Harvesting body heat through clay-based ionic thermoelectric devices

Direct conversion of body heat into electricity through thermoelectric (TE) devices is emerging as an attractive option to power wearable electronics. As semiconducting TE devices suffer from the trade-off between electronic and thermal conductivity and high operating temperature, ionic thermoelectric devices relying on atmospheric humidity perfectly fit this low-temperature operating condition. Here, atomically thin 2D channels of reconstructed clay membranes were applied to demonstrate the possibility of harvesting electricity from body heat through the ionic thermoelectric (i-TE) effect. Nanofluidic membranes prepared by reconstructing layers of montmorillonite clay (MMT) displayed outstanding i-TE characteristics. Thermal transport of intercalating cations through an interconnected network of 2D channels yielded a Seebeck coefficient (S_i) of up to 13.63 ± 1.13 mV K^-1. As the hydration of molecularly thin 2D channels relies on atmospheric water molecules, the ionic conductivity and S_i of MMT increase with increasing humidity levels in the atmosphere. In contrast to polymer-based i-TE devices, clay membranes sustain exposure to high temperatures (∼200 °C, 5 min) and self-repair physical damages with the help of water droplets. The MMT membrane deposited on a PET film generated voltages of up to 63 mV (ΔT = 1.8 K) at 85% RH upon being pasted on human skin.