High‐Performance Freshwater Harvesting System by Coupling Solar Desalination and Fog Collection with Hierarchical Porous Microneedle Arrays

The maximizing daily freshwater yield on the ocean surface necessitates all‐day water harvesting technologies and materials. This is realizable by taking advantage of the natural sunlight and humid air, which can drive daytime solar desalination and nighttime fog collection, respectively. To this end, two types of hierarchically porous microneedle array structures, which demonstrate superior capabilities for efficient fog capturing and photothermal evaporation, respectively, are prepared. The gel‐forged microneedle arrays with Janus wettability are fabricated via a simple and controllable top‐down micro‐molding process on a porous platform, and porosity within microneedles is further achieved readily by additional freeze‐drying treatment. The developed microneedle structure shows an ultrahigh fog harvesting rate up to 30.5 kg m−2 h−1, enabling high flux water droplet harvesting from moisture during nighttime. In the daytime, a solar evaporation rate of 2.46 kg m−2 h−1 is realized due to the increased evaporative area of the porous microneedle arrays and enhanced photothermal conversion. By uniting these two water‐harvesting routes, a daily cycle can ideally deliver an overall water yield close to 200 kg m−2, which will offer a promising solution for sustaining future low‐cost and decentralized clean water production. Porous hydrogel‐based microneedle array structures are developed through a top‐down micro‐molding process. Additional freeze‐drying treatment readily creates pores in the microneedles, and further blending the microneedle structure with carbon nanotube enables broadband light absorption to make it befit solar thermal conversion. The microneedle arrays demonstrate great promise for water production technologies like atmospheric fog harvesting and solar interfacial evaporation.