Scalable Aqueous Processing-Based Passive Daytime Radiative Cooling Coatings

Passive daytime radiative cooling (PDRC) can realize electricity-free cooling by reflecting sunlight and emitting heat to the cold space. Current PDRC designs often involve costly vacuum processing or a large quantity of harmful organic solvents. Aqueous and paint-like processing is cost-effective and environmentally benign, thereby highly attractive for green manufacturing of PDRC coatings. However, common polymers explored in PDRC are difficult to disperse in water, let alone forming porous structures for efficient cooling. Here, a simple "bottom-up" ball milling approach to create uniform microassembly of poly(vinylidene fluoride-co-hexafluoropropene) nanoparticles is reported. The micro- and nanopores among secondary particles and primary particles substantially enhance light scattering and results in excellent PDRC performance. A high solar reflectance of 0.94 and high emittance of 0.97 are achieved, making the coating 3.3 and 1.7 degrees C cooler than commercial white paints and the ambient temperature, under a high solar flux of approximate to 1100 W m(-2). More importantly, the volatile organic compound content in the aqueous paint is only 71 g L-1. This satisfies the general regulatory requirements, which are critical to sustainability and practical applications.