Electrolyte and temperature effects on the electron transfer kinetics of Fe(CN)₆^-3/-4 at boron-doped diamond thin film electrodes

Cyclic and linear sweep voltammetry were used to investigate the effects of the electrolyte composition and temperature on the electrontransfer kinetics of Fe(CN)₆^-3/-4 at well-characterized, boron-doped diamond thin-film electrodes. Highly conductive films were employed, which were first cleaned of any adventitious nondiamond carbon impurity by a twostep chemical-oxidation, and subsequently hydrogenated in hydrogen microwave plasma. The apparent heterogeneous electron-transfer rate constant, k° _app, depended on the electrolyte concentration and the electrolyte cation type, increasing in order of Li⁺ < Na⁺ < K⁺ < Cs⁺. However, the dependence of k°_app on the electrolyte cation was less than the dependence observed for other electrodes, like glassy carbon and gold. For example, k°_app at the 1.0Mconcentration was only a factor of 1.6 greater in KCl than in LiCl for diamond. This is less than the factor of 5-10 seen for other electrodes, like glassy carbon and gold. The transfer coefficient for the oxidation was largely independent of the temperature and the electrolyte composition with a value ranging from 0.52 to 0.55. The activation energy for electron transfer was found to be 14.3, 15.6, and 16.5 kJ/mol respectively for KCl, NaCl, and LiCl. The results suggest that the electric double layer structure at sp³ diamond may be different from that found at sp ² glassy carbon.