Highly Porous Yet Transparent Mechanically Flexible Aerogels Realizing Solar-Thermal Regulatory Cooling

Highlights A lamellar-structured fluorinated cellulose nanofiber aerogel film is prepared by filtration-induced delaminated gelation and ambient drying. The aerogel film demonstrates exceptional mechanical flexibility and resistance to complex deformations. The aerogel film displays low thermal conductivity, high visible-light transmittance and superior selective infrared emissivity, rendering it high solar-thermal regulatory cooling performance. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01356-x.


S1 FTIR and XPS Characterizations
FTIR was utilized to analyze surface modification and characterize the molecular composition of FCNF. Figure S5a shows the infrared spectra of FCNF and CNF at 3800-600 cm -1 .The FCNF retains the characteristic absorption peak of CNF, and the broad absorption peak at 3395 cm -1 is attributed to the O-H stretching vibration in the molecular composition of CNF.The peak at 2874 and 1581 cm -1 are attributed to the C-H stretching vibration of CH2 and the bending vibration of H-OH.The absorption peak at 1013 cm -1 is ascribed to C-O stretching vibration of the alcohol hydroxyl group of CNF [S1].As shown in Fig. S5b, new absorption peaks appear in the infrared spectra at 1400-800 cm -1 , the peaks at 1255 and 1196 cm -1 are attributed to the characteristic stretching vibration of -CF2 and -CF3, respectively.In addition, the peaks at 1005 cm -1 and 890 cm -1 are attributed to Si-O-Si and Si-O-C [S2], respectively.XPS was used for qualitative and semi-quantitative analysis of FCNF and CNF.The peaks at 533.1 and 288.4 eV shown in Fig. S6a correspond to the oxygen and carbon in the CNF before modification.Three new peaks located at 689.5, 154.2, and 103 eV in the spectrum of F-CNF are attributed to F1s, Si2s and Si2p [S3, S4].As shown in Fig. S6b, a new peak appears at 290 eV in the FCNF spectrum and is attributed to the C-F bond in the PFOTES chain, indicating that PFOTES is grafted onto the surface of CNF.The restricted spectrum of C1s was compared to the binding energy, which further confirmed the surface modification of CNF. Figure S6c, d shows the C1s narrow scan spectra of CNF and FCNF, respectively.The peaks at 282.9, 284.6, and 286.1 eV are assigned to C1: C-C/C-H, C2: C-O and C3: O-C-O (three types of carbon bonds) [S5].CNF is mainly a C-O bond (C2 type), while the new peaks of FCNF at 291.9 and 294.0 eV correspond to -CF2 and -CF3, respectively.This further confirms that PFOTES have been successfully grafted onto the surface of CNF.

S2 Calculation of Porosity
Porosity () of the CNFA is calculated by Eq. ( S1): where  is the porosity,  0 is the apparent density of CNFA, and  is the volume density of polymer that is estimated from the average density of neat CNF of 1.6 g cm -3 .

S4 Calculation of Radiative Cooling Power
Radiative cooling performance is influenced by solar irradiance and atmospheric downward thermal radiation.Meanwhile, heat can be transferred from the ambient surrounding to the radiative cooling sample through the thermal conduction and convection due to the temperature differences between the cooling sample and the ambient environment.The device meets the constraints of the power balance equation and the net cooling power (  ()) is calculated by Eq. ( S3) [6]: is the surface temperature of cooling sample and   is the ambient temperature.  () is the power radiated by the DAF, and   (  ) is the absorbed atmospheric thermal radiation at   .  is the incident solar irradiation absorbed by the DAF, and  + is the power lost due to thermal convection and conduction.The net cooling power defined in Eq. ( S3) could reach a high value by increasing the radiative power of DAF and reducing either the solar absorption or the parasitic heat gain [S7].These parameters are calculated by the following Equations [S8, S9]: is the spectral radiance of a blackbody at the temperature (T).h is the Planck's constant,  is the speed of light in a vacuum and  is the Boltzmann constant.The (,  ) is the directional emissivity of the surface at wavelength .The   (,  ) = 1 − () 1/ is the angle-dependent emissivity of the atmosphere, and () is the atmospheric transmittance in the zenith direction.The   () and   (  ) are determined by both spectral data of the DAF and the emissivity spectra of the atmosphere according to MODTRAN of the Mid-Latitude Summer Atmosphere Model.The solar illumination is represented by the AM1.5 spectrum ( 1.5 ()) in Eq. ( S6) [S10].In Eq. ( S7), ℎ  = ℎ  + ℎ  is a combined nonradiative heat coefficient that captures the collective effect of conductive and convective heating, which is limited to a range between 0 and 12 W m -2 K -1 .
The average solar reflectance ( solar ) is defined as:  S9) where I BB (λ, T) is the spectral intensity emitted by a blackbody and ε LWIR (, ) is the surface's angular spectral thermal emittance in the range of 8-13 m.

Fig. S1 S9 Fig. S3
Fig. S1 SEM image of the freeze-dried FFCNF hydrogel sample taken halfway during the filtration

Fig. S10
Fig. S10 Typical strain-stress curve of the CF is the wavelength of incident light in the range of 0.3-2.5 m, I solar (λ) is the normalized ASTM G173 Global solar intensity spectrum, and ρ solar (, ) is angular spectral reflectance of the surface.The average emittance (  LWIR ) in the LWIR atmospheric transmittance window is defined as: