Suppressing vanadium crossover using sulfonated aromatic ion exchange membranes for high performance flow batteries†
Liquid flow batteries have potential to achieve high energy efficiency as a large-scale energy storage system. However, the ion exchange membranes (IEMs) currently used in flow batteries do not have high ion selectivity and conductance at the same time, owing to the trade-off between ionic membrane resistance and ion selectivity. Here, we report a rationally designed sulfonated aromatic polymer membrane which can greatly mitigate the trade-off limitation and achieve high performance vanadium RFB. Small-angle X-ray scattering studies and density functional theory calculations indicated that the narrowly distributed aqueous ionic domain of just the right width (<7 Å) and the strong hydrogen bond interaction of vanadium species with a unique polymer side chain structure play a key role in improving the ion selectivity. Our systematic studies of the polymer structures, morphologies, and transport properties provide valuable insight that can aid in elucidating the structure–property relationship of IEMs and in establishing design criteria for the development of high-performance membranes.