(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 V₂O₅ 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.