The Electrochemical Properties of Diamond and Tetrahedral Amorphous Carbon Electrodes in Room Temperature Ionic Liquids

Room temperature ionic liquids (RTILs) are emerging as a new medium for redox reactions as these liquids possess high solubilizing  power, electrical conductivity, wide breakdown voltage and low volatility. Being composed entirely of ions (no solvent) and possessing wide electrochemical windows (ca. 5 V), it is not difficult to see why these liquids are viewed by electrochemists to be attractive electrolytes for investigation. Our group is (i) studying the wetting of these ionic fluids on different carbon electrode surfaces and how this wetting affects the capacitance-potential relationships and (ii) investigating how the carbon electrode surface chemistry and microstructure affect the heterogeneous electron-transfer rate constant of different inorganic and organic redox systems in these ionic liquids. It is well established that the in aqueous electrolyte solutions, the heterogeneous electron-transfer rate constant for certain redox-systems at ordered HOPG and more microstructurally-disordered glassy carbon is strongly affected  by the electrode surface microstructure and chemistry. We seek to learn if these same trends hold in ionic liquids.

In this presentation, we will compare the electrochemical behavior of boron-doped nanocrystalline diamond and nitrogen-incorporated tetrahedral amorphous carbon thin-film electrodes. Specifically, contact angle measurements of the surface wettability, measurements of the double-layer capacitance (C_dl) and its variation with potential, and heterogeneous electron-transfer rate constants for redox systems in structurally distinct RTILs will be presented. These two carbon electrodes have distinct microstructures as compared to their sp² carbon counterparts. Nitrogen-incorporated tetrahedral amorphous carbon (ta-C:N) is a composite material consisting of a mixture of sp² and sp³-bonded carbon. Impurities can be incorporated during growth (e.g., N) further adding to their complex structure. These films typically possess 40-60% sp³-bonded carbon. It has been widely used as a protective coating due to its hardness, high wear resistance and low coefficient of friction. The growth temperature for ta-C is usually from 25 to about 100 °C. This means that non-traditional materials, such as plastics, can be used as substrates for deposition. Importantly, these carbon materials possess some electrochemical properties that are similar to those of diamond.