Electrolyte Stability in Vanadium Flow Batteries

The stability of VFB catholytes was investigated using both light-scattering measurements and visual observation. V₂O₅ precipitates after an induction time τ which shows an Arrhenius variation with temperature. The value of τ increases with increasing [S] and with decreasing [V^V] but the activation energy remains constant with a value of (1.791±0.020) eV. Plots of ln τ against [S] and [V^V] show good linearity and the slopes give values of β_S = 2.073 M^-1 and β_V5 = -3.434 M^-1 for the fractional rates of variation of τ with [S] and [V^V], respectively. Combining the Arrhenius Equation with the observed log-linear variation of τ with [S] and [V^V] provides a model for simulating the stability of catholytes. The addition of H₃PO₄ has a strong stabilizing effect on catholytes at higher temperatures. For example, at 50°C the induction time for precipitation for a typical catholyte is enhanced ~ 12.5-fold by 0.1 M added H₃PO₄. At concentrations of H₃PO₄ less than ~0.04 M, the precipitation time increases with increasing concentration at all temperatures investigated (30-70°C). At higher concentrations, induction time begins to decrease with increasing concentration of H₃PO₄: the changeover concentration depends on the temperature. Experiments at 70°C using other phosphate additives (sodium triphosphate, Na₅P₃O₁₀, and sodium hexametaphosphate, (NaPO₃)₆) showed similar results to H₃PO₄.