A transient model for charge and mass transfer through anion exchange membranes in vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) with anion exchange membranes (AEMs) exhibit a high columbic efficiency and slow capacity decay due to the reduced crossover rate of vanadium ions. Modeling of this promising type of flow battery is challenging because of the difficulty in the description of charge and mass transfer through AEMs. In this work, we propose a transient model for VRFBs with AEM, which incorporates the ion selective adsorption to overcome the inaccurate description of ion distributions in AEM, and improves the transport coefficients to avoid their changes with different electrolytes, thereby introducing AEM parameters such as porosity and fixed charges into the description of charge and mass transfer. It shows that the simulation results of ion selective adsorption are highly consistent with the experiment; the effects of AEM parameters on transport processes and battery performance are well captured. Interestingly, it is found that the current density in AEM is carried by both H⁺ and HSO₄⁻ ions, and the proportion of H⁺ ions is quite large. At high current densities, an increase in AEM porosity greatly improve the voltage efficiency while maintaining a high coulombic efficiency. Meanwhile, small capacity decay rate can be obtained by adjusting fixed concentration.