Tandem Ion‐Selective Barriers Enable Directional Ion Gating: Injectable Ion‐Asymmetric Hydrogel Sponges for Hypersaline Desalination

ABSTRACT Solar desalination of hypersaline brines holds promise for addressing water scarcity, yet salt crystallization at the evaporator interface remains a barrier to its practical viability. Here, we report an injectable ion‐asymmetric hydrogel sponge (IAHS) that regulates salt transport through directional ion gating. Leveraging a mold‐free injection strategy, we integrate oppositely charged polymer networks within a sponge skeleton to form a Donnan‐governed tandem ion‐selective barrier, where the positively charged bottom layer excludes cations and the negatively charged top layer excludes anions, enabling directional ion gating. Under the constraint of local electroneutrality, co‐ion exclusion in each layer restricts counter‐ion transport, fundamentally inhibiting salt nucleation and growth at the evaporation interface. Ion transport simulations combined with experiments demonstrate that this asymmetric charge architecture restricts salt‐ion migration towards the evaporation interface, validating the directional ion‐gating mechanism. The rigid sponge matrix sustains unobstructed water transport under high osmotic pressure, and the IAHS achieves an evaporation rate of 4.05 kg m −2 h −1 in 20 wt% brine with no salt accumulation throughout 12 h of continuous operation. Enabled by a mold‐free injection technique, this work introduces an ion‐selective salt‐regulation strategy based on directional ion gating, offering a practical pathway for hypersaline wastewater treatment and scalable solar desalination.