Bioinspired Cellulose‐Based Hygroscopic Aerogel with Hierarchical Encapsulation Design for Sustainable Passive Daytime Cooling <i>via</i> Synergistic Dual Mechanisms

Passive radiative cooling technology shows great application in next-generation thermal regulation fields, but still suffers from thermodynamic limits. Combining evaporation cooling and radiative cooling offers a promising solution to address this drawback, however, existing dual-functional devices exhibit poor efficiency and structure stability. Inspired by the vapor transfer process in the tree, an encapsulate-structured cellulose hygroscopic aerogel (CHA) is proposed via unidirectional freeze casting of crosslinked cellulose nanofiber/Al₂O₃ suspension and assembly of LiCl in 3D network of aerogel, featuring daytime radiative cooling integrated with hygroscopic-evaporation cooling functionality. It can efficiently eliminate water nucleation near the 3D network, boost vapor transfer kinetics in aligned channels, and address trade-offs between cooling efficiency and structure durability, thereby resulting in high water absorption of 2.7 g g^-1 in 90% RH, high solar reflectance of 96.4%, high infrared emissivity of 0.94, and well-structured stability. Field test demonstrated that it can achieve subambient cooling of 10.6 °C under direct sunlight during daytime with RH of 73%, higher than that of previous reports. Most importantly, CHA also exhibited unique structure stability and longtime use ability. This work paves the way for developing high and stable outdoor cooling materials toward energy savings.