Effect of Pretreatment on the Rate of the VO2+/VO2+ and V2+/V3+ Reactions at a Carbon Electrode

It was shown by cyclic voltammetry, current measurements at constant potential and electrochemical impedance spectroscopy that polarization pretreatment of glassy carbon electrodes affects the kinetics of the VO2+/VO2+ and V2+/V3+ redox reactions. After pretreatment at potentials more positive than ~0.7 V (sat. Hg/Hg2SO4), electrodes were less active both for oxidation of VO2+ and for reduction of VO2+. The activity of the electrode remained depressed indefinitely but was recovered by pretreatment at negative potentials. However, the converse was observed for the V2+/V3+ redox reactions: after pretreatment at positive potentials electrodes were more active for oxidation of V2+ and reduction of V3+ than after pretreatment at negative potentials. Both activation and deactivation (for VO2+/VO2+) occurred relatively quickly, typically within ~60 s. and eventually approached a steady state characteristic of the particular pretreatment potential. It is suggested that oxygen species formed on the electrode during anodization inhibit the VO2+/VO2+ electrode reaction but enhance the V2+/V3+ electrode reaction.