Review and Perspectives of Sulfonated Poly(ether ether ketone) Proton Exchange Membrane for Vanadium Flow Batteries

The vanadium flow battery (VFB) has been implemented as an alternative energy storage technology for wind and solar energy systems due to its advantages of high energy conversion efficiency, facile design and fabrication, extended battery lifespan, high power, and low cost. The proton exchange membrane (PEM), as one of the key VFB components, is critical in determining the application value and practical efficiency of battery systems. Among various PEMs, sulfonated poly(ether ether ketone) (SPEEK) membranes have received widespread attention due to their simple structure, facile preparation, superior thermal and mechanical stability, low cost, and easy modification. However, their large-scale application is still plagued, where membrane performance heavily depends upon the sulfonation degree and the higher sulfonation leads to worse ion permeation and an excessive swelling behavior, decreasing the membrane stability and battery performance. Therefore, structural regulation, functionalization modification, and filler enhancement strategies of SPEEK have been applied to improve its comprehensive performance. Here, the fundamental research and practical development status of SPEEK in the VFB are comprehensively reviewed, including recent advances in various modification strategies. Potential interactions among the structure-activity relationship, modification methods, and chemical stability protection of the SPEEK membrane are established. Finally, the challenges and potential future research directions for the development of SPEEK membranes are critically appraised.