An overview of amphoteric ion exchange membranes for vanadium redox flow batteries

Vanadium redox flow battery (VRB), as the most promising large-scale electrical energy storage units, has attracted extensive attention. Amphoteric ion exchange membrane (AIEM), as the core part of VRB, separates electrolyte on both sides of electrolytic tank and conducts H+. The AIEM with cation and anion groups possesses excellent performances, such as high ion conductivity (σ), low vanadium ion permeability (PVn+), relative stability and low cost. However, the performance of AIEM directly depends on the chemical structure of polymers. In addition to ensuring foundational physical performance, ion selectivity of AIEM is significant since the crossover of vanadium ion with various valences may reduce the battery capacity. In this paper, AIEMs for VRB and their chemical structures as well as synthesis approaches to realize all kinds of high-performing AIEMs are reviewed. The current trend and future direction of prospective materials for the VRB separators are documented in detail as well.