Hydrogel-Based Stimuli-Responsive Radiative and/or Evaporative Cooling Materials for Carbon Neutrality

Radiative cooling cools by radiating heat toward outer space. Evaporative cooling cools by dispatching the latent heat of water via evaporation. These two types of cooling operate without additional pollutant generation. Hydrogel is the one substance that can combine the above two cooling strategies and thus holds high promise to facilitate the construction of a carbon neutral society. This review article first documents the basic principles related to passive cooling, evaporative cooling, heat transfer, and light modulation. Then, the general considerations for hydrogel-based cooling materials are summarized. Third, stimuli-responsive hydrogel-based cooling systems are systematically discussed from aspects of operation mechanisms as well as material selections. Lastly, typical applications of hydrogel-based radiative and/or evaporative cooling materials in buildings, electronic devices, and wearable devices are discussed. Challenges for further development of the hydrogel-based cooling materials are discussed as ending remarks.