Hygroscopic salt-embedded composite materials for sorption-based atmospheric water harvesting

Sorption-based atmospheric water harvesting has the potential to address water scarcity by extracting fresh water from the air. The performance of this technology largely depends on the sorbent used. Hygroscopic salt-embedded composite materials (HSCMs) are promising sorbents for sorption-based atmospheric water harvesting because they combine the high sorption capacities of hygroscopic salts across all relative humidity levels with the salt-retaining structure and kinetics-enhancing properties of a porous or networked matrix. However, the interactions between the matrix and salts in HSCMs are not yet fully understood, which hinders the rational design of their sorption performance. This Review introduces a framework for understanding key sorption characteristics — capacity, enthalpy, kinetics and stability — of HSCMs, through an in-depth thermodynamic analysis of the interactions among hygroscopic salts, water and salt solutions. Using this framework, we analyse reported HSCMs and guide the design of future composites by considering factors such as salt content, pore structure and the carrying capacity of the matrix. We also examine the energy flow within the sorption and desorption cycles to explore potential designs for the matrix that could enhance both aspects. Looking forward, we emphasize the importance of designing sorbent materials and multifunctional device systems in tandem, integrating material design needs, local water demand and energy efficiency to fully leverage the untapped capabilities of atmospheric humidity. Sorption-based atmospheric water harvesting offers a potential solution to address global water scarcity. This Review provides a framework for guiding future designs of sorbent materials through understanding key sorption characteristics — capacity, enthalpy, kinetics and stability — derived from the thermodynamic analysis of the interactions among hygroscopic salts, water and salt solutions.