Modelling and accelerated testing of catholyte stability in vanadium flow batteries

Using our standard methodology, we examined the thermal stability of vanadium flow battery positive electrolytes over a range of temperature from 30 to 70 °C with stable lifetimes from 11 min to 87 days. At higher temperatures (45 °C–70 °C) measurements showed excellent reproducibility but at lower temperatures (30 °C–45 °C) showed some scatter. Measurements at higher temperatures are in good agreement with our (single-slope) model which is based on earlier data but there is some divergence from the model at lower temperatures. Arrhenius plots of the data show two linear regimes: one in the range 45 °C–70 °C and another in the range 30 °C–45 °C, the latter having a higher Arrhenius slope. Based on linear least-squares best fits in these two regimes, we have formulated an improved stability model (two-slope model). We use our models to derive expressions for accelerated testing of thermal stability using increased temperature, increased vanadium concentration and decreased sulfate concentration and estimate values for the acceleration factors over a range of test and use temperatures and concentrations. We analyse the effect of changing concentration to counteract the decrease in electrolyte stability at higher temperatures and derive expressions to calculate the necessary concentrations.