From hygroscopic hydrogel to sustainable water nexus: a critical review of hydrogel-based atmospheric harvesters

Freshwater scarcity poses a critical global challenge, particularly in arid and semi-arid regions. Sorption-based atmospheric water harvesting (SAWH) has emerged as a promising solution due to its feasibility in low-humidity environments. Among various hygroscopic materials, hygroscopic hydrogels have drawn great attention due to their superior water uptake capacity, relative lower desorption temperature, and scalable synthesis routes. Considering the significant progress in hydrogel-based SAWH, it is imperative to systematically summarize the innovations and achievements in this important and rapidly developing field. In this review, the structure design and synthesis strategies of hygroscopic hydrogels are summarized. Then, three key points for SAWH are discussed in detail: (1) How to accelerate water capture kinetics to enhance atmospheric water uptake performance of hydrogels; (2) how to facilitate the desorption process to obtain fresh water; and (3) how to reinforce the stability of hydrogels to keep their high performance. Various strategies including chemical modification, integration of inorganic hygroscopic agents, and structural engineering for addressing these key points, as well as the underlying mechanisms, are introduced and analyzed. Finally, the challenges and opportunities in this field are prospected to promote the development and practical application of hydrogel-based SAWH.