Effective daytime radiative cooling via a template method based PDMS sponge emitter with synergistic thermo-optical activity

Radiative cooling, a passive cooling method without energy consumption, provides a sustainable alternative for existing cooling strategies. Microstructures in porous polymers have enabled further optimization of radiative cooling efficiency through selective reflection and emission. While numerous fabrication strategies have been reported, there is still a critical need for scalable method with simplicity, low cost, and designable microstructures. Herein, we report a low-cost eco-friendly template method towards scalable production of porosity-controlled sponge emitter with high radiative cooling efficiency. We demonstrate the feasibility by preparing porous polydimethylsiloxane sponge emitter (P-PDMS) with NaCl template. By modulating the template-to-PDMS mass ratio, facile porosity control is realized and the obtained sponge-like porous structure endows PDMS with higher sub-ambient cooling ability enabled by enhanced emissivity and solar reflectivity. Furthermore, the heat transfer between hot air and the object is efficiently inhibited by the micropores. Combined theoretical and experimental results demonstrate that the PDMS sponge has a remarkable reflectivity of 95% in solar spectrum and averaged emissivity of 96.5% within the 8–13 μm range, thus realizing excellent sub-ambient temperature drop of 8.0 °C.