Rectangular Bismuth Oxychloride With High Mid-Wave Infrared Reflectance to Strengthen Radiative Cooling Performance

Abstract Radiative cooling materials (RCMs) based on polymers, which reflect solar irradiance (0.24–2.5 μm) strongly and emit radiation within atmospheric window (8–13 μm) intensively, have yielded obvious cooling performance. However, RCMs reported failed to realize high mid-wave infrared (2.5–8 μm) reflectance (i.e., spectral selectivity), which play an important role in rejecting the radiation from the ambient, especially in high relative humidity. Thus, enhancing the reflectance of RCMs in mid-wave infrared can further strengthen the cooling effect in an environment of high relative humidity. The additive of fillers with high reflectance in mid-wave infrared is effective, low cost, and convenient to modify the cooling performance of polymeric RCMs. In this article, micron rectangular bismuth oxychloride (BiOCl) powders were proposed and synthesized using the hydrothermal method. Rectangular BiOCl boasts excellent mid-wave infrared reflective capacity (85%). Then, rectangular BiOCl was added into the top layer of flexible hierarchically porous complex membrane and improved the reflectance of porous membrane within 2.5–8 μm from 48.3% to 58.1%. However, the emittance of the obtained bilayer membrane within the atmospheric window was hardly affected. The membrane with BiOCl exhibited better cooling performance in humid ambient (70%), demonstrating the importance of spectral selectivity experimentally. This work proposed an effective strategy to modify the spectral selectivity of RCMs, and it could expand the application of radiative cooling materials in hot and humid areas.