Superhydrophobic Hierarchical Polymer Film for All-day Passive Radiative Cooling Atmospheric Water Harvesting

Passive radiative cooling atmospheric water harvesting (PRC-AWH) is a promising alternative to overcome water scarcity issues as an energy-saving technique. However, it is a challenging task to ensure all-day PRC-AWH operation. Here, we report a self-cleaning, scalable, flexible, and optically effective hierarchical polymer film to achieve continuous (all-day) PRC-AWH which exhibits high solar reflectivity, high emissivity, and a high water uptake performance. Owing to its unique hybrid photonic and hierarchical structured surface, the polymer film exhibits high solar reflectivity and ideal infrared emissivity, and achieves superb average subcooling throughout the day. Outdoor tests demonstrate that daytime PRC-AWH is realized in natural weather conditions to collect large amounts of water from the atmosphere. Controlled environment tests further elucidate the droplet capture mechanisms that contribute to the performance enhancement with the use of hierarchical structures at both micro and macro scales. Besides, the hierarchical structures ensure the surfaces to achieve superhydrophobic surfaces where the dust collected on the surface is cleared out by the condensed droplet to keep it clean at all times. Our results demonstrate that such a hierarchical polymer film shows great potential of all-day PRC-AWH that can at least double the yield of water, while removing and collecting water passively and efficiently.