Fluorine-Engineered Membranes Break the Conductivity-Selectivity Trade-Off in Aqueous Organic Redox Flow Batteries

Ion exchange membranes constitute critical components in aqueous organic redox flow batteries (AORFBs), yet face a fundamental trade-off. High-ion-affinity membranes achieve high conductivity but endure swelling-induced low selectivity due to co-uptake of water and organic active species. To address it, we develop a fluorine-engineered polymer architecture strategy, demonstrated via fluorinated poly(arylene alkylene) anion exchange membranes. Fluorine incorporation 1) establishes interconnected ion channels with suppressed swelling, enabling competitive conductivity at low hydration (λ < 4.5), and 2) simultaneously reduces redox-active material affinity, yielding low permeabilities of, for example, 4.0 × 10^-12 cm² s^-1 for methyl viologen, representing 50-fold and 425-fold lower permeability than those of fluorine-free counterpart and commercial DSV membranes, respectively. The optimized membrane enables a pH-neutral AORFB exhibiting a low-capacity decay rate (0.00077% per cycle)-outperforming existing systems by 1-3 orders of magnitude. Our observations provide fundamental guidance for developing advanced membranes in electrochemical energy technologies.