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.
