Interfacial Salt Redistribution Enables Rapid Atmospheric Water Harvesting in a Biomimetic Multi‐Layer Hygroscopic Membrane

Sorption‐based atmospheric water harvesting (SAWH) offers a promising route for decentralized freshwater production, but practical water output is often limited by slow sorption kinetics in salt‐loaded hydrogel sorbents. Here, a biomimetic multi‐layer heterogeneous hygroscopic (MHH) membrane is developed to decouple vapor capture from bulk liquid storage. Inspired by the salt‐management strategy of Avicennia marina leaves, the membrane integrates an air‐accessible nanofiber interface with a brush‐like hydrogel reservoir. During drying, lithium chloride (LiCl) becomes enriched within the nanofiber layer, enabling rapid interfacial deliquescence upon exposure to humid air. The resulting salt solution is then transferred into the underlying hydrogel, where polyethylene glycol (PEG) side chains promote water transport by reducing internal diffusion resistance. The MHH membrane reaches approximately 85% of its equilibrium uptake within 90 min at 70% relative humidity (RH), while maintaining a water uptake of approximately 2.4 g g −1 . In a semi‐continuous three‐bed SAWH device, the membrane delivers 3.47 L m −2 over 8 h at 25°C and 90% RH and demonstrates outdoor operation under naturally variable conditions. This work establishes interfacial salt redistribution coupled with polymer‐network engineering as an effective strategy for fast‐cycling atmospheric water harvesting.