Ion Exchange Membranes: Latest Developments toward High-Performance Vanadium Redox Flow Batteries

Redox flow batteries (RFBs) are one of the hopes for grid energy storage applications. Among the various RFBs, the vanadium redox flow battery (VRFB) has the specific advantage of deploying the same element, i.e., vanadium in different oxidation states in both negolyte and posolyte. However, its major unmet concern is the poor charge retention during cycling, attributed to cross-contamination of vanadium across the separator. Perfluorosulfonic acid-based cation exchange membranes (CEMs) are the preferred separators for VRFB. Nevertheless, for the negatively charged matrix of CEMs, redox-active vanadium species are the counterions and easily diffuse through, leading to capacity decay. To counter the crossover, the benefit of Donnan exclusion has been considered. For this reason, anion exchange membranes (AEMs) are encouraged for VRFB. The positive charge on AEMs can effectively exclude the vanadium ions, but AEMs have the limitation of low ionic conductivity and questionable chemical stability in a highly oxidative flow battery environment. In recent years, the membrane research community has adopted different strategies to counter the cross-contamination of the vanadium ions between the electrodes and boost the overall performance of the battery. In this review, we will focus on the various approaches developed for the advancement of VRFB membranes.