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

Abstract

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 0.92, 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 occurred 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.