ELECTROCHEMICAL STUDIES OF REDOX REACTIONS AT NANOSTRUCTURED CARBON ELECTRODES IN AQUEOUS ELECTROLYTE SOLUTIONS AND ROOM TEMPERATURE IONIC LIQUIDS

ABSTRACTELECTROCHEMICAL STUDIES OF REDOX REACTIONS AT NANOSTRUCTURED CARBON ELECTRODES IN AQUEOUS ELECTROLYTE SOLUTIONS AND ROOM TEMPERATURE IONIC LIQUIDS By Fatemehsadat Parvis Carbon electrodes are often used in electroanalytical chemistry. There are several reasons for this including low cost, high mechanical strength, wide usable potential range, rich surface chemistry, chemical inertness, and compatibility with a wide variety of reaction conditions.[1-3] There are different allotropes of carbon including diamond, graphite, glassy carbon, and fullerenes.[3] Carbon materials are used in metal production, batteries, supercapacitors, and as electrocatalyst supports.[3] Room temperature ionic liquids (RTILs) are solventless salts with a low melting point. They are liquid at room temperature and composed of pure ions. They usually contain a large asymmetric organic cation, such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, or N-methyl-N-alkylpyrrolidinium, and a small symmetric inorganic anion, such as tetrafluoroborate (BF4-) or hexafluorophosphate (PF6-). The physiochemical properties of these liquid salts can be manipulated based on the molecular structure of cationic and anionic parts. Their low vapor pressure, high thermal and chemical stability, moderate electrical conductivity, and non-flammability make them a unique choice for electroanalytical measurements offering huge advantages over organic solvents and aqueous electrolyte solutions. For instance, pure RTILs have a broad and stable working potential window (~ 4-6 V).[4] This wide working potential window allows the study of a larger number of redox systems at extreme positive/negative potentials, which is not possible in an aqueous electrolyte solutions. Their wide working potential window, high viscosity, and conductivity are key properties for electrochemical measurements.[5-7] High viscosity and the presence of impurities limit their electrochemical applications. The high viscosity of RTILs arises from strong van der Waals interactions between the cations and anions. Their high viscosity suppresses the diffusional mass transport and slows down the heterogeneous electron-transfer kinetics.[8-12] The presence of impurities has a noticeable impact on electrochemical processes in RTILs. One of the most abundant impurities in RTILs is water. Water decreases the viscosity[13-16], density[17], working potential window[13-15, 18], and increases the electrical conductivity[13-16]. Also, the presence of water can influence the solubility of redox analytes, the structure of the electrical double-layer, and the solvent environment around a redox system. Therefore, the effective removal of water is a vital step if researchers aim to study the electron-transfer processes in RTILs.[6, 16, 17, 19] There are fundamental differences between room-temperature ionic liquids and aqueous electrolyte solutions. RTILs contain no dielectric solvent to separate the ions. Their ionic nature makes the columbic interaction the principal force in this environment. As such, ions exist with significant ion pairing. Béguin and coworkers demonstrated the formation of ion-pair complexes using NMR and DFT/PCM-based chemical shift calculations.[8, 9] Cations and anions in an RTIL surround the redox system with some particular organization. However, there is a solvation shell around redox systems in an aqueous electrolyte solution.[11, 17, 20] Therefore, the solvation dynamics in RTILs is distinctly different from the aqueous electrolyte environment, and consequently, the process of electron transfer in RTILs differs from that aqueous solution. The structure of the double-layer also influences electrochemical processes. In 1933, Frumkin described the effect of the double-layer structure on the kinetics of electrochemical reactions. For instance, the heterogeneous electron-transfer rate constant can be strongly dependent on the nature and concentration of electrolyte ions.[6] The electrode-solution interfacial region in RTILs and aqueous solutions is different. Electrochemists usually employ the “Gouy-Chapman-Stern” model to describe the double-layer for aqueous electrolyte solutions. The application of this model for RTILs is arguable for two reasons. First, the total ionic concentration of RTILs is approximately 3-6 M. However, the concentration of free ions is less than this because of ion-pairing. A study by Tokuda’s group showed that the true ionic concentration of RTILs is in the range of 50 to 80 % of their total ionic concentration. [6] Second, there are a few layers of oscillatory charges at the interface in RTILs, which is not compatible with the “Gouy-Chapman-Stern” model.[11, 13, 14] There is a large body of research on how the electronic properties, microstructure, and surface chemistry affect electron-transfer kinetics of various redox systems at sp2 carbon electrodes in aqueous electrolyte solutions. However, more limited research is available about electron transfer kinetics at sp2 and sp3 carbon electrodes in room temperature ionic liquids. The dissertation research reported herein addresses this knowledge gap. The following questions were investigated: 1) How do the double-layer capacitance and background voltametric current for different carbon electrodes in RTILs compare with aqueous electrolyte solutions? 2) How does the molecular composition of the RTIL affect electron-transfer and mass-transfer kinetics for ferrocene derivatives at different carbon electrodes? 3) How is the electron-transfer rate for surface confined redox systems affected by the molecular composition of the RTIL in comparison with an aqueous electrolyte solution? The central hypothesis of this research is that the distinct microstructure of the carbon materials studied, glassy carbon and boron-doped nanocrystalline diamond thin films will affect the capacitance, molecular adsorption, and heterogeneous electron-transfer rate constants of several soluble redox systems in RTILs in ways that are different from the trends found for the same electrodes in aqueous electrolyte solutions.