Star-Polyelectrolyte Interfaces Regulate Ion–Water Transport for Continuous Solar-to-Vapor Conversion in Hypersaline Brines

Hypersaline brines are difficult to treat by solar evaporation because rapid interfacial water loss inevitably drives salt accumulation and crystallization at the evaporation surface, blocking transport pathways and suppressing energy-conversion efficiency. Here, we report a sandwich-structured solar evaporator featuring a star-polyelectrolyte multilayer (sPEM) as an ion–water regulating interface. The multivalent star architecture forms a densely interconnected yet hydrated polymer network that maintains persistent ion exclusion under extreme salinity while simultaneously activating interfacial water. Quantitative ion-partitioning measurements show that the optimized sPEM excludes more than 65% of ions across 1.83–4.88 M NaCl and preserves this behavior for up to 192 h. Mechanistically, electrostatic overcharging and dielectric exclusion establish a salt-depleted interface, whereas polymer-induced reorganization of the hydrogen-bond network increases water mobility; even near NaCl saturation, water diffusivity within sPEM remains nearly one order of magnitude higher than in the bulk brine. When integrated between a bacterial-cellulose water-supply layer and a porous Ti₃O₅ photothermal layer, the sPEM enables water evaporation from a salt-depleted interface while displacing crystallization into the bulk solution. The optimized device sustains an evaporation rate of 7.8 kg m⁻² h⁻¹ under 1 sun at 4.88 M NaCl, operates for approximately 300 h without surface salt accumulation, and achieves continuous co-recovery of water and crystalline salt with efficiencies above 95%. Outdoor operation further demonstrates stable performance over 72 days. This work establishes star-polyelectrolyte interfaces as a general platform for coupling molecular-scale ion–water regulation with efficient solar-to-vapor energy conversion, providing a route toward continuous zero-liquid-discharge treatment of hypersaline brines.