Protective-enhanced passive cooling epoxy aerogel coating with an asymmetric dense skin/porous core structure

Passive thermal management coatings are pivotal for energy efficiency; however, their practical implementation is often hampered by the inherent porosity of insulating fillers, which compromises barrier integrity and renders substrates vulnerable to environmental corrosion. This study aims to develop an epoxy-based passive cooling coating that decouples the highly porous internal structure from surface permeability. By utilizing a water-induced phase inversion (WIPI) process to form an asymmetric single-layer structure, the coating simultaneously integrates thermal insulation, broadband solar reflection, mid-infrared emission, and environmental barrier protection. By incorporating superhydrophobic silica aerogel into the microporous epoxy matrix, a synergistic multiscale pore network was established, integrating microvoids (∼960 nm) with aerogel-derived nanopores (∼50 nm). The combined effect of macroscopic gas insulation within micropores and the Knudsen effect in aerogel nanopores suppresses heat conduction. Meanwhile, the multiscale air/polymer/silica interfaces enhanced broadband solar scattering, yielding a total solar reflectance of 88.3%, including 93.2% in the visible region and 85.1% in the near-infrared region. The coating also exhibited a high weighted thermal emissivity of 95.78% in the 8-13 μm atmospheric window, providing favorable optical properties for passive daytime cooling. Electrochemical impedance spectroscopy (EIS) indicated that the aerogel-modified porous coating partially recovered the barrier performance lost by pore formation, showing improved corrosion resistance compared with the porous epoxy coating without aerogel. Outdoor box tests in Chengdu demonstrated a maximum internal temperature reduction of 7.5 °C relative to ambient air. In addition, EnergyPlus simulations suggested that applying the optimized coating to building envelopes could reduce cooling energy demand in different Chinese climate zones, with relative savings of up to 23.4% under radiation-dominated conditions. These results demonstrate that the as-prepared coating offers a feasible strategy for epoxy-based coatings integrating thermal insulation, solar reflection, mid-infrared emission, and enhanced environmental barrier performance.