Development of a Thermally Insulating Passive Daytime Radiative Cooler Using Hierarchical Silica Aerogels

Passive radiative cooling is an energy-free technique for dissipating heat to space (3 K) through the atmospheric transparency window (8–13 μm); it can be used for applications such as enhancing thermal efficiency in buildings or refrigeration. Silica is a promising radiative cooling material due to its wide bandgap, high mid-IR emissivity, and low cost. However, it has two major disadvantages. First, its moderate refractive index across the solar spectrum limits optical reflection and allows for transmission of solar radiation, and second, the Reststrahlen effect is observed in the atmospheric transparency window, which limits IR emittance of bulk silica. In this study, we developed a hierarchical organosilica-based material that mitigates both disadvantages by integrating solar-scattering silica spheres into an IR-emissive and thermally insulating silica aerogel. The large particles effectively backscatter solar radiation due to their high refractive index mismatch with the low effective refractive index aerogel matrix. Furthermore, the porous silica-based aerogel matrix emits in the mid-IR almost ideally by mitigating the Reststrahlen effect through its reduced refractive index. A 3.5 mm-thick ambiently dried aerogel with 15.6 particle vol % of silica showed large solar-weighted reflectance (97.8 ± 0.5%) and high emissivity (99.2 ± 0.3%) in the atmospheric transparency window while maintaining the thermally insulating properties of aerogels (0.055 ± 0.010 W/m·K). In the proof-of-concept outdoor experiment, the surface of the aerogel exposed to direct sunlight was 10 °C lower than the ambient temperature and 26 °C lower than a bare Al plate under the same condition, demonstrating significant passive daytime radiative cooling promising for refrigerated transport. In addition, the aerogel showed resistance to UV light exposure, water, dirt, and steam, indicating exciting potential for practical applications.