Sustainable Cellulose‐Based Aerogels with a Hierarchical “Foam‐Flake” Structure for Ultrahigh Emissivity Radiative Cooling

Biomass‐derived radiative cooling systems have garnered significant attention due to their inherent sustainability. However, biomass‐derived aerogels fabricated using a single ice‐templating strategy often suffer from compromised radiative cooling performance, primarily stemming from non‐tunable micro/nanostructures or inadequately designed crosslinking systems. Herein, a cellulose‐based aerogel featuring a unique “foam‐flake” hierarchical architecture has been developed to enable efficient daytime radiative cooling. This hierarchical porous structure is achieved by utilizing hydroxypropyl methylcellulose as both an intrinsic surfactant and a crosslinking component, in combination with directional freezing technology. The aerogel exhibits exceptional optical and thermal properties, characterized by a high visible‐light reflectance of ≈96.0%, an ultrahigh infrared emittance of ≈99.0%, a low thermal conductivity (λ = 59 mW·m −1 K −1 ), and a high limiting oxygen index (LOI = 31.5%). Notably, its infrared emissivity exceeds that of most reported radiative cooling materials. Field testing conducted outdoors in Guangzhou, China, achieves a significant sub‐ambient daytime cooling effect (≈8.8°C). Furthermore, the aerogel fabricated via the “foam‐flake” strategy exhibits excellent self‐healing capability, rapid solution recyclability, and biodegradability, thereby enabling a truly environmentally benign closed‐loop life cycle. This work provides an innovative and advanced dual‐templating strategy for the scalable manufacturing of sustainable aerogels, positioning them as a promising candidate for next‐generation, low‐carbon radiative cooling applications.