A hydrogel-based passive cooling Janus material with both high solar reflectance and remarkable self-cleaning capability

Abstract

Passive evaporative cooling technology is an environmentally friendly and promising strategy for thermal management, requiring no additional energy input and achieving cooling power exceeding 300 W m⁻². Among various materials, hydrogels stand out due to their high water content and stretchable structure, making them suitable candidates for evaporative cooling. However, conventional hydrogel-based cooling materials often face challenges, such as low solar reflectance, poor durability, and a hydrophilic surface susceptible to rain flushing and dust pollution, limiting their practical application. Herein, a monolayer hydrogel-based passive cooling material with Janus wettability and high solar reflectance, as well as exceptional flexibility and mechanical durability, is proposed. The hydrophobicity of the upper surface (contact angle 133°) provides the hydrogel with excellent self-cleaning capability, while the high solar reflectance (80.06%) effectively reduces the accumulation of heat from solar radiation. Outdoor tests further support the material's potential for practical application, as it achieves a remarkable sub-ambient temperature drop of 27.4 °C under an average solar irradiance of 709.3 W m⁻². This research provides a novel strategy for developing efficient passive cooling materials capable of adapting to complex environments and holds great promise for contributing to global warming mitigation and carbon emission reduction.