Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling

TLDR

Passive daytime radiative cooling cools surfaces by reflecting sunlight and emitting heat to space, yet existing designs are either inefficient or limited in application. The authors aim to develop a simple, inexpensive, scalable method to produce hierarchically porous P(VdF‑HFP)HP coatings with superior radiative cooling performance. They employ a phase‑inversion process to fabricate the porous polymer coatings, achieving high solar reflectance and long‑wave infrared emittance. The coatings exhibit 0.96 ± 0.03 hemispherical solar reflectance and 0.97 ± 0.02 infrared emittance, enabling ~6 °C subambient cooling and ~96 W m⁻² cooling power under 890 W m⁻² sunlight, matching or exceeding state‑of‑the‑art PDRC systems while being paint‑like in application.

Abstract

Passive daytime radiative cooling (PDRC) involves spontaneously cooling a surface by reflecting sunlight and radiating heat to the cold outer space. Current PDRC designs are promising alternatives to electrical cooling but are either inefficient or have limited applicability. We present a simple, inexpensive, and scalable phase inversion-based method for fabricating hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) [P(VdF-HFP)HP] coatings with excellent PDRC capability. High, substrate-independent hemispherical solar reflectances (0.96 ± 0.03) and long-wave infrared emittances (0.97 ± 0.02) allow for subambient temperature drops of ~6°C and cooling powers of ~96 watts per square meter (W m-2) under solar intensities of 890 and 750 W m-2, respectively. The performance equals or surpasses those of state-of-the-art PDRC designs, and the technique offers a paint-like simplicity.