Ultrathin membranes prepared through interfacial polymer cross-linking for selective and fast ion transport

Ion-selective membranes are widely used in water treatment and batteries. However, it is challenging to obtain membranes that are both selective and permeable. Here, we report an interfacial polymer cross-linking strategy to produce ultrathin but robust polymeric membranes that are simultaneously permeable and selective. Cross-linking the polymer at the interface of two immiscible solvents followed by nonsolvent exchange produces a 3-µm-thick ultrathin membrane that contains a nanoscale separation layer with a quasi-ordered reticular cross-linking structure. Besides conferring strength, the cross-linked structures have angstrom-scale channels and ion-selective sites that can precisely separate ions of similar sizes and charges. We show that these membranes enable increased working current density and power density of various aqueous flow batteries. This strategy resolves a long-standing challenge in polymeric membranes. Ionic or molecular transport in conventional polymeric membranes often suffers from a trade-off between permeability and selectivity. The authors report on an interfacial polymer cross-linking strategy to produce a robust, permeable and selective 3-µm-thick ultrathin polymeric membrane containing quasi-ordered reticular cross-linking structures.