(Invited) Some Aspects of Electrode Kinetics and Electrolyte Stability in Vanadium Flow Batteries

The electrochemical kinetics of the V II /V III redox reaction on carbon electrodes is enhanced by anodic treatment of the electrode and inhibited by cathodic treatment. Contrastingly, the kinetics of the V IV /V V redox reaction is inhibited by anodic treatment of the electrode and enhanced by cathodic treatment. The observed effects are attributed to oxygen-containing species on the surface of carbon electrodes and it is likely that enhancement of both V II /V III and V IV /V V occurs on the same (meso) state of the electrode. Oxidation of this active meso state leads to inhibition of V IV /V V while reduction of the active meso state leads to inhibition of V II /V III .We have investigated the thermal stability of VFB catholytes using a standard methodology. The induction time for precipitation of V 2 O 5 decreases exponentially with increasing temperature or V V concentration but increases exponentially with increasing sulphate concentration. Arrhenius plots show two linear regimes, at 45–70 °C and 30–45 °C, respectively. We model this behavior to estimate acceleration factors for testing of electrolyte stability over a range of test and use temperatures. Group V additives (phosphate and arsenate) are effective for improving the thermal stability of VFB catholytes.