Geopolymer-based sub-ambient daytime radiative cooling coating

Sub-ambient daytime radiative cooling coating (SDRCC) is an appealing thermal management technology with great potential for alleviating the global warming and urban heat island effect. Over the past few years, various polymeric SDRCCs have been developed. However, they may face problems of environmental aging under UV and moisture due to their organic nature. In this study, an ambient-cured inorganic geopolymer-based SDRCC was synthesized with the modification of barium sulfate (BaSO4) and nano-silica (SiO2) particles. The optical and physicochemical properties were systematically investigated. The chemical composition, functional groups, surface morphologies of the raw materials, and the formed geopolymer coating were characterized by XRD, FTIR, SEM, and EDS. The developed coating exhibits a high infrared emissivity of 0.9491 and solar reflectance of 97.6%. When exposed to direct sunlight, the coating's surface could cool down up to 8.9 degrees C below the ambient air temperature under Hong Kong's climate. In addition, the coating could retain well its performance under a variety of harsh environments, including high temperature, water immersion, mechanical wearing, and exposure to sunlight.