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.