Achieving a Porous PDMS Film for Passive Cooling through the Utilization of Ultrafine NaCl Sacrificial Templates

Passive radiative cooling technology serves as an energy-free alternative to traditional cooling systems. Porous polymer structures are frequently employed for radiative cooling by leveraging the refractive index mismatch between the polymer and the pores, enabling the scattering of incoming sunlight. Recently, water-soluble and readily available Sodium chloride (NaCl) particles have been utilized as sacrificial templates for sustainable pore creation. Nevertheless, optimizing NaCl particle size, and thus the polymer pore size to enhance scattering capabilities remains a challenge. Here, we report a simple, scalable, and sustainable approach to creating an optimized porous polydimethylsiloxane (PDMS) film. This approach utilizes ultrafine NaCl powders as sacrificial templates, which were synthesized via ultrasonic precipitation to ensure their small size. The ultrafine NaCl particles have a size distribution centered around 6-8 μm, and the as-fabricated porous PDMS film achieves a high thermal emissivity of 0.95 within the atmospheric window (8-13 μm) and exhibits a reflectivity of 0.95 within the visible range (0.4-0.78 μm). Due to the desired dual-spectrum properties, the porous PDMS film exhibits a superior subambient cooling capacity over that fabricated with typically larger NaCl particles under strong sunlight. This study offers a scalable and practical radiative cooling solution for sustainable thermal management.