Ether-free Poly(arylene methylimidazole) Membranes with High Performance for Vanadium Redox Flow Battery Applications

Achieving superior ionic selectivity in membranes is vital for enhancing the performance of vanadium redox flow batteries (VRFBs). In this study, we synthesize a series of ether-free poly(arylene methylimidazole) copolymers (P(MeIm-co-X)) enriched with methylimidazole groups. These copolymers are prepared via a straightforward superacid-catalyzed polymerization of 1-methyl-2-imidazolecarboxaldehyde with five distinct aromatic monomers: biphenyl, fluorene, 1,2-diphenylethane, diphenyl sulfide, and p-terphenyl (used as a reference). Our objective is to investigate how variations in the polymer backbone’s chemical structure affect membrane properties relevant to VRFB applications. The incorporated basic methylimidazole groups facilitate ion transport through hydrogen bonding, while modifications in the aromatic monomer structures adjust the polymer microstructure to optimize area resistance and ionic selectivity. Among the synthesized membranes, the fluorene-based P(MeIm-co-Flu) exhibits the most outstanding performance, displaying excellent chemical stability, high tensile strength (22.4 MPa), and low area resistance (0.32 Ω cm2). When evaluated in VRFBs at a current density of 100 mA cm–2, the P(MeIm-co-Flu) membrane achieves an energy efficiency (EE) of 85.7%, surpassing that of Nafion 115 (76.5%). Additionally, this membrane demonstrates exceptional capacity retention over 570 cycles at 100 mA cm–2, maintaining Coulombic efficiencies above 99%, with energy efficiencies decreasing slightly from 85.4% to 80.9%. Therefore, this work presents a high-performance, easily synthesized, and cost-effective P(MeIm-co-Flu) membrane for potential application in VRFBs.