Electrode Kinetics of Vanadium Flow Batteries: Contrasting Responses of VII-VIIIand VIV-VVto Electrochemical Pretreatment of Carbon

Electrochemical impedance spectroscopy and cyclic voltammetry were used to investigate the electrode kinetics of V II -V III and V IV -V V in H 2 SO 4 on glassy carbon, carbon paper, carbon xerogel, and carbon fibers.It was shown that, for all carbon materials investigated, the kinetics of V II -V III is enhanced by anodic, and inhibited by cathodic, treatment of the electrode; in contrast, the kinetics of V IV -V V is inhibited by anodic, and enhanced by cathodic, treatment.The potential region for each of these effects varied only slightly with carbon material.Rate constants were always greater for V IV -V V than for V II -V III except when anodized electrodes were compared, which may explain discrepancies in the literature.The observed effects are attributed to oxygen-containing functional-groups on the electrode surface.The considerable differences between the potentials at which enhancement of V II -V III and inhibition of V IV -V V occur indicates that they do not correspond to a common oxidized state of the electrode.Likewise inhibition of V II -V III and enhancement of V IV -V V do not correspond to a common reduced state of the electrode.It is possible that enhancement of both V II -V III and V IV -V V is due to the same (active) state of the electrode.