Evaluation of mitigation of capacity decay in vanadium redox flow batteries for cation- and anion-exchange membrane by validated mathematical modelling

Vanadium redox flow battery (VRFB) is a potential electrochemical energy storage solution for residential accumulation and grid stabilization. Long-term durability, non-flammability and high overall efficiency represent the main advantages of the technology. The ion-exchange membrane, an essential component of the battery stack, is largely responsible for the efficiency of the battery and capacity losses caused by asymmetric cross-over of vanadium ions and a solvent. To mitigate these losses, we developed a mathematical model of the VRFB single-cell for both cation-exchange membrane (CEM) and anion-exchange membrane (AEM) and validated it against our own experimental data. Our model simulates the charge-discharge cycling of a VRFB single-cell under selected sets of operating conditions differing in the following parameters: applied current density, initial volume and concentration of electrolytes, arrangement of storage tanks (hydraulic shunt) and option of periodic rebalancing of electrolytes. The model includes a description of vanadium ions permeation and osmotic flux across the membrane and kinetics of electrode reactions. The hydraulic connection of electrolyte tanks appears to be the most promising mitigating strategy, reducing capacity losses by 69 % over 150 cycles when compared to standard VRFB set-up, which we have also confirmed experimentally. Moreover, by combining the operation methods, our model shows that using AEM with the hydraulic electrolyte connection and periodic rebalancing, the overall battery utilization can be increased by 80 % compared to a standard operation of VRFB using CEM. The developed model offers useful optimization tool for the construction and operation of flow batteries and can be easily adapted for other chemistries.