Factors controlling the electrochemical potential window for diamond electrodes in non-aqueous electrolytes

The electrochemical behavior of as-deposited diamond was examined in various non-aqueous solvents and supporting electrolytes. The potential for the onset of anodic current was found to be dependent on the nature of the organic solvent, whereas that for the onset of cathodic current was dependent on the type of supporting salt cation. These results indicate that outer-sphere decomposition reactions of the organic solvents are the most important in determining the oxidation potential limit, and those involving cations are the most important in determining the reduction potential limit. These oxidations and reductions were correlated to the HOMO and LUMO energies of the solvents and the supporting electrolyte cations, respectively. The correlations were found to be linear. Electrical double-layer capacitance values were also found to be dependent on the type of electrolyte, and correlations were observed between the capacitance values and the molecular volumes of the solvent as well as between the capacitance and the volumes of the supporting electrolyte cations. The experimental results clearly indicate that the electrical double-layer capacitance values decrease, whereas the thickness of the Helmholz layer increases with increasing volume of the solvated cations.