Measurement and computer simulation of catholyte stability in vanadium flow batteries (VFBs)

Based on careful experimental measurements, a model for the stability of vanadium flow battery (VFB) catholytes was developed which quantifies their precipitation behavior as a function of temperature and composition. The model enables simulation of the induction time for precipitation at a temperature T for any catholyte with concentrations of sulfate and V^V within the range of applicability. The results of such simulations are in good agreement with experiment. The model can predict catholyte stability using any of three alternative metrics: the induction time τ, the relative stability parameter ρ and the stability temperature T_W. The induction time is a good measure of overall stability; the relative stability parameter compares the stability of any catholyte to a standard in a temperature-independent manner; and the stability temperature estimates the upper temperature limit at which a catholyte is stable for practical purposes. Equations are derived for these parameters and the behavior of each parameter is simulated and plotted under a variety of conditions. Likewise, the effect of state of charge is simulated and plotted. The plots and the associated equations provide detailed stability data that can be useful in the design of practical flow batteries.