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

Abstract

Abstract 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 cm 2 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.