Double-network thermocells with extraordinary toughness and boosted power density for continuous heat harvesting

Thermocells hold great potential for continually harvesting waste heat to power ubiquitous electronics, but their integration is hampered by leakage risk and poor mechanical properties of liquid electrolytes. Although some physically crosslinked networks have been introduced to prepare leak-free quasi-solid thermocells, they are easy to break because of extremely small fracture energies of about 10 J m−2 and limited by low output power densities of 0.01–0.06 mW m−2 K−2. Herein, this study designs double chemically crosslinked networks to address the mechanical challenges while also improving power density. The thermocell shows a large stretchability of 217%, impressive strength of 1,190 kPa, notch insensitivity, extraordinary toughness of 2,770 J m−2, and a boosted output power density of 0.61 mW m−2 K−2. It works stably even when being sliced and stretched. This study breaks the mechanical limitations of thermocells and will benefit the realization of sustainable self-powered wearable electronics in the era of the internet of things.