Maximizing flow battery membrane performance via pseudo-nanophase separation enhanced by polymer supramolecular sidechain

Abstract

Pseudo-nanophase separation, enabled by noncovalently grafted sidechains, offers a promising approach for constructing high-performance membranes, featuring rapid ion transport, robust chemical stability, and simplified manufacturing. However, striking a balance between ionic conductivity and mechanical/chemical stability proves challenging since excessive hydrophilic grafting leads to overswelling and compromised integrity of the membranes, rendering them unsuitable for demanding applications like vanadium redox flow batteries (VRFBs). In this study, we describe a new approach for achieving high-performance VRFB membranes via employing polymer as supramolecular sidechains, rather than small molecules. This strategy achieves remarkable pseudo-nanophase separation while minimizing the utilization of functional (hydrophilic) sites. As a result, the resulting membranes exhibit exceptional robustness and proton conductivity, with an extraordinarily low area resistance of merely 0.11 Ω cm2, thus circumventing the prevailing trade-off between ionic conductivity and mechanical/chemical stability. Ultimately, VRFBs integrated with these membranes achieve energy efficiencies up to 80 % even at high current densities of 240 mA cm−2, accompanied by a remarkably low capacity decay rate of 0.064 % per cycle during long-cycle tests. This work not only achieves ultra-high conductivity with minimal functional groups, but also advances pseudo-nanophase separation strategies and provides valuable insights into optimized utilization of limited functional groups in membrane design.