Study of Manipulative Pore Formation upon Polymeric Coating for the Endowment of the Switchable Property between Passive Daytime Radiative Cooling and Heating

Passive daytime radiative cooling (PDRC) emerges as a promising cooling strategy with an attractive feature of no energy and refrigerant consumption. In the current study, for the purpose of achieving cost-efficient fabrication of a PDRC polymeric material, a microporous polymeric coating is prepared by a novel "inverse emulsion"-"breath figure" (Ie-BF) method using water droplets as pore-formation template, and the porous morphologies of both the surface and bulk layer can be dynamically manipulated by tuning the emulsion composition as well as environmental conditions. Therefore, the solar reflectivity of the Ie-BF coating can be efficiently tuned within a rather wide range (21-91%) by facile modulation of porosity and thickness. The Ie-BF coating with a thickness of only 125 μm exhibits a high solar reflectance of 85.4% and a long-wave infrared emissivity of 96.3%, realizing a subambient radiative cooling of 6.7 °C and a cooling power of ∼76 W m^-2 in the open air. Moreover, by employing the reversible feature of in situ pore formation and erasure combined with the additional attachment of a carbon black layer, the composite film could be easily switched between cooling and heating modes by solvent post-treatment. This research establishes a cost-efficient strategy with high flexibility in the structural manipulation concerning the construction of porous polymeric PDRC coating.