Porous frameworks for effective water adsorption: from 3D bulk to 2D nanosheets

The generation of freshwater from ubiquitous atmospheric moisture via using appropriate water adsorbents in atmospheric water generators has the potential to serve as a powerful strategy to effectively address global water shortages that are threatening the lives of humans. In this regard, the preparation and selection of water adsorbents are the essential premise. In this review, we summarize the latest progress in the development of porous frameworks for water harvesting. First, we introduce systems engineering for hygroscopic salts and the fabrication of nano-porous super-hygroscopic hydrogels, followed by the design of nanomaterials with controlled morphologies and a structural design strategy for metal-organic frameworks (MOFs). Porous adsorbents with new forms (porous organic polymers (POPs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and two-dimensional (2D) materials) are then summarized in detail. Finally, future challenges and directions relating to this emerging field are discussed.