Self‐Layering Porous Cellulose Acetate/Ag Janus Film for Dual‐Mode Radiative Cooling and Heating with High Thermal Conductivity and Electromagnetic Shielding

ABSTRACT The dual‐mode Janus film integrating daytime radiative cooling and heating enables dynamic thermal management. However, its practical application remains limited by the low electrical conductivity and poor thermal conductivity, particularly for systems requiring efficient heat dissipation and electromagnetic shielding. Here, we report a novel porous cellulose acetate (PCA) /Ag Janus film prepared through the synergistic effects of cellulose acetate phase inversion and a self‐layering process driven by high‐density Ag fillers. The spontaneously formed PCA and Ag layers exhibit opposite optical properties, with the Ag layer also providing high thermal conductivity (10.4 W/(m·K)) and superior electrical conductivity (sheet resistance of 0.06 Ω/sq). Thus, the PCA/Ag Janus film not only enables the rapid diffusion of the radiative cooling or heating effect into the underlying space, but also achieves 70 dB of electromagnetic shielding efficiency. In heating mode, the PCA/Ag Janus film exhibits a photothermal effect of 17.5°C. More significantly, it achieves sub‐ambient cooling of 9.8°C in cooling mode and provides a substantial cooling effect of 16.8°C for heat‐dissipating electronic equipment operating during daytime in hot urban environments. Thus, this self‐layering PCA/Ag Janus film design offers a practical and efficient solution for multifunctional thermal management materials.