Anisotropic nanocellulose-based aerogels for radiative cooling.

To this day, energy conservation, emission reduction, and environmental protection continue to be goals pursued by humanity. Passive radiation cooling, as a zero-consumption refrigeration technology, offers substantial opportunities for reducing global energy consumption and carbon dioxide emissions. It is of great significance to develop high-performance passive radiation cooling materials from sustainable materials. In this work, polyvinyl alcohol (PVA) and nanocellulose fibers (CNFs) were used as raw materials to build a hydrophobic aerogel with high strength, satisfied heat insulation and qualified radiative cooling effect by freeze-casting. The obtained aerogel with borax/CNFs = 2: 5 by directional freezing processed excellent mechanical property (compressive strength = 291 KPa) under low density of 38.1 mg·cm-3. Furthermore, the hybrid aerogel exhibited hydrophobicity, enhancing its durability and practicability. Significantly, the aerogel exhibited high solar reflectivity (96.8 %) at 0.2-2.5 μm, high emissivity (97.2 %) across the entire atmospheric window, and good thermal insulation performance (34.56 mW·m-1·K-1). Subsequently, the radiative cooling effect of composites were evaluated, and the hybrid aerogel under sunlight was approximately 9 °C cooler than EPS in hot summer. Our research will provide insights into developing radiative cooling materials, and also provides great possibilities of cellulose-based aerogels for applications in energy conservation.