Dual Molecular Confinement Engineering of Graphene Oxide‐Based Monovalent Cation‐Selective Membrane for Osmotic Energy Harvesting and Chlor‐Alkali Process

Ion‐selective membranes are widely employed to extract energy and resources from seawater, yet their performance is significantly compromised by divalent ions present in natural seawater. Here, we present a graphene oxide‐based monovalent cation‐selective membrane with high ion selectivity and permeability that can reject divalent ions by leveraging steric exclusion within nanoconfined interlayers. The graphene oxide nanochannels are dual‐confined with charged small molecules tannic acid and the amphiphilic molecule through cross‐linking and electrostatic modulation, respectively. The obtained membrane achieves ion selectivity ratios of 87 for K + /Mg 2+ , 74 for Na + /Mg 2+ , and 67 for Li + /Mg 2+ . The output power density can reach 30.2 W m −2 under a 50‐fold salinity gradient in osmotic energy harvesting, with a conversion efficiency of 43.8%. Furthermore, the membrane can be used in the chlor‐alkali process, enabling the production of high‐purity sodium hydroxide with brine containing divalent ions. This study provides a novel perspective for the development of next‐generation ion‐selective membranes, with potential applications in advanced resource and energy recovery technologies.