Effect of polarization treatment on V^II/V^III and V^IV/V^V kinetics at carbon electrodes

It is shown by cyclic voltammetry and electrochemical impedance spectroscopy that polarization treatment of carbon electrodes affects the kinetics of the V^II/V^III and the V^IV/V^V redox reactions. After treatment at potentials more negative than ∼-0.5 V (sat. Hg/Hg₂SO₄), electrodes are less active for the V^II/V^III reactions than after treatment at positive potentials. The activity of the electrode for the V^II/V^III reactions is recovered by treatment at positive potentials. However, the converse is observed for the V^IV/V^V reactions: after treatment at positive potentials electrodes are less active for the V^IV/V^V reactions than after treatment at negative potentials. Thus, depending on the polarization treatment of each half-cell electrode, either the positive or negative half-cell can be less active (i.e. have slower kinetics). In general, however, the positive half-cell is more active than the negative half-cell: the rate constant of an activated electrode in the positive half-cell is over two orders of magnitude larger than that of a deactivated electrode in the negative half-cell. It is suggested that oxygen species formed on the electrode during anodization inhibit the V^IV/V^V electrode reaction but enhance the V^II/V^III electrode reaction.