3D porous polymer film with designed pore architecture and auto-deposited SiO2 for highly efficient passive radiative cooling

Passive radiative cooling (PRC) is the most promising technique to address future cooling requirements as it cools a surface without any energy input by reflecting sunlight and radiating heat, which will have a great impact on the global energy landscape. Here, we report a significant advance toward the design and fabrication of a novel hybrid material for outdoor PRC based on three-dimensionally porous cellulose acetate (3DPCA) film with rationally designed pore sizes centered at ~5 µm and auto-deposited resonant polar dielectric SiO2 microspheres. The side of 3DPCA/SiO2 film with enriched SiO2 shows both ultrahigh average solar reflectance R̅solar of ~96% and enhanced average infrared emittance ε̅IR of ~95%, reaching up to the state-of-the-art levels. Especially the ε̅IR value is greater than that of the reported solid polymer film with randomly distributed SiO2 microspheres (93%). The excellent PRC capability is further demonstrated by outdoor tests where practically attainable cooling temperatures are ~8.6 °C for nighttime and ~6.2 °C for daytime. The performance equals or surpasses that of state-of-the-art PRC designs, while the developed technique offers paint-like simplicity for low-cost and large-scale production. Owing to its superior PRC capability and scalability, this composite film is of great potential for promoting radiative cooling as a viable energy technology.