Efficient Passive Cooling Over a Novel Bifunctional Polymer Bilayer Composite Simultaneously Possessing Radiative and Evaporative Cooling Properties

Passive cooling technologies being free from additional energy consumption, offer significant advantages in reducing carbon emissions and mitigating global warming. However, radiative cooling has a thermodynamic limit, and evaporative cooling requires additional components, which restricts their large-scale application, respectively. Herein, a radiative/evaporative bifunctional cooling bilayer is presented, featuring an upper layer composed of poly(vinylidene fluoride-cohexafluoropropene) [P(VdF-HFP)] integrated with a hygroscopic hydrogel lower layer [polyacrylamide (PAAm)/alginate-CaCl₂]. High solar reflectance (0.916) and long-wave infrared emittance (0.900) of the P(VdF-HFP) combined with hydrogel evaporation enable a notable 15.4 °C temperature drop under 706.3 W·m⁻² solar radiation. The bilayer is demonstrated to be effective under outdoor conditions for continuous three cloudy days, achieving an average temperature reduction ranging from 6.3 to 15.7 °C. Considering the low cost and simplicity of the preparation method, ease of large-scale fabrication, and good cooling performance, this bilayer structure provides a promising strategy for the application of passive cooling.