Bright-white hydrogels for on-demand passive cooling

Passive cooling permits thermal management of near-zero energy consumption and low CO 2 emissions. Despite significant progress of passive radiative coolers, comfortable and steady temperatures can hardly be achieved due to their inadequate daytime cooling power (below 0.2 kW m − 2 ) yet over-cooling at night. Here, we provide a bright-white hydrogel that enables on-demand passive cooling by virtue of adaptive water evaporation and high solar reflectance up to 86.1%. Notably, theoretical cooling power determined by the evaporating rate can reach 1.25 kW m −2 in daytime but decreases dramatically at night. Hence sub-ambient temperature reduction of 11–13 °C at noon yet nearly none at night are realized, with the diurnal temperature difference narrowed significantly. Moreover, effective cooling using colored hydrogels, and transition from evaporative cooling to solar heating have been demonstrated. This novel evaporative cooling approach will pave the way for smart passive coolers of high efficiency, colorful appearance, and low cost.