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Designing Ion‐Selective Membranes for Vanadium Redox Flow Batteries

It is predicted that the future of energy will mainly rely on batteries such as vanadium redox flow batteries (VRFBs), and its related research has already attracted significant attention. The primary function of a membrane in VRFBs is to control proton transport between the half‐cells and to hinder admixing the anolyte and catholyte at the same time. However, to develop a low‐cost and energy‐efficient VRFB, other membrane roles are crucial. The combination of a highly stable backbone of polytetrafluoroethylene with hydrophilic perfluorinated‐vinyl‐polyether side chains equipped with sulfonic acid groups (Nafion membranes) has led to a breakthrough in the field. However, suffering from high cost and low selectivity, these perflurinated membranes are not properly qualified for VRFBs. Sulfonation of aromatic hydrocarbon polymers is suggested as cost‐effective alternative chemistry for VRFBs’ membrane design. Further tunning the performance of the membrane and VRFB is obtained through designing their microstructure by different tools, especially adjusting the degree of sulfonation and degree of branching, utilizing additional membrane layers, and incorporation of particles in the polymer matrix. In this review, the studies performed to develop membranes for VRFBs are discussed as a road map for the development of advanced membranes qualified for VRFBs.