Ultrawhite and Lightweight Boron Nitride Nanoplatelet Paints for Daytime Radiative Cooling

According to the United States Department of Energy, 6% of the average household’s energy use goes towards space cooling. This is due to cooling technologies that rely on electricity generated via processes with high carbon emissions. At a time when many are interested in working to address climate change affecting our planet, it is vital to look into ways to reduce our carbon-footprint in everyday life. Radiative Cooling is a passive cooling technology that accomplishes this by reflecting solar irradiation and emitting thermal radiation into deep space, therefore not relying on electricity generation. Conventional air conditioning removes heat from buildings into the ambient air so heat still stays in the city and on the earth. Radiative cooling, on the other hand, directly loses heat to the deep space, reducing the heat island effect and cooling down the earth. Radiative cooling technology has rapidly advanced as a promising energy-saving and climate-friendly approach. Yet a paintable, lightweight technology for radiative cooling remains a challenge. By exploiting bandgap and morphology, this work develops ultrawhite hBN nanoplatelet-acrylic paints, and experimentally demonstrates full daytime cooling with thin and lightweight coatings. hBN offers a moderately wide bandgap (5.96eV) that eliminates UV absorption while maintaining a high solar spectrum refractive index. Nanoplatelet morphology provides effective light scattering. Experimental samples achieved solar reflectance of 98.1% at 150-micron layer thickness, reducing thickness and weight by 62.5% and 80% respectively compared to state-of-the-art BaSO₄ paint. The sky window emissivity is 0.83. Field tests show full daytime cooling under direct sunlight, reaching 5-6 C below ambient on average. The hBN-acrylic paint demonstrates cooling performance among the highest achieved by recent technologies at a fraction of the thickness and weight.