Phosphate activated geopolymer-based coating with high temperature resistance for sub-ambient radiative cooling

Passive sub-ambient daytime radiative cooling (SDRC) is an energy-free method enabling heat dissipation away from buildings or infrastructures to cold outer space (∼3 K). However, although organic coatings (based on polymer matrices) have been proven to be a cost-effective means to achieve the SDRC effect by incorporating various functional fillers, organic coatings may have environmental concerns and aging problems. Herein, an inorganic phosphate activated geopolymer-based (PAGP) SDRC coating was synthesized using nano-silica (SiO2) particles and barium sulfate (BaSO4) sheets as the modifiers. The synthesized coating could achieve high infrared emissivity of 0.9634 and solar reflectance of 0.9471, which was also found to exhibit high-temperature resistance and optical stability up to 1000 °C due to its covalently bonded structure and electrically neutral system. The chemical composition, surface morphology, and elemental distribution of the coating were characterized by XRD, FTIR, SEM, BET, and EDS. The evolution of condensed structures and weight loss analysis during the high-temperature treatment was conducted by TGA and XRD to analyze the dehydration process and phase change during the heat process. The outdoor field tests showed a maximum sub-ambient temperature reduction of 3.8 °C by the developed coating without any energy input under direct sunlight in Hong Kong. Furthermore, the energy-saving performance of the PAGP-based SDRC coating is analyzed under different climatic conditions in China with the aid of EnergyPlus. Simulation results demonstrated that the PAGP-based SDRC coating can significantly save the cooling load in all five selected cities, which shows great potential in building energy conservation and carbon emission reduction.