Gallate-based metal-organic frameworks for atmospheric water harvesting under ultra-low humidity

Water scarcity affects the survival of over 2.2 billion people worldwide, emerging as a pressing global challenge. Atmospheric water harvesting based on porous adsorbents presents a promising solution to this crisis. However, the current adsorption materials exhibit quite low capturing capacity under extremely low atmospheric humidity. Herein, we synthesized a class of M-gallate (M = Mg, Co, and Ni) metal-organic frameworks (MOFs) and evaluated their atmospheric water harvesting performance. Impressively, Mg-gallate MOF was stable for at least 28 days in water, and exhibited exceptional water uptake capacity of 170.0 mg/g at 0.2% relative humidity (RH), which exceeds the highest value of the previously reported porous materials under the same conditions. Even at 5.0% RH, its water adsorption capacity is lower than that of the record-breaking water-stable Ni 2 Cl 2 BBTA (BBTA: 1H,5H-benzo(1,2-d),(4,5-d′)bistriazole). Spectral studies and density functional theory‌ (DFT) calculations suggest that the water adsorption process follows a multi-layer mechanism. The main driving forces underlying the outstanding atmospheric water harvesting performance of the MOF are the hydrogen bonding interactions between water molecules and the oxygen-containing functional groups (carboxylate and phenolic-OH) in MOF, the hydrogen bonding among adsorbed water molecules, and the synergistic effect of pore filling. Thus, the strategy developed here provides a simple and effective way for the development of high-performance atmospheric water adsorbents under ultra-low humidity environments.