Phosphorylated cellulose-based hybrid aerogels with flame retardancy and hydrophobicity for passive radiative cooling

Growing energy demands for indoor cooling necessitate the development of sustainable building envelope materials. Passive daytime radiative cooling is a promising strategy; however, integrating excellent optical properties with fire safety, thermal insulation, and environmental durability remains a critical challenge. Herein, a multifunctional inorganic-organic hybrid aerogel (M-PCB-1) is rationally designed using phosphorylated cellulose nanofibrils and barium sulfate (BaSO₄) nanoparticles, followed by a hydrophobic methyltrimethoxysilane coating. The composite achieves an exceptional solar reflectance of 93.7% via broadband light scattering from BaSO₄ scattering centers and a high mid-infrared emittance of 95.0% associated with multiple infrared-active vibrational modes within the atmospheric window. Consequently, the M-PCB-1 aerogel delivers outstanding daytime cooling performance, achieving a temperature difference of up to 19.2 °C relative to the chamber ambient temperature under controlled xenon-lamp irradiation. Furthermore, the aerogel exhibits excellent thermal insulation (0.054 W/m·K) and robust chemically integrated fire safety. Phosphorylation endows the PCNF framework with phosphate-promoted char-forming flame retardancy, while the thermally stable BaSO₄ particles provide an additional physical barrier effect, resulting in a residual mass of 65.4% at 800 °C and an 88.6% reduction in total heat release. Additionally, the Cassie-Baxter wetting state endows the surface with high hydrophobicity, a water contact angle of approximately 145°, low water adhesion, and self-cleaning properties. Global building energy simulations suggest that integrating this aerogel into the modeled building envelope could reduce annual cooling-energy consumption by up to 13% under the specified assumptions. This work presents a sustainable, high-performance material paradigm for next-generation energy-efficient building thermal management.