Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling

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Science  19 Oct 2018:
Vol. 362, Issue 6412, pp. 315-319
DOI: 10.1126/science.aat9513
  • Fig. 1 The formation and optical properties of P(VdF-HFP)HP.

    (A) Schematic of the phase inversion process, showing the formation of a hierarchically porous polymer coating from a solution of acetone (solvent), water (nonsolvent), and P(VdF-HFP) (polymer). (B) Micrographs showing top and cross-section views of P(VdF-HFP)HP. Inset shows the nanoporous features. (C) Photograph superimposed with schematics to show that high Embedded Image and Embedded Image enable a net radiative loss and PDRC. (D) Spectral reflectance Embedded Image of a 300-μm-thick P(VdF-HFP)HP coating presented against normalized ASTM G173 Global solar spectrum and the LWIR atmospheric transparency window. Embedded Image (0.96) and Embedded Image (0.97) are exceptionally high, especially given that they are achieved on a black selective solar absorber (fig. S2) (29). (E) P(VdF-HFP)HP’s high Embedded Image and (F) Embedded Image across angles result in excellent hemispherical Embedded Image and Embedded Image.

  • Fig. 2 The optical properties of P(VdF-HFP)HP.

    (A) A wireframe showing the structure of P(VdF-HFP), with the VdF and HFP repeating units shown. (B) Experimental complex spectral refractive index (n + iκ) of P(VdF-HFP), showing negligible absorptivity in the solar, and high emissivity in the LWIR, wavelengths. The peaks in κ correspond to the vibrational modes of different molecular components (e.g., CF3, CF2, CF, C-C, CH2, C-H, and carbon backbone) (1719). (C) Size distributions of nano- and micropores in P(VdF-HFP)HP, showing number-weighted mean pore sizes of ~0.2 μm for nanopores and ~5.5 μm for micropores. (D) Simulated scattering cross-section spectra of circular micro- and nanovoids in P(VdF-HFP)HP. Voids of different sizes collectively scatter all solar wavelengths, resulting in a high Embedded Image. (E) Spectral LWIR emittance and (F) Embedded Image of P(VdF-HFP)HP compared to a solid P(VdF-HFP) slab of the same volume. As evident, the former has a higher spectral and angular emittance. Further details are provided in the supplementary materials (4).

  • Fig. 3 Passive daytime radiative cooling performance of P(VdF-HFP)HP.

    (A) Schematic of the setup for testing performance under sunlight. (B) Topographic and meteorological information of the test locations. (C) Average solar intensity (Isolar) and subambient temperature drops (ΔT) of P(VdF-HFP)HP coatings in New York, Phoenix, and Chattogram. (D) Detailed Isolar and (E) temperature data of the result for Phoenix in (C). (F) Isolar and cooling powers (Pcooling) of P(VdF-HFP)HP coatings measured in New York and Phoenix. (G) Detailed Isolar, (H) temperature tracking, and (I) Pcooling data of the result for Phoenix in (F). Dotted line in (I) indicates average Pcooling over the duration of the experiment. Additional information is provided in the supplementary materials (4).

  • Fig. 4 Versatility of P(VdF-HFP)HP coatings.

    P(VdF-HFP)HP can be (A) painted onto plastics, (B) spray-coated on copper, (C) dip-coated on wood, and (D) made into strong, flexible, and freestanding sheets for tarpaulin-like designs. (E) Spectral reflectances of ~350-μm-thick blue and yellow P(VdF-HFP)HP coatings and (F) of a black P(VdF-HFP)HP coating compared to a commercial black pigment on reflective and black substrates. (G) Despite being on black substrates, their Embedded Image surpasses those of similarly colored “IR-reflective” pigments (~25-μm-thick films) on both black and reflective substrates.

Supplementary Materials

  • Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling

    J. Mandal, Y. Fu, A. Overvig, M. Jia, K. Sun, N. Shi, H. Zhou, X. Xiao, N. Yu, Y. Yang

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

    Download Supplement
    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S16
    • Tables S1 to S3
    • References

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