Hierarchically structured electrospun MWCNTs/h-BN bilayer membranes with passive radiative cooling and microwave absorption

Sequential electrospinning creates spatially separated MWCNT/h-BN bilayers. • M-B-30 delivers an RL min of −34.26 dB and an EAB max of 4.48 GHz. • M-B-30 reaches 91.52% solar reflectance and emissivity up to 0.979. • Outdoor cooling lowers M-B-30 to 16.45 °C below ambient temperature. • Bilayer design balances impedance matching, dielectric loss, and radiative cooling. Integrating passive radiative cooling with microwave absorption in a single membrane remains challenging because conductive carbon-based absorbers generally exhibit strong solar heating, whereas highly reflective components alter the dielectric response and disrupt the impedance-matching balance required for efficient microwave attenuation. Here, bilayer MWCNTs/h-BN nanofibrous composite membranes were fabricated by sequential electrospinning. The h-BN-containing polyacrylonitrile (PAN) upper layer was designed to enhance solar reflection and mid-infrared emission while moderating the impedance transition from free space to the lossy MWCNTs/PAN lower layer. By adjusting the h-BN content, the optical and electromagnetic responses of the bilayer membranes were effectively coordinated. The optimized M-B-30 membrane achieved a minimum reflection loss of −34.26 dB at 1.5 mm and a maximum effective absorption bandwidth of 4.48 GHz at 1.9 mm. It also exhibited a solar reflectance of 91.52% and an emissivity of up to 0.979 within the 8–13 μm atmospheric window, resulting in a maximum sub-ambient temperature reduction of 16.45 °C during outdoor testing. The microwave absorption was attributed to the balance among layered impedance matching, conductive dissipation, polarization relaxation, and multiple scattering, whereas the radiative-cooling performance resulted from enhanced solar scattering and efficient mid-infrared thermal emission. This spatially separated bilayer design provides an effective route for integrating microwave attenuation and passive thermal regulation in multifunctional composite membranes. Download: Download high-res image (589KB) Download: Download full-size image