The vanadium redox flow battery (VRB) has received wide attention due to its attractive features for large scale energy storage. The key material of a VRB is an ion exchange membrane (IEM) that prevents cross mixing of the positive and negative electrolytes, while still allowing the transport of ions to complete the circuit during the passage of current. This review focuses on all aspects related to IEMs that are of relevance to understand IEMs better. An overview of the general issues of VRBs will be given first, after which the role of the IEM will be outlined together with the material requirements for advanced alternative IEMs. Finally, the recent progress of IEMs in VRBs will be reviewed and directions will be given for the development of next-generation materials.

NF membranes, as an alternative to traditional ion exchange membranes, were first proposed and successfully prepared for VRBs based on a totally new concept of tuning the vanadium/proton selectivity via pore size exclusion. The results showed that membranes show increasing vanadium ion/proton (V/H) selectivity with decreasing pore size distribution. VRBs assembled with prepared NF membranes exhibited comparable performance to that of commercialized Nafion. The concept could potentially overcome the traditional restriction from ion exchange membranes and provide much more material options for VRB membranes.