Recent advances in sulfonated poly(ether ether ketone) membrane for vanadium redox flow batteries

Abstract

Vanadium redox flow batteries (VRFBs) have emerged as a viable solution for large-scale energy storage, valued for their high efficiency, safety, scalability, design flexibility, and long operational lifespan. The proton exchange membrane (PEM) is a pivotal component in VRFBs, exerting a profound influence on battery efficiency and economic viability. Sulfonated poly(ether ether ketone) (SPEEK) membranes have garnered considerable attention as promising PEM candidates for VRFBs, due to their simple structure, straightforward synthesis, superior thermal and mechanical stability, cost-effectiveness, and amenability for modification. However, the large-scale application of SPEEK membranes in VRFBs is impeded by the inherent tradeoff between proton conductivity and vanadium ions permeability. The degree of sulfonation in SPEEK membranes is a critical parameter influencing their performance, as increased sulfonation improves proton conductivity but also augments ions permeability and membrane swelling simultaneously, compromising both membrane and battery performance. Addressing these limitations requires innovative strategies, such as structural regulation, functionalization, surface modification, and composite structure to enhance SPEEK membrane performance. In this review, we examine the recent research progress in the development of SPEEK membranes for VRFBs, encompassing recent advancements in optimizing their structure-performance relationship and chemical stability. The review culminates with a critical evaluation of the challenges and potential future research directions for advancing the development of SPEEK membranes for VRFB applications.