Smart Flexible Porous Bilayer for All-Day Dynamic Passive Cooling

Radiative cooling and evaporative cooling are sustainable cooling techniques without additional energy input. However, radiative cooling lacks dynamic cooling ability, while evaporative cooling demands external water replenishment, hindering their applications. Herein, a smart radiative/evaporative cooling bilayer combining a polydimethylsiloxane (PDMS) upper layer with a hydrogel lower layer is presented for efficient all-day dynamic passive cooling. The PDMS layer with high solar reflectivity (0.930) and emissivity (0.952) provides excellent all-day radiative cooling and protects the hydrogel from solar exposure, while the hydrogel layer demonstrates remarkable water evaporation and absorption, achieving dynamic evaporative cooling. Thus, the synergy of the two layers significantly enhances the overall cooling performance. Specifically, the bilayer can achieve the peak cooling power values of 424.4 and 650.6 W m^-2 as well as the maximum subambient cooling temperatures of 10.4 and 3.7 °C during sunny and cloudy mid-days, respectively. Moreover, the bilayer obtains 3.2 °C warmer temperature compared with the PDMS alone during cold nighttime, while the two structures exhibit comparable cooling performance during hot nighttime, indicating the self-adaptive cooling property of the bilayer. In addition, the bilayer can achieve good cooling performance even under continuous cloudy days, offering a promising strategy for efficient all-day dynamic passive cooling.