Half-Cell State of Charge Monitoring for Determination of Crossover in VRFB-Considerations and Results Concerning Crossover Direction and Amount

Membranes play a crucial role in efficiency and longevity of flow batteries. Vanadium flow batteries suffer self-discharge and capacity fading due to crossover of electrolyte components through the membrane from one battery half-cell to the other. We consider the impact of vanadium species crossing ion exchange membranes on state of charge of the battery and we present a simple method to determine crossoverll open circuit potential measurements. State of s. State of charge for the negative and positive half-cell is simulated based on assumptions and simplifications for cation and anion exchange membranes and different crossover parameters. We introduce a crossover index IndXovr which enables the determination of crossover direction from state of charge data for the negative and positive half-cell and therewith identification of the half-cell in which predominant self-discharge occurs. Furthermore IndXovr allows statements on crossover amount in dependence on state of operation. Simulated case studies are compared to experimental state of charge values estimated from half-cell potential measurements. Our results reveal that half-cell potential monitoring respectively half-cell SOC estimation, is a simple and suitable tool for the identification of crossover direction and relative amount of crossover in VFB.

Automated summary

The study examines the impact of membrane crossover in vanadium flow batteries on the battery’s state of charge, and presents a method to determine crossover direction and amount. An index is introduced to aid in identifying the predominant self-discharge in the battery half-cells. The research suggests that monitoring half-cell potential or estimating state of charge is an effective tool for understanding crossover in VFB systems.