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.
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