Recent advances in the selective membrane for aqueous redox flow batteries

Redox flow batteries (RFBs) have gained intensive attention and are regarded as the ideal choice for large-scale energy storage owing to their attractive features such as excellent electrochemical reversibility, long life, high efficiency, and decoupling of energy and power density. Ion-exchange membranes in the flow battery act as a physical barrier to separate the positive and negative half-cell and allow migration of charge-balancing ions from one side to the other to complete the internal circuit of the cell. Certainly, the overall performance of the RFBs heavily depends on the properties of the ion-exchange membranes. To prevent power loss and minimize the crossover of the active species, it is essential for the ion-selective membrane to acquire high ionic conductivity to ensure low area specific resistance and high selectivity. This review summarizes recent research advances on improvement methods to enhance the selectivity of membranes in RFBs, mainly including the modifications on the pore size, hydrophilicity, and some other aspects. The relationship between performances and structures of these membranes is analyzed, and the advantages and limitations are discussed. Based on the recent advances, corresponding perspectives on future development in this field are also discussed.