A Critical Update on the Design of Dense Ion‐Conducting Membranes for Redox Flow Batteries

Abstract Recent progress in the design and preparation of dense ion‐conducting membranes, to improve redox flow batteries (RFBs) performance are critically examined. The ideal membrane has to balance a high ionic conductivity, a low crossover of ion/redox‐active species, and high coulombic and voltage efficiencies. Several ion exchange membranes are analyzed, with a focus on proton exchange membranes (PEMs), that are the most mature membrane technology in RFBs, led by the gold standard Nafion. Key developments in the design of membranes include the synthesis of tailored (co)polymers, the post‐functionalization of commercially available ones, membrane formation techniques like electrospinning, polymer blending, the additions of organic and inorganic fillers, and surface modification. Dense, asymmetric and composite membranes are reported and discussed. The effects on the membrane properties of macromolecular parameters (polymer backbone, type and length of the side chains, and the acidity of the ion‐exchanging group) are highlighted. Correlations between chemical structure, properties and performance are discussed, targeting the trade‐off between conductivity, selectivity and overall performance of the RFB cell. Although significant steps forward in the development of ion‐conducting membranes were made, improvements in electrochemical properties and long‐term stability, while reducing costs, are still challenging and necessary to a large‐scale application of RFBs.