(Invited) Factors Influencing the Performance of Vanadium Flow Batteries: Electrodes and Electrolytes

The electrochemical kinetics of the V II -V III reaction on carbon electrodes is enhanced by anodic treatment and inhibited by cathodic treatment. In contrast, the kinetics of V IV -V V is inhibited by anodic treatment and enhanced by cathodic treatment. Rate constants are greater for V IV -V V than for V II -V III under comparable conditions. 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 is due to the same (active) state of the electrode. Oxidation of this active state leads to inhibition of V IV -V V while reduction of the same active state leads to inhibition of V II -V III . Using our standard methodology, we investigated the thermal stability of VFB catholytes. We showed that 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 to estimate acceleration factors for testing of electrolyte stability over a range of test and use temperatures. Both arsenate and phosphate are effective additives for improving the thermal stability of VFB catholytes.