Maximizing Uptake of Hygroscopic Hydrogels Through Extreme Swelling-Induced Salt Loading
Hygroscopic hydrogels have emerged as scalable and low-cost materials with the potential of high-performance vapor sorption for atmospheric water harvesting, dehumidification, and passive cooling. Despite extensive research interest aimed at improving hygroscopic hydrogels, devices using these materials still exhibit insufficient performance, partly due to limited water uptake of the hydrogels. In this work, we study the swelling dynamics of hydrogels in aqueous lithium chloride solutions and use this to achieve extreme salt loading of hygroscopic hydrogels. By rationally tuning the salt concentration used for swelling we achieve successful synthesis of hygroscopic hydrogels with high water uptake of 1.76 g/g, 2.53 g/g, 3.80 g/g at relative humidites of 30%, 50%, and 70%, respectively, exceeding previous hydrogels by over 19%. These water uptakes bring the performance of hydrogels within ≈91% of the fundamental limit of commonly used hygroscopic salts, while still avoiding leakage problems common in salt solutions. Furthermore, we elucidate via models the maximum uptake achievable without leakage as a function of the operating relative humidity and hydrogel swelling ratio. The insights from this work can be used to design hygroscopic hydrogels with exceptional performance for a wide variety of sorption applications to tackle global challenges such as water scarcity and energy efficiency.