An ultrathin ionic thermoelectric cell design utilizing near body heat for self-powered wearable electronics

Harvesting low-grade heat is a sustainable way to power wearable electronics, and quasi-solid-state ionic thermoelectric cells offer a flexible, low-cost option. Their use, however, has been limited by a key trade-off: miniaturization reduces the internal thermal gradient and compromises performance. Here, we address this challenge with an ultrathin asymmetric architecture that separates thermal energy harvesting from the conventional reliance on a sustained through-plane temperature gradient. The design couples thermally driven ionic modulation at one interface with engineered pseudocapacitive charge storage at the other. Our 1-mm-thick device delivers an open-circuit voltage of 0.1 V, a power density of 1.6 W m−2, and an energy density of 1500 J m−2 using near-body heat. An array of 20 cells generates 1.9 V and a peak power of 23 W m−2, enabling continuous smartwatch operation. This strategy provides a practical route to ultrathin ionic thermoelectric cells for self-powered wearable systems. Ionic thermoelectric cells suffer from compromised performance upon miniaturization due to reduced thermal gradients. Here, the authors propose an ultrathin asymmetric architecture that couples ionic modulation with pseudocapacitive storage, enabling high power density for self-powered wearables.