Highly Compressible and Efficient CNT/rGO/PDMS Thermoelectric Generator Based on a 3D Sponge-Structured Network for Harvesting Energy from a Shoe Sole

A compressible and highly elastic sponge thermoelectric generator (S-TEG) that exhibits excellent mechanical properties under an applied compressive force was fabricated. Carbon nanotubes were surface-modified via oxidation (m-CNTs) and combined with reduced graphene oxide (rGO) to form an m-CNT@rGO composite. A thermoelectric composite with a cylindrical sponge structure was fabricated by adding m-CNT@rGO to polydimethylsiloxane (PDMS) and sodium chloride, followed by chemical doping with FeCl₃ (p-type) and poly(ether imide) (n-type) solutions to improve the thermoelectric effect. A stand-alone continuous thermoelectric module design was realized by connecting the cylinders in series; the electrodes were connected by attaching copper sheets to the top and bottom of the S-TEGs. The system generated a voltage of 24 mV at a temperature difference of 20 K. The sponge cylinder with a 20 mm diameter, 15 mm height, and 1.5 cm distance between two cylinders resulted in the best thermoelectric effect, producing a voltage of 300 mV and maintaining its original elasticity under 50% strain. The developed S-TEG with a three-dimensional sponge structure was applied to insoles to convert the wearer’s body heat into electrical energy. The S-TEGs are expected to sustainably power various wearable electronic devices, realizing a truly self-powered system. This innovation is anticipated to not only improve the performance of wearable devices but also promote the development of smart insoles.