An inorganic water-based paint for high-durability passive radiative cooling

Due to the extreme reflectivity requirements of radiative cooling coatings, these materials often employ ceramic nanoparticles such as TiO2 due to their high refractive index and scattering efficiency. However, the bandgap of TiO2 (3.2 eV) is lower than the energy of the most energetic solar light, leading to significant absorption in the UV range (0.25–0.4 μm) and subsequent oxidation aging and yellowing of particles and/or organic binders. To overcome the conflict between high reflective efficiency and UV durability, formulations using high-bandgap materials can be used despite their lower refractive index. In this work, we describe an optimized PRC-Al2O3 coating by adjusting the ratio of low refractive index alumina particles to sodium methylsilicate adhesive. The PRC-Al2O3 exhibits a high solar reflectance above 0.96 and a high mid-infrared emissivity of 0.91, enabling it to achieve a maximum theoretical cooling power of 109 W m−2. Following continuous UV irradiation with a power of 0.7 kW m−2 for 72 hours, only a marginal 0.2% decline in solar reflectance compared to the unaged coatings. The resulting anti-aging cooling paint is scalable and can be spray-coated onto outdoor structures and containers, providing durable radiative cooling towards real-world applications.