Bio-inspired BC aerogel/PVA hydrogel bilayer gel for enhanced daytime sub-ambient building cooling

Radiative cooling and evaporative cooling, as the common heat dissipation strategies in the nature, are considered as promising alternatives to conventional ways for providing huge cooling capacity without energy consumption and waste discharge. Herein, inspired by natural creatures’ intelligently utilization of their own unique functionalities to fight against the extremely hot surroundings, a hydrophobized bacterial cellulose (BC) aerogel/polyvinyl alcohol (PVA) hydrogel bilayer gel combining radiative and evaporative cooling together was fabricated through facile and effortless solvent exchange, freeze dry, hydrophobization and dip-coating. The as-obtained bilayer gel shows high visible reflectance (98.8%) and emissivity in atmospheric window band (0.86) due to the inherent fiber structures of BC aerogel, contributing to the huge reduction in heat input and strong dissipation of thermal aggregation. Experimental observations indicated that the evaporation of the water swollen in hydrogel allow for sub-ambient temperature drops of ~ 18 °C under direct sunlight. After combining hydrophobized BC aerogel and PVA hydrogel, the followed validation tests demonstrated that a temperature difference of 16.4 °C could be obtained by covering the roof with bilayer gel attributing to the synergistic effect of radiative cooling and evaporative cooling. Moreover, the water contact angle of 142° gained from the hydrophobization endows the bilayer gel with the great resistance against the collapse of structure and failure of cooling properties resulting from water. Additionally, the facile and effortless fabrication approach makes scalable preparation possible. Therefore, this easily fabricated bilayer gel with great radiative and evaporative properties shows promising applications in building cooling and maintenance and residential thermal comfort.