Sustainable and self-cleaning bilayer coatings for high-efficiency daytime radiative cooling

Abstract

Passive daytime radiative cooling (PDRC) without additional energy input to cool objects by directly radiating energy into outer space and reflecting sunlight provides a promising pathway to replace current compression-based cooling systems. However, the state-of-art PDRC coating still suffers from contamination during long-term operation to maintain the high solar reflectance (_solar) and thermal emittance (_LWIR) in the long-wavelength infrared (LWIR) atmospheric transmission window. Here, to balance the super-hydrophobicity with micro-pores and the high-efficiency PDRC performance with nano-pores, we report a sustainable and scalable method to prepare a self-cleaning bilayer porous coating to achieve a static water contact angle (CA) ∼163°, _solar = 0.97 and _LWIR = 0.96. Results show that _solar changes little after being rubbed and exposed to air. And _solar only decreases slightly from 0.97 to 0.92 in muddy water, illustrating the excellent contamination resistance of the super-hydrophobic bilayer coating. Under direct sunlight of 774 W m⁻², the bilayer coating achieves an average temperature of 4.0 °C below the ambient temperature and 4.7 °C below the commercial coating temperature. This remarkable cooling performance can be achieved even after contamination, which can be 3.1 °C below the ambient temperature after muddy water treatment. The super-hydrophobic bilayer coating remains clean and dry outdoors since the dust or contaminant can be easily carried away by rainwater, indicating that energy-free and labor-free cleaning of the coatings by rain favors long-term practical applications.