Mitigation of Capacity Decay in Vanadium Redox Flow Batteries through Initial Imbalance of Electrolytes State of Charge

Abstract

Capacity decay due to vanadium cross-over is a key technical challenge for Vanadium Redox Flow Batteries (VRFBs). To mitigate this effect this study investigates an operating strategy based on the imbalance of electrolyte solutions State of Charge (SoC). The strategy was conceived starting from the analysis of a VRFB during charge-discharge cycles adopting through-plate Reference Hydrogen Electrodes (RHE), which allowed to continuously monitor the evolution of the electrolytes SoCs during the operation. The obtained information, combined with a modelling analysis to estimate cross-over fluxes and net-vanadium transfer, suggested that starting the operation of the battery with positive electrolyte at a higher SoC compared to the negative one can be an effective strategy. Simulations of battery operation with different initial values of positive electrolyte SoC confirmed that this approach mitigated both battery capacity decay and net vanadium transfer. Model results were then validated by experimental testing. Starting the operation of the battery with negative and positive electrolyte SoC equal to 0% and 15% respectively, permitted to increase the discharged energy per cycle by 15% and to reduce the net vanadium transfer by over 30% without penalizing the efficiency of the battery.