Nanocomposite hydrogel for daytime passive cooling enabled by combined effects of radiative and evaporative cooling

Both radiative and evaporative cooling as passive cooling technologies can reduce dependence on active cooling technologies. However, most reported studies investigated radiative and evaporative cooling separately, and significant challenges still remain in achieving highly efficient combined radiative and evaporative cooling. Hence, a polyacrylamide/polyvinyl alcohol hydrogel with nanoparticles (NPs/NADES@PAAm/PVA) was fabricated by natural deep eutectic solvent for highly efficient daytime passive cooling. The NPs/NADES@PAAm/PVA with a solar reflectance of ∼0.95 and an atmospheric emissivity of ∼0.92 could realize daytime radiative cooling. Besides, the NPs/NADES@PAAm/PVA possessed robust mechanical strength, excellent flexibility, and great rehydration. After water absorption, both swollen NPs/NADES@PAAm/PVA and hygroscopic NPs/NADES@PAAm/PVA could simultaneously achieve radiative and evaporative cooling. Outdoor experiments demonstrated that the NPs/NADES@PAAm/PVA, swollen NPs/NADES@PAAm/PVA, and hygroscopic NPs/NADES@PAAm/PVA obtained an average temperature drop (ΔT) of ∼2.9, ∼6.2, and ∼3.6 °C under the sunlight, respectively, compared with the ambient temperature of ∼35.0 °C, indicating that the NPs/NADES@PAAm/PVA could achieve a sub-ambient temperature while the swollen and hygroscopic NPs/NADES@PAAm/PVA possessed better cooling performances due to the combined cooling effects. Another outdoor experiment proved that the NPs/NADES@PAAm/PVA had great application potential in building cooling and iced food preservation under sunlight, further promoting passive cooling technology.