Strategy towards high ion selectivity membranes for all-vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) have become the most promising and commercially exploited flow batteries among the range of technical solutions for stationary electrical energy storage. Although the technology has reached pre-commercial level and a series of VRFB implementations have been demonstrated, ion exchange membranes (IEMs) with higher ion selectivity, possessing high (proton) conductivity while maintaining low vanadium ion crossover, are lacking in the market. Such membranes are highly desirable in order to achieve higher power densities while minimizing capacity decay. To address this critical technical challenge for IEMs specific to VRFB applications, this review provides polymer scientists with valuable guidance to the specific requirements of IEMs for VRFBs as well as a clear picture of the challenges facing various IEM modification strategies for ion selectivity improvement. Recommendations for future exploration, design, and optimization of IEMs with superior ion selectivity are also offered for consideration.