Study of the Mechanism of the Vanadium 4+/5+ Redox Reaction in Acidic Solutions

Abstract

The mechanism of the vanadium VO 2ϩ /VO 2 ϩ redox couple has been examined in acidic aqueous solutions.A detailed understanding of this chemistry is of interest for improving and optimizing the performance of vanadium redox-flow batteries, a promising electrochemical electricity storage technology.The vanadium 4ϩ/5ϩ redox reactions were studied at a rotating disk graphite electrode and polarization curves were obtained in sulfuric acid and perchloric acid, with varying pH and vanadium concentrations.The results were compared to model predictions for different mechanisms.The data were consistent with a model with a multistep chemical-electrochemical-chemical mechanism at low overpotentials, which changes to a multistep electrochemicalchemical-chemical mechanism at higher anodic or cathodic overpotentials.Unusually high Tafel slopes ͑350-450 mV/decade͒ were observed for the reduction of VO 2 ϩ at higher overpotentials.While this could not be directly explained by the model, insights gained through the use of the model can provide the basis for some suggestions.