A phosphoric acid pre-swelling strategy to construct poly(fluorenyl-co-imidazoliumisatin) membranes with high ion selectivity and stability for vanadium redox flow battery.

The development of advanced membranes combining high ion conductivity with long-term stability remains a critical challenge in vanadium redox flow battery (VRFB) technology. In this work, we report a novel class of ether-free poly(fluorenyl-co-imidazoliumisatin) (PFIMIS-x%) membranes prepared through rational copolymerization of fluorene, methylimidazole and isatin units, followed by a phosphoric acid (PA) pre-swelling strategy. The rigid fluorene core enhances dimensional and chemical stability, while methylimidazole provides protonation sites and dense hydrogen-bond networks. Meanwhile, the highly conjugated and sterically hindered isatin moieties introduce intrinsic porosity and oxidative resistance, optimizing microphase separation and extended ionic pathways. Protonated imidazolium and hydrophobic isatin cooperatively contribute to the exclusion of vanadium ions. The PA pre-swelling strategy further promotes continuous channel formation. Synergistic structural regulation optimizes chain packing and microphase separation of the copolymer, leading to improved performance and a well-balanced structure-property relationship. The optimized PFIMIS-20 %-5 M membrane demonstrates a low area resistance of 0.27 Ω cm2, extremely low vanadium permeability (6.85 × 10-8 cm2 min-1) and excellent ion selectivity of 2.97 × 105 S min cm-2, 16 times higher than that of Nafion 212. It also exhibits excellent chemical stability over 400 h and achieves superior performance in VRFB single battery tests, including an energy efficiency of 72.7 % at 300 mA cm-2, a 196 h self-discharge duration, a peak power density of 521.2 mW cm-2 and exceptional cycling stability over 600 cycles at 150 mA cm-2. The demonstrated performance breakthroughs offer new insights into molecular design strategy for developing high-performance membranes in advanced energy storage systems.