Recyclable hygroscopic hydrogels with dynamic networks enable efficient solar-driven atmospheric water harvesting

Salt-based hygroscopic polymeric gel materials are widely utilized in atmospheric water harvesting to address the global challenge of water scarcity. However, due to the limited swelling capacity of their networks, these materials tend to leak salt solution under high relative humidity conditions. Moreover, the presence of non-cleavable covalent bonds in the gel networks poses significant challenges for recyclability. In this study, a dynamic network hygroscopic hydrogel (DNH) with low cross-linking density was developed based on poly(vinyl alcohol) incorporating boric ester crosslink sites. The boric ester bonds exhibit reversible hydrolysis, which allows unrestricted swelling of the polymer network and effectively prevents leakage of the salt solution. Due to the low density of dynamic crosslink sites in the polymer network, the hygroscopic hydrogel can be completely dissolved in water, thereby facilitating efficient recovery of raw materials through simple processing procedures. The gel demonstrates excellent hygroscopic performance; when combined with photothermal materials, it enables efficient solar-driven atmospheric water harvesting, achieving a water yield of 1.832 L/m²·d (3.52 kg_water/kg_sorbent) of water within 8 h under 70% relative humidity. This study presents a novel strategy for the design of recyclable and salt-leakage-resistant hygroscopic hydrogels tailored specifically for atmospheric water harvesting applications.