Kinetics of VIII and VII Sulfate Precipitation Processes in Negative Half‐Cell Electrolyte of the Vanadium Redox Flow Battery

Abstract The low‐temperature stability and precipitation behaviour of V III and V II sulfate solutions are critical for the optimal operation of vanadium redox flow batteries. In this study, the kinetics of the V III and V II sulfate precipitation processes have been studied at different temperatures and sulfate concentrations. Kinetic studies of the precipitation of stagnant vanadium(III) sulfate solutions followed a first‐order rate law with rate constants on the order of 0.007 h −1 and an activation energy of about 14.6 kJ mol −1 . Stirring the solutions at low supersaturation causes a ten‐fold increase in the rate of precipitation and also resulted in a change in the precipitation mechanism from diffusion controlled to a surface reaction controlled mechanism with an activation energy of 37.0 kJ mol −1 . Stagnant solutions at high supersaturation followed a second‐order rate law with an activation energy of 27.0 kJ mol −1 for the first 30 % of the precipitation process. The process becomes first order in the last 70 % with an activation energy of 14.3 kJ mol −1 , indicating a change in the reaction mechanism to a diffusion‐controlled mechanism. Kinetic studies of the precipitation of vanadium(II) sulfate solutions showed that both stagnant and stirred solutions follow a first‐order rate law with a low activation energy (10.7 and 14.5 kJ mol −1 , respectively). This indicates a diffusion‐controlled precipitation mechanism under both conditions.