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

The electrochemical kinetics of the VII/VIII redox reaction on carbon electrodes is enhanced by anodic treatment of the electrode and inhibited by cathodic treatment. Contrastingly, the kinetics of the VIV/VV 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 VII/VIII and VIV/VV occurs on the same (meso) state of the electrode. Oxidation of this active meso state leads to inhibition of VIV/VV while reduction of the active meso state leads to inhibition of VII/VIII.We have investigated the thermal stability of VFB catholytes using a standard methodology. 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 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.