Ion regulation in double-network hydrogel module with ultrahigh thermopower for low-grade heat harvesting

Harvesting low-grade heat as source of electrical power has emerged as a research frontier for self-powered wearable devices, as a promising route to overcome challenges associated with limited access to grid power. However, such promise is compromised by current attainable thermopowers and constraints of rigid or complicated thermoelectric systems. We report an ultrahigh thermopower of 19.32 mV K-1 on a stretchable thermoelectric module by the assembly of porous electrodes and hybrid hydrogel, containing 1-ethyl-3-methylimidazolium and tetrafluoroborate ions and polyethylene glycol. The anions act as charge carrier; for the first time, distinct ion mobilities are directly measured by 2D-diffusion-ordered nuclear magnetic resonance spectroscopy. By regulating ion transport via the synergy of selective ion-localization and thermo-osmotic mechanism, such design provides an effective strategy to increase thermopower, and our device is endowed with high output power density, tailorable architecture, and excellent stretchability, which is showcased in a thermoelectric wristband for body heat recovery.