Effects of temperature and composition on catholyte stability in vanadium flow batteries: Measurement and modeling

The stability of typical vanadium flow battery (VFB) catholytes was investigated at temperatures in the range 30-60°C for V^V concentrations of 1.4-2.2 mol dm^-3 and sulfate concentrations of 3.6-5.4 mol dm^-3. In all cases, V₂O₅ precipitates after an induction time, which decreases with increasing temperature. Plots of the logarithm of induction time versus the inverse of temperature (equivalent to Arrhenius plots) show excellent linearity and all have similar slopes. The logarithm of induction time also increases linearly with sulfate concentration and decreases linearly with V^V concentration. The slopes of these plots give values of concentration coefficients P_S and P_V5 which were used to normalize induction times to reference concentrations of sulfate and V^V. An Arrhenius plot of the normalized induction times gives a good straight line, the slope of which yields a value of 1.791 ± 0.020 eV for the activation energy. Combining the Arrhenius equation with the observed variation with sulfate and V^V concentrations, an equation was derived for the induction time for any catholyte at any temperature in the range investigated. Although the mechanism of precipitation of V^V from catholytes is not yet well understood, a precise activation energy can now be assigned to the induction process.