Geometry-engineered ion transport enables over 4000-h salt-free solar evaporation using Janus electrospun membranes

Abstract Salt accumulation and surface fouling remain major barriers to long-term solar-driven interfacial evaporation. Here we report a geometry-engineered Janus electrospun membrane that integrates asymmetric wetting with structural design to suppress salt buildup. The membrane combines a hydrophobic carbon black–polystyrene photothermal layer for broadband solar absorption and localized heating with a hydrophilic polyacrylonitrile transport layer and a cotton scaffold ensuring continuous capillary water supply. A systematic comparison of nine geometries fabricated from the same membrane reveals that evaporator geometry governs heat localization, ion transport pathways, and salinity tolerance. Among different configurations, an inverted-cone architecture exhibits exceptional durability, sustaining evaporation fluxes of 1.03–1.36 kg m⁻² h⁻¹ and solar thermal conversion efficiencies of 70%–92% under 1-sun illumination in 3.5 wt% NaCl without visible salt deposition for over 4224 h. These findings reveal macroscopic geometry-controlled ion transport as a key design principle for scalable, salt-resistant solar evaporation.