Kinetics of V-III and V-II Sulfate Precipitation Processes in Negative Half-Cell Electrolyte of the Vanadium Redox Flow Battery

摘要

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 0.007h(-1) and an activation energy of about 14.6kJmol(-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 37.0kJmol(-1). Stagnant solutions at high supersaturation followed a second-order rate law with an activation energy of 27.0kJmol(-1) for the first 30% of the precipitation process. The process becomes first order in the last 70% with an activation energy of 14.3kJmol(-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 14.5kJmol(-1), respectively). This indicates a diffusion-controlled precipitation mechanism under both conditions.