Hygroscopic photothermal sorbents for atmospheric water harvesting: From preparation to applications

Considering the current situation of water scarcity in arid regions worldwide, atmospheric water harvesting (AWH) has been widely discussed in that atmospheric moisture takes up a big part of freshwater in the global hydrological cycle. Compared with other strategies for AWH, sorption based on polymer hydrogels has attracted great interest for its tunable chemical and physical properties and promising applications for AWH. In this review, we started with the mechanism of water sorption and the fabrication of hygroscopic hydrogels for AWH, including the inherently hygroscopic polymeric gels, hydrogels as the matrix with other hygroscopic materials, and stimuli-resposive hydrogels. Moreover, the mechanism of water desorption and the most commonly used materials which could help enhance the photothermal performance of the hydrogel were illustrated. Furthermore, in order to create hydrogels with high porosity, several strategies have been introduced to optimize the structure of the hydrogel. In addition, the extended applications of hygroscopic photothermal hydrogels have been discussed, such as agricultural irrigation and dehumidification. Finally, the challenges and prospects of polymeric hydrogels for AWH are concluded.