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 electron-transfer 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 two-step 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 o app on the electrolyte cation was less than the dependence observed for other electrodes, like glassy carbon and gold. For example, k o app at the 1.0 M concentration 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 sp 3 diamond may be different from that found at sp 2 glassy carbon.