Electrochemical behavior of highly conductive boron-doped diamond electrodes for oxygen reduction in alkaline solution

Highly conductive boron-doped polycrystalline diamond thin films (rho approximate to 10(-3) Ohm cm) were prepared via microwave plasma chemical vapor deposition (CVD). The electrochemical behavior for oxygen reduction was examined in 0.1 M KOH using linear sweep voltammetry. Oxygen reduction was found to be highly inhibited, the cathodic voltammetric peak being observed at similar to -1.2 V vs. Ag/AgCl, compared with the standard potential for the two-electron reduction of oxygen (O-2 + H2O + 2e(-) = HO2- + OH-, E-0' = -0.234 V vs. Ag/AgCl at pH 13). This demonstrates that, even in the presence of dissolved oxygen, diamond retains a relatively wide potential window, which could be advantageous in certain types of analytical applications. Possible interpretations for the high overpotential for oxygen reduction include a lack of adsorption sites for oxygen and/or reduced intermediates, a low density of states or a potential drop within a thin (similar to 2 nm) surface layer, all of which have also been proposed for highly ordered pyrolytic graphite. The experimental data were fitted using digital simulation, which showed that the reduction peak appearing at ca. -1.2 V vs. Ag/AgCl is predominantly due to the reduction of oxygen to peroxide. Rotating disk electrode measurements were also consistent with an overall two-electron process. Experiments involving the addition of superoxide dismutase also supported this conclusion. The oxygen reduction reaction is proposed to occur on the sp(3) carbon component of the surface, with a very small contribution from sp(2) carbon impurities at smaller overpotentials.