Hydrophobic 1D and 3D Anodic Alumina Radiative Coolers on Aluminum: Humidity‐Resilient Daytime Sub‐Ambient Cooling

ABSTRACT

Passive radiative coolers (PRCs) operating under realistic outdoor conditions must preserve their optical response under humidity, condensation, and soiling. Here, we demonstrate hydrophobic one‐dimensional (1D) and three‐dimensional (3D) anodic aluminum oxide (AAO) radiative coolers on Al via surface functionalization with stearic acid (STA), increasing the contact angle while preserving morphology and spectral properties. The 1D‐AAO in sulfuric acid with STA exhibits a solar reflectance of 77% and a mid‐infrared emissivity of 90% in the 8–13 µm atmospheric window, while 3D‐AAO in sulfuric acid reaches emissivities up to 99%. Under outdoor summer conditions in Madrid, 1D‐AAO with STA achieves a maximum temperature drop of 12.9°C at 958 W m ⁻² , compared to an empty box using the same experimental setup, together with a theoretical cooling power of 161.6 W m ⁻² . Analysis under varying environmental conditions shows that through the STA functionalization ( P _cool ) cooling temperature (ΔT) increases ∼0.8°C per 100 W m ⁻² of solar irradiance and decreases ∼0.2°C per 10% relative humidity. Hydrophobic functionalization enhances cooling in both structures, with a ∼4‐fold increase in porosity (from 10% to 36%) leading to an ~8‐fold enhancement of the cooling effect under humid conditions. This hydrophobic functionalization strategy enhances the humidity robustness of AAO‐based radiative coolers without sacrificing their optical performance or requiring additional metallic coatings.