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Hygroscopic hydrogels are emerging as scalable and low‐cost sorbents for atmospheric water harvesting, dehumidification, passive cooling, and thermal energy storage. However, devices using these materials still exhibit insufficient performance, partly due to the limited water vapor uptake of the hydrogels. Here, the swelling dynamics of hydrogels in aqueous lithiumchloride solutions, the implications on hydrogel salt loading, and the resulting vapor uptake of the synthesized hydrogel–salt composites are characterized. By tuning the salt concentration of the swelling solutions and the cross‐linking properties of the gels, hygroscopic hydrogels with extremely high salt loadings are synthesized, which enable unprecedented water uptakes of 1.79 and 3.86 gg−1 at relative humidity (RH) of 30% and 70%, respectively. At 30% RH, this exceeds previously reported water uptakes of metal–organic frameworks by over 100% and of hydrogels by 15%, bringing the uptake within 93% of the fundamental limit of hygroscopic salts while avoiding leakage problems common in salt solutions. By modeling the salt‐vapor equilibria, the maximum leakage‐free RH is elucidated as a function of hydrogel uptake and swelling ratio. These insights guide the design of hydrogels with exceptional hygroscopicity that enable sorption‐based devices to tackle water scarcity and the global energy crisis.

Adsorption-based atmospheric water harvesting (AWH) has vast potential for addressing global water shortage. Despite innovations in adsorbent materials, fundamental understanding of the physical processes involved in the AWH cycle and how material properties impact the theoretical limits of AWH is lacking. Here, we develop a generalized thermodynamic framework to elucidate the interplay between adsorbent properties and operating conditions for optimal AWH performance. Our analysis considers the temperature dependence of adsorption, which is critical but has largely been overlooked in past work. Using metal-organic framework (MOF) as an example, we show that the peak energy efficiencies of single-stage and dual-stage AWH devices, after considering temperature-dependent adsorption, increased by 30% and 100%, respectively, compared with previous studies. Moreover, in contrast to common understanding, we show that the adsorption enthalpy of MOFs can also be optimized to further improve the peak energy efficiency by 40%. This work bridges an important knowledge gap between adsorbent materials development and device design, providing insight toward high-performance adsorption-based AWH technologies.

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