Moisture thermal battery with autonomous water harvesting for passive electronics cooling

Passive cooling of high-power electronics with minimum energy and water input is critical for the global water-energy nexus but has been challenging because of the large fluctuation in power and heat loads between the on/off-peak hours. Here we develop a moisture thermal battery (MTB) by coating superabsorbent hydrogel onto a pin fin heat sink with a large surface. The MTB leverages large latent heat and high thermal conductance of water evaporation for electronics cooling during on-peak hours and, importantly, autonomously harvests atmospheric moisture and stores the water during off hours. The MTB provides a thermal capacity of up to ∼200 kWh m−2 for evaporative cooling with a record-high effective heat transfer coefficient of up to ∼1,000 W m−2 K−1 for a passive device (no external power). The MTB suppresses the temperature fluctuation of a high-power field effect transistor and a computer CPU between the on- and off-peak hours by ∼15 K.