Ultra‐Emissive MgO‐PVDF Polymer Nanocomposite Paint for Passive Daytime Radiative Cooling

Abstract Passive daytime radiative cooling (PDRC) holds enormous potential to provide low‐cost, electricity‐free cooling in hot environments and to overcome the urban heat island effects. Conventional radiative cooling devices consist of complicated multilayer structure, and reflective back metallic plates that are unsuitable for household applications. Most single‐layer polymer nanocomposites are also not practically applicable due to their high cost, frailty, and sub‐optimum cooling performances arising from low thermal emissivity. Herein, it is shown that ultra‐white and ultra‐emissive magnesium oxide (MgO)‐polyvinylidene fluoride (PVDF) nanocomposite that exhibits an average ≈7°C temperature decrease below the sub‐ambient conditions under direct sunlight. The optimized MgO‐PVDF metamaterials with a dielectric particle size of ≈50 nm exhibit a large solar reflectance of 96.3% due to the Mie‐scattering and a record high thermal emission of 98.5% at the atmospheric transmission window due to the anharmonic multiphonon Mg─O bond vibrations, and other stretching/bonding vibrations from the polymer. The nanocomposite paint exhibits water‐resistant hydrophobic properties and can be easily coated on pavers, wood sticks, etc., with high uniformity and good adhesion. This work provides a low‐cost, scalable, and solution‐processed nanocomposite coating with excellent cooling performance for pavers, tiles, and building cooling applications, especially in equatorial regions, southeast Asia, middle‐east, and African regions.