Radiative Cooling Hierarchically Porous Fluoropolymer Film by Ambient Humidity-Induced Phase Separation

Daytime passive radiative cooling is a highly desirable technology for cooling objects exposed to direct sunlight, offering a sustainable and energy-efficient solution without the need for electricity. However, widespread implementation of this cooling method is hindered by the complex preparation process and limited cooling power of existing radiative cooling materials. Herein, a simple and sustainable ambient humidity-induced phase separation approach was proposed for the fabrication of porous fluoropolymer films with unique hierarchical structures. This method is based on the exchange from DMSO (good solvent) to H2O (poor solvent), where interactions within and between the poly(vinylidene fluoride) (PVDF) and poly(vinyl alcohol) (PVA) are first inhibited and then restored, leading to polymer dissolution and cross-linking. The PVA-coated granular PVDF was phase-separated from the solution by controlling the rate of solvent exchange, resulting in the formation of film with graded micro- and nanoporosity upon freeze-drying. The porous fluoropolymer film exhibits an exceptional spectral performance with an infrared emissivity exceeding 97.6% within the atmospheric window and a high sunlight scattering capability of 95.3%. When subjected to direct sunlight, the porous fluoropolymer film is capable of achieving an average net radiative cooling power of 76.09 W m–2, rendering it to cool objects by 7.5 °C. Moreover, the film demonstrates mechanical flexibility and environmental stability, enabling its application on irregularly shaped cooling surfaces. This study therefore introduces a promising avenue for the cost-effective and large-scale production of radiative cooling porous materials.