Ion-Track Polycarbonate Membrane for Sustainable All-Day Passive Bilateral Thermal Management

Passive daytime radiative cooling (PDRC) materials face the pervasive challenge of overcooling at night, which increases heating energy consumption, particularly in climates with high diurnal temperature variation. Here, we demonstrate an ion-track polycarbonate (PC) membrane radiative cooling system with Janus mid-infrared optical properties (J-ITMRC) that achieves all-day passive bilateral thermal management without external energy input. The membrane comprises a PC layer with randomly distributed high-aspect-ratio air columns engineered by multidirectional ion-irradiation and chemical etching on the front side and a thin metallic reflective coating on the back side. This asymmetric structure exhibits distinctive Janus mid-infrared optical properties, with high solar reflectance (R⃖solar = 95.7%) and MIR emissivity (ε⃖rad = 96.6%) of the front side for daytime cooling, and high MIR reflectivity (R⃖rad = 95.4%) of the back side to minimize nocturnal heat loss. Outdoor experiments showed that the J-ITMRC provides a temperature reduction of up to 7.6 °C below ambient during the day, whereas its cavity temperature is up to 2.0 °C above ambient during the night. Building energy simulations across 34 Chinese cities revealed an average 20% reduction in the total HVAC energy consumption. This work highlights the potential of microstructure-engineered polymer membranes based on ion-track technology as high-performance passive materials for intelligent thermal management in energy-saving buildings.