Influence of surface inhomogeneities of boron doped CVD-diamond electrodes on reversible charge transfer reactions

The electrochemical behaviour of reversible charge transfer reactions on boron doped diamond (BDD) was studied by cyclic voltammetry and electrochemical impedance spectroscopy using rotating disc electrodes under defined convection. Diamond films of 5 mum thickness with doping levels of 200, 3000 and 6000 ppm were prepared by hot. lament chemical vapour deposition on niobium substrate. The electrochemical measurements were carried out on BDD electrodes in deaerated 0.5 M Na2SO4 + 5 mM K-3[Fe(CN)(6)]/ K-4[Fe(CN)(6)] solution at rotation frequencies 0 < f(rot) < 4000 rpm. Comparative measurements were carried out on an active Pt electrode. The BDD electrodes exhibit distinct irreversibilities indicated by a larger peak potential difference in the cyclic voltammograms, lower diffusion limiting current densities and an additional impedance element at high frequencies. Mechanical polishing with carbon fleece and SiC paper strongly affects the irreversible behaviour of the BDD electrodes. The experimental results are explained by a partial blocking of the diamond surface with reversible charge transfer at active sites. The impedance spectra are analysed quantitatively using a transport impedance model for reversible reactions on partially blocked electrode surfaces.