Proton exchange membranes with perfluorobenzenesulfonic acid groups for vanadium redox flow battery applications

Abstract

Proton exchange membranes (PEMs) are critical to the performance of vanadium redox flow batteries (VRFBs). Still, conventional perfluorosulfonic acid membranes such as Nafion® suffer from poor ion selectivity and high cost. In this study, we introduce PEMs prepared from four types of poly(arylene perfluorobenzenesulfonic acid)s, synthesized via polyhydroxyalkylations of biphenyl (BP) or p-terphenyl (TP) with pentafluorobenzaldehyde (BA) or perfluoroacetophenone (AP), named by their monomer contents (e.g., sBPBA). The combination of rigid and ether-free polymer backbones and densely distributed highly acidic sulfonic acid groups led to high proton conductivity and improved ion selectivity. In addition, the –CF₃ substitution in the PEMs derived from perfluoroacetophenone likely increased the free volume and enhanced chemical stability. These membranes displayed a reduced area resistance and markedly lower vanadium ion permeability compared with Nafion®115. In VRFB single-cell tests, the membranes sBPBA, sTPBA, and sBPAP consistently delivered higher voltage and energy efficiencies than Nafion®115 across 40–160 mA cm⁻², with the former PEM achieving the highest energy efficiency at all current densities. Long-term cycling demonstrated outstanding stability for sBPAP (∼99.5% CE, ∼82% EE over 450 cycles), moderate stability for sTPAP (∼98% CE over 250 cycles), and rapid performance degradation for sBPBA and sTPBA, prepared from pentafluorobenzaldehyde. This study demonstrates that CF₃-containing poly(arylene perfluorophenyl) PEMs, with only a small fraction of the perfluoroalkyl (PFAS) content of Nafion®, are promising candidates for high-efficiency long-term VRFB operation. It also provides a clear molecular design framework for developing advanced membrane materials.