Electric-Eel-Inspired Aqueous Polyelectrolyte Membranes for Osmotic Energy Conversion.

Salinity-gradient energy is a promising renewable resource, yet its practical conversion is constrained by the coupled limitations of ion selectivity and transport resistance in conventional solid-state membranes. Here, we present a liquid-dominated ion-selective membrane in which ion discrimination is established within a confined aqueous polyelectrolyte phase rather than rigid nanostructures. Cation-selective liquid membranes deliver power densities up to 6.7 W m-2 at a 50-fold NaCl gradient and 28.7 W m-2 at high salinity while maintaining stable performance over centimeter-scale thicknesses and in complex ionic environments. Anion-selective liquid membranes are realized within the same framework using cationic polyelectrolytes. By integration of both membrane types into a liquid-based reverse electrodialysis architecture, cooperative osmotic energy harvesting with linear voltage scaling is achieved, enabling direct powering of electronic devices. The liquid membrane platform further offers chemical tunability, operational robustness, and closed-loop recyclability, establishing liquid-phase electrostatics as a versatile strategy for salinity-gradient energy conversion.