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

Abstract

The electrochemical kinetics of the VII-VIII reaction on carbon electrodes is enhanced by anodic treatment and inhibited by cathodic treatment. In contrast, the kinetics of VIV-VV is inhibited by anodic treatment and enhanced by cathodic treatment. Rate constants are greater for VIV-VV than for VII-VIII 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 VII-VIII and VIV-VV is due to the same (active) state of the electrode. Oxidation of this active state leads to inhibition of VIV-VV while reduction of the same active state leads to inhibition of VII-VIII. 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 VV 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.