Hierarchical SiO ₂ @Cellulose Nanofiber Aerogels With Synergistic Optical–Thermal Regulation for High‐Performance Passive Cooling

Passive radiative cooling offers a sustainable pathway for thermal management by minimizing solar absorption while maximizing mid‐infrared (MIR) emission through the atmospheric transparency window. However, parasitic heat gain substantially compromises its net cooling efficiency. Here, we report a reversible xanthation‐mediated strategy to fabricate hierarchical SiO 2 @cellulose nanofiber aerogels that synergistically integrate broadband solar reflectivity, high MIR emissivity, and low thermal conductivity. The xanthation chemistry enables uniform, in situ anchoring of ∼300 nm SiO 2 nanospheres along nanofibers, creating a distinctive ‘pearl‐necklace’ morphology, while directional ice‐templating further constructs lamellar hierarchical porous networks that suppress nonradiative heat transfer. The optimized aerogel exhibits an average solar reflectance of 95.6%, a MIR emissivity of 95.3% within the 8–13 µm atmospheric window, and an ultralow thermal conductivity of 0.028 W m −1 K −1 . Under 1000 W m −2 solar irradiance, it achieves a time‐averaged subambient cooling of 3.7°C and a net temperature reduction of 24.9°C compared to polystyrene foam, while extending refrigeration thermal cycling by 47.3%. This work provides a scalable material design framework for monolithic integration of optical selectivity and thermal insulation, offering a promising sustainable solution for energy‐efficient buildings, cold‐chain logistics, and next‐generation thermal management systems.