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.