Sodium Alginate/Polyacrylamide Sponge-Based Hydrogel Containing Calcium Chloride for Atmospheric Water Harvesting with High Water Release Rates

Atmospheric water harvesting (AWH) technology is important to alleviate the growing freshwater crisis. Currently, the most widely used atmospheric water harvesting (AWH) materials are salt compounds combined with hygroscopic hydrogel networks. However, they often expand with water absorption, leading to a loss of water absorbency. At the same time, a strong interaction between the salt and water molecule causes a slow water release rate. Herein, we designed a sponge-based sodium alginate/polyacrylamide (SA/PAM) hydrogel with rich hydrophilic groups that provides vacancies for water molecules. Multiwalled carbon nanotubes (MWCNTs) were integrated into the SA/PAM hydrogel matrix, originating from melamine sponge, to enhance the kinetics of water release. Besides, the stabilized skeleton of a melamine sponge can solve the problem of deterioration of the hydrogel pore structure due to water absorption and saturation. With the introduction of CaCl2, first, Ca2+ can cross-link the SA molecular chains and increase the strength of the material and, second, CaCl2 can be used as a hygroscopic salt to form hexahydrate CaCl2 with water molecules, enhancing the material’s hygroscopicity. The rate of water release from the sponge-based SA/PAM hydrogel can be achieved at 3.35 kg m–2 h–1 under an illumination intensity of 1 kW m–2. Finally, we collected 15.36 kg m–2 of clean water using a homemade device under natural light for 7 h. This work provides an effective technical support for atmospheric water harvesting.