Solar-driven sorption-based atmospheric water harvesting: Sorbent mechanisms, materials and performance metrics

Solar-driven sorption-based atmospheric water harvesting (AWH) offers a decentralized route to freshwater production by coupling hygroscopic sorbents with solar or low-grade thermal energy. However, material performance is still frequently judged by maximum equilibrium water uptake, which does not directly represent recoverable liquid-water production under realistic operating conditions. This review examines the relationships between sorbent mechanisms, effective working capacity, and liquid-water recovery, with particular attention to material-related constraints on device performance. The thermodynamic and microscopic origins of water capture are first analyzed in terms of sorption isotherms, effective working capacity, hydrophilic-site binding, ionic hydration, water-cluster growth, bulk absorption, pore filling, capillary condensation, and hysteresis. The structure–mechanism–performance relationships of three major sorbent families—crystalline porous frameworks (MOFs and COFs), hygroscopic polymer gels and aerogels, and salt-based composites—are then compared, with emphasis on working humidity window, transport resistance, regeneration demand, swelling or salt migration, cycling stability, and manufacturability. We show that high practical productivity requires coordinated optimization of water affinity, reversible storage capacity, hierarchical mass-transfer pathways, heat transfer, desorption kinetics, and condensation efficiency rather than maximization of a single uptake metric. Effective working capacity, release efficiency, condensation efficiency, cycle duration, durability, water quality, and areal productivity are identified as key metrics for meaningful cross-study comparison. Future progress depends on integrating sorbent chemistry with device-scale thermal and mass management and on standardized testing under realistic environmental conditions to translate laboratory sorption performance into reliable, scalable water production.