Electrode Kinetics of Vanadium Flow Batteries: Contrasting Responses of V-II-V-III and V-IV-V-V to 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 H2SO4 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. (C) The Author(s) 2015. Published by ECS. All rights reserved.