A Flexible Passive Daytime Radiative Cooling Composite Membrane With High Performance Based on Synergistic Mesoporous‐Structure Scattering and Phonon Emission Constructed by Micro‐/Nano‐Silica

ABSTRACT Passive daytime radiative cooling (PDRC) offers a pathway to zero‐energy cooling and mitigation of urban heat islands. Current PDRC materials mainly utilize the first atmospheric window (8–13 µm), leaving the second window (16–25 µm) under‐exploited, limiting cooling performance. Here, we report a high‐performance multi‐level porous flexible composite fiber membrane fabricated via electrospinning, using alumina‐modified mesoporous glass (Al‐MG) as a functional filler incorporated into a polylactic acid (PLA) matrix, and synergistically combined with micro/nano‐silica (SiO 2 ) and polydimethylsiloxane (PDMS) (abbreviated as AMPS). The AMPS membrane exhibits a solar reflectance of 98.12% (0.3–2.5 µm) and infrared emissivities of 95.92% and 89.03% in the first and second atmospheric windows, respectively. Outdoor tests show average sub‐ambient temperature drops of 12.24°C on sunny days and 11.20°C on cloudy days. In simulated automotive thermal management, the surface temperature decreased by up to 23.8°C. EnergyPlus simulations across five Chinese climate zones indicate summer cooling energy savings of 8.34–34.72% with a cooling power of 116.29 W/m 2 . This study presents a high‐performance, weather‐resistant radiative cooling material and establishes a systematic design strategy, providing a framework for next‐generation PDRC materials.