Efficient passive cooling by a novel cellulose foam-hydrogel bilayer composite

Green and energy-free cooling serves as an effective strategy to address the global energy crisis. Radiative and evaporative cooling can be simultaneously achieved by integrating radiative materials and hydrogel into a bilayer system. However, hygroscopic hydrogel may reduce their cooling performance in outdoor environments due to dust contamination, rain erosion, and other factors, which limits their widespread application.Herein, based on the principle of interface design, a novel type of radiative/evaporative cooling bilayer is proposed, featuring an upper layer of regenerated cellulose (RC) foam and the lower layer of hydrogel for efficient passive cooling. The RC foam coated with SiO 2 and then encapsulated with polydimethylsiloxane (PDMS/SiO 2 @RC) has high visible reflectance (96%), high mid-infrared emissivity (90%), low thermal conductivity (0.044 W m −1  K −1 ) and exceptional hydrophobicity (water contact angle of 146°). After two weeks of outdoor exposure, the PDMS/SiO 2 @RC maintains a high visible reflectivity of 95%, demonstrating superior long-term cooling stability. By synergistically integrating radiative cooling, evaporative cooling, and thermal insulation, the bilayer cooler achieves a maximum cooling effect exceeding 15 °C under 1000 W m −2 solar irradiance. Owing to its low cost and facile fabrication, this bilayer system presents a promising pathway for passive cooling applications.