Photothermal-Responsive Cr/Al Bimetallic MOF–Hydrogel@CaCl ₂ Composite for Enhanced Atmospheric Moisture Sorption and Water Release

Bimetallic MOF-hydrogel composites offer a promising strategy for efficient atmospheric water harvesting (AWH) through synergistic moisture sorption and solar-driven water release. Herein, a multifunctional composite (VHN_(Cr_dAl)_M/CaCl₂) is developed by integrating a Cr/Al bimetallic metal-organic framework (BMOF) into a thermoresponsive Valine-HEMA-NIPAM (VHN) hydrogel matrix and further loaded with CaCl₂. This hybrid material leverages the high surface area and hydrophilicity of the MOF, the porous nature and swelling-deswelling responsiveness of the hydrogel, and the hygroscopic properties of CaCl₂. The composite demonstrates superior atmospheric water uptake capability under highly humid conditions (90% RH), reaching up to 4.9 g·g^-1. Under 1 kW m^-2 solar irradiation, the composite rapidly heats to ∼46 °C within 2 min, enabling efficient water release with a desorption rate of 0.8 g·g^-1·h^-1. The composite maintains stable performance across multiple sorption-desorption cycles, indicating excellent durability and reusability. Under low-to-moderate humidity conditions, the CaCl₂ nanocrystals efficiently incorporated within the porous network demonstrate effective atmospheric water sorption, achieving uptake capacities of 0.85 g·g^-1, 1.01 g·g^-1, 1.53 g·g^-1, and 3.04 g·g^-1 at 20%, 40%, 60%, and 80% RH, respectively. These findings establish the developed hybrid as a scalable and energy-efficient material for atmospheric water harvesting in arid environments, offering strong potential for real-world deployment in sustainable water technologies.