'Small' Technology, Big Power: Micropore Engineering for High-Performance Flow Battery Membranes

Achieving carbon neutrality demands large-scale deployment of renewable energy, which in turn requires efficient, durable, and low-cost electrochemical energy storage systems. Redox flow batteries (RFBs) have emerged as a leading technology for grid-scale storage owing to their decoupled power and energy, long cycle life, and intrinsic safety. At the heart of RFB performance lies the membrane, which governs ion transport, selectivity, stability, and overall system cost. Optimizing membrane properties is therefore central to advancing RFB technology. This Review examines recent progress in flow battery membranes, emphasizing their working mechanisms, performance criteria, and key challenges. We discuss the structural characteristics, ion transport behavior, and modification strategies of diverse membrane types, including ion-exchange membranes, non-ion-exchange membranes, porous membranes, and emerging functional materials such as covalent organic frameworks, metal-organic frameworks, and polymers of intrinsic microporosity. Particular attention is given to strategies that enhance selectivity and ionic conductivity through synergistic effects, such as size exclusion, Donnan exclusion, and dielectric regulation. Finally, we outline future directions for membrane design, including multi-mechanism coupling, sub-nanometer pore engineering, defect modulation, and composite functionalization, providing a framework for developing high-performance, low-cost, and long-life membranes for next-generation flow batteries.