Photothermal MOF-Alginate Hydrogel Beads for Solar-Driven Atmospheric Water Harvesting

Atmospheric water harvesting (AWH) has emerged as a promising strategy for decentralised freshwater production in regions where conventional water infrastructure is limited. Among the various approaches, sorption-based systems employing metal-organic frameworks (MOFs) are particularly attractive due to their high surface area and tunable pore chemistry. In this work, we report the development of structured photothermal composite beads based on Ca²⁺-crosslinked sodium alginate (SA) incorporating MIL-101(Cr) and MIL-100(Fe) MOFs together with graphene oxide (GO) as a solar absorber. The resulting MOF-alginate-GO beads combine high water adsorption capacity with efficient photothermal regeneration and good mechanical robustness. The Ca²⁺-crosslinked alginate network acts as a water-permeable hydrogel scaffold, enabling repeated swelling-drying cycles while integrating MOF sorption capacity and GO-mediated light-to-heat conversion. Dynamic vapour sorption measurements reveal characteristic cooperative adsorption behaviour, with MIL-101(Cr) beads exhibiting an earlier adsorption step at ~30-40% RH, while MIL-100(Fe) beads activate at ~45-55% RH. Under simulated solar irradiation (1 sun), the incorporation of GO enhances solar-to-thermal conversion, increasing the fraction of desorbed water by approximately 22%, 12%, and 8% relative to the corresponding GO-free SA, Fe/SA, and Cr/SA formulations, respectively. These results highlight the potential of MOF-alginate photothermal composites as scalable sorbents for solar-driven atmospheric water harvesting, particularly in moderate-humidity environments.