]. When comparing the solution resistance between CH3CN and CH3OH-containing solutions, no significant was apparent. This suggests that the hydrogen bond formation of CH3OH 89 has minimal impact on altering the viscosity of [BMIM][BF4] and the true ion concentrations of [BMIM][BF4]. The conductivity of [BMIM][BF4]/organic binary systems increases with the inclusion of organic solvents in the medium.60 Prezhdo et al. have confirmed this trend, demonstrating that for CH3CN and CH3OH, conductivity reached a maximum before declining with further dilution. Their findings indicate that the maximum conductivity of the mixture is around 50 wt. % of CH3CN.61 These observations are anticipated since in ionic liquid + organic mixtures (simply solutions of a liquid salt in an organic solvent), the ions of the ionic liquids are separated by neutral solvent molecules, thereby enhancing RTIL dissociation. The addition of the organic solvents resulted in an alteration in ∆Ep. In neat [BMIM][BF4], this value was 99 mV, whereas for pure CH3CN and CH3OH, it decreased to 61 ± 2 and 62 ± 2 mV, respectively. As the amount of CH3CN or CH3OH in the mixture increased, ∆Ep decreased accordingly. The redox process of Fc in both neat [BMIM][BF4] and [BMIM][BF4]/organic solvent mixtures exhibited quasi-reversibility. In the absence of any solvent in the [BMIM][BF4], the solution environment surrounding Fc is quite different from conventional solvents (aqueous and non-aqueous).In neat [BMIM][BF4], the chemical environment around neutral Fc and Fc+ consists of an extended organization of ions and clusters, rather than organized solvent dipoles shielding the Fc from discrete anions and cations as in pure CH3CN and CH3OH solutions.48 With the addition of dielectric solvents, the ions and ion clusters surrounding FC and FC+ become separated. This implies a larger reorganizational barrier in RTIL when Fc molecules undergo electron transfer and form Fc+ cations.10 In our group, we observed a similar trend when water was added to the RTIL; the peak currents increased alongside the decreasing ∆Ep, indicating a 90 substantial reorganizational barrier in the pure RTIL, which was mitigated when diluted with dielectric solvents.43 Chronoamperometry experiments were conducted to determine the diffusion coefficient of Fc in both neat [BMIM][BF4] and [BMIM][BF4]/organic mixtures. As the mole fraction of organic solvent in the mixture increased, the diffusion coefficient increases in a non- linear manner. This suggests a considerable difference in the diffusion of Fc in [BMIM][BF4]/organic mixtures compared to neat [BMIM][BF4], similar to the deviation observed with microelectrodes. The data indicate that this non-linearity is linked to the heterogeneous structure of [BMIM][BF4]/organic mixtures. Abbott and coworkers previously observed the nonlinear diffusion of Fc+ in ionic liquid media.59 Similar behavior was observed by Compton et al. for the diffusion of Fc in RTIL /water binary mixtures. They proposed that an increased presence of water in RTIL alters the diffusion pathways of Fc towards the electrode, resulting in pocket-to-pocket diffusion rather than linear diffusion.63 Although the calculated diffusion coefficients of Fc in the mixtures exhibit an increment with the volume of organic solvents, the change is quite marginal. This suggests that the environment surrounding most Fc molecules differs significantly, even at 0.4 mole fractions. As a result of this, the diffusion of the Fc molecule in the organic pocket should not noticeably differ from the diffusion of Fc in neat [BMIM][BF4] as the organic pocket must diffuse through more viscous ionic liquid media to reach the electrode, similar to other Fc molecules in the ionic liquid domain. The enhancement in electron transfer kinetics is attributed to the extensive reorganization of the RTIL in neat [BMIM][BF4] around the Fc and Fc + compared to the binary mixtures. Moreover, it is anticipated that the dynamics of the ionic reorganization (λ) around a redox system 91 in response to an electron transfer event will be slower in the more viscous [BMIM][BF4] than in binary mixtures. ko value can be expressed for conventional solvent systems as follows,61. 𝑘𝑜 = 𝑘𝑝𝑘𝑒𝑙𝑣𝑛 𝑒𝑥𝑝 (− 𝐺∗ 𝑅𝑇 ) (6) Where G* is solvents reorganization energy, kp is the precursor equilibrium constant, κel is the electronic transmission coefficient, υn is the nuclear frequency factor, and R and T are the gas constant and temperature, respectively. The parameter υn represents the frequency of attempts the reactant makes to reach the transition state and transfer an electron which is affected by higher viscosity and the absence of a dielectric solvent in neat [BMIM][BF4]. Higher viscosity leads to a lower υn value resulting in a lower ko app. Further, when Fc+ is present in [BMIM][BF4] due to the absence of dielectric solvents, Fc+ may pair with the BF- anion which may electrostatically interact with neighboring BMIM+ in an extended fashion. These changes lead to substantial reorganization of the BMIM+ and BF- when transitioning from the Fc to Fc+ resulting in a larger ΔG* in neat [BMIM][BF4] compared to [BMIM][BF4] binary mixtures. Moreover, the larger ΔG* should be directly related to the lower ko app in neat [BMIM][BF4]. 92 4.4.1. Possible Mechanism of Fc Diffusion and Electron Transfer Process in Neat [BMIM][BF4] and [BMIM][BF4]/Organic Mixtures A C B D Figure 4.12. A simplified illustration of diffusion pathways and electron transfer process of Fc in (A) neat [BMIM][BF4] (B) [BMIM][BF4] /organic binary mixtures (0.1, 0.2 of CH3CN or CH3OH) (C) [BMIM][BF4] /organic binary mixtures (0.3 - ~ 0.7 of CH3CN or CH3OH) (B) [BMIM][BF4] /organic binary mixtures (> 0.7 of CH3CN or CH3OH). Based on the findings, we propose a mechanism to describe the diffusion of Fc molecules in the RTIL/organic binary mixtures. Intriguingly, the calculated diffusion coefficients remain in proximity to the diffusion coefficient of neat [BMIM][BF4], while the diffusion pathways of Fc shift from linear to non-linear with the addition of organic solvents. This suggests that the binary mixtures exhibit a heterogeneous behavior rather than a homogeneous behavior, indicating a 93 tendency to form nanodomains within the RTIL (pocket formation).27,42 This phenomenon has been proven theoretically for water and organic solvents (such as CH3OH, CH3CN) using molecular dynamics simulations, and experimentally validated using small-angle neutron scattering for diluted binary mixtures.65-68 Figure 4.12 illustrates the predicted mechanism for diluted RTIL/organic binary mixtures. According to the data, the diffusion coefficient of Fc is expected to undergo a drastic change in binary mixtures when the mole fraction exceeds ~ 0.7, leading to rapid diffusion through a less viscous pathway. As discussed above, if the organic domains remain separate, diffusion should not be more prominent than in the neat RTIL, since Fc molecules in the organic pocket still need to diffuse through the viscous neat RTIL. However, the observed rapid increase in diffusion coefficient suggests otherwise. This suggests that Fc diffuses through less viscous pathways in the diluted systems. We anticipate that the number of organic pockets and volume per pocket will increase with the increasing organic volume in the binary mixture. Eventually, to stabilize the system these isolated pockets should aggregate and begin to form a network-like structure (Figure 4.12 D). Subsequent addition of organic solvent would extend the organic network within the RTIL, thereby providing a less viscous pathway for Fc molecules to diffuse through the network to the electrode surface. Overall, our study demonstrates that diluting imidazolium RTIL with organic solvents creates heterogeneity in the mixture, resulting to the formation of nanoaggregates in the binary system. The non-Cottrell behavior observed in binary mixtures further confirms the heterogeneous nature of binary mixtures. The drastic decrease in ∆Ep indicates that most Fc molecules tend to partition into and occupy the organic nanoaggregates, leading to rapid electron transfer for Fc at Au electrode. Although Fc tends to occupy nanosized organic sites, it still needs to diffuse through 94 the highly viscous RTIL regions, resulting in lower diffusion coefficients even after addition a substantial volume of organic solvents to the RTIL. Upon reaching the electrode surface, some Fc molecules undergo oxidation in an ionic liquid environment, while others are oxidized within the organic phase. The higher the organic volume in the medium higher the amount of Fc present in the organic environment, leading to lower ∆Ep and larger ko app values. Due to the higher affinity of Fc for the organic solvents, we observe significantly lower ∆Ep (almost reversible) even at 0.4 mole fraction. This lower ∆Ep demonstrates that the environment around the Fc molecules undergoes significant changes, even with the addition of a small volume of organic solvent to the imidazolium RTIL. Similar trends are observed with protic CH3OH and aprotic CH3CN binary mixtures suggesting a negligible effect arising from the hydrogen-bond formation between the organic solvents and RTIL on the electrochemical properties of the RTIL. 4.5. Conclusions We have presented findings on the effects of the diluting imidazolium-based RTIL on both polar aprotic and protic solvents. Our results demonstrate similar electrochemical behavior when imidazolium RTIL is diluted with both CH3CN or CH3OH. The addition of organic solvents to [BMIM][BF4] exhibits non-Cottrell behavior, confirming the heterogeneous nature of the binary mixtures; further dilution does not produce a homogeneous solution to the same extent as when diluted [BMIM][BF4]. Even with an increase in organic mole fraction to 0.4 in the mixture, molecular diffusion substantially increases, indicating that although Fc tends to occupy the organic region, it still must diffuse through the highly viscous ionic liquid region to reach electrode surface. Incorporating even a small volume of organic solvent into [BMIM][BF4] significantly enhances the electron transfer process. Increasing dilution enlarges the size of aggregates and the amount of Fc present in the organic media. Therefore, further dilution of the ionic liquid facilitates a substantially faster electron transfer process for Fc due to environmental changes around the Fc 95 molecules. Further work can focus on determining how the anions and cations of imidazolium ionic liquids influence the electrochemical behavior of RTIL/organic binary mixtures. This could offer deeper understanding of the mechanisms underlying the observed phenomena. 96 REFERENCES 1. 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"Direct evidence of confined water in room-temperature ionic liquids by complementary use of small-angle X-ray and neutron scattering." The Journal of Physical Chemistry Letters 5, no. 7 (2014): 1175-1180. 103 CHAPTER 5. CONCLUSIONS AND FUTURE WORKS 5.1. Conclusions Given the promising potential of RTILs and RTIL/organic solvent mixtures for energy storage and sensing, a comprehensive understanding of the double layer capacitance, molecular diffusion, and electron transfer mechanisms in these systems is essential for advancing practical applications. This dissertation work investigated the influence of microstructure and surface chemistry of ta-C and ta-C:N films on key electrochemical properties, such as voltametric background current, double-layer capacitance, and electron transfer kinetics in RTILs and organic solvent electrolytes. Additionally, it explored the effect of organic solvent addition on electron transfer, molecular diffusion, and double layer formation in RTILs. In chapter 3, it was noted that the double-layer current and double-layer capacitance of ta- C electrodes in organic electrolytes media were lower than those observed for ta-C:N (10) and ta- C:N (30). The capacitance values for solutions containing acetonitrile and propionitrile ranged from 17 to 26 µFcm-2. In RTILs, the capacitance was affected by the cation size of RTILs and was lower than that observed in organic solvent systems. The electrochemical oxidation of anthracene and its derivatives exhibited slow electron transfer at ta-C, ta-C:N (10) and ta-C:N (30) in both solvents at low scan rates (0.1 - 0.5 Vs-1), indicating sluggish kinetics. This effect was more pronounced in RTILs due to their higher viscosity, which hindered molecular diffusion, and the absence of a dielectric medium, which resulted in a highly charged environment surrounding the anthracene molecules. Nitrogen doping had minimal impact on the electron transfer kinetics of anthracenes, indicating that the electrode microstructure does not significantly influence their electron transfer processes. Chapter 4 highlights that the addition of organic solvents into RTILs leads to non-Cotterall 104 behavior, suggesting that dilution of RTILs creates a heterogeneous RTIL/organic solvent binary mixture, rather than a simple homogeneous mixture. Although most of the Fc resides in the organic region, they must still diffuse through the viscous RTIL to reach the electrode surface when the organic mole fraction in the mixture is low. Based on these findings, we predicted that diffusion could change drastically when the organic mole fraction increases. The significant change in the electron tranter rate constant of Fc with addition of organic solvents indicates that even small amounts of organic solvent enhance electron transfer, while further dilution accelerates the process by altering the environment around Fc. Additionally, both protic and non-protic solvents exhibited similar electrochemical behavior. 5.2. Future Works An important direction for future research is to gain a deeper understanding of the electron transfer behavior of anthracene derivatives in RTILs with significantly different viscosities and dielectric constants. Expanding this work to include studies involving organic solvents with varying dielectric constants would provide valuable insights and broaden the scope of the research. Additionally, future investigations should explore the effects of temperature on the electron transfer process of anthracene by varying temperature conditions. Another avenue worth exploring is to examine the electron transfer and molecular diffusion of anthracenes derivatives in RTIL/organic solvent binary mixtures at ta-C electrodes. Combing these studies will provide a comprehensive understanding of the electron transfer behavior of anthracene derivatives in highly viscous, charged RTIL media. Another promising direction is to fully explore the mechanisms of the electron transfer, molecular diffusion, and double-layer formation in RTIL/organic solvent binary mixtures using less viscous RTILs. Integrating these findings with simulation techniques, such as molecular dynamics, would significantly enhance our understanding of these complex 105 systems. 106