A tubing shaped, flexible thermal energy harvester based on a carbon nanotube sheet electrode

A tubing-shaped, flexible electrochemical thermal energy harvester (thermocell), which can be wound around various types of waste heat sources, was fabricated. The thermocell utilizes the temperature dependence of the ferri/ferrocyanide (Fe(CN)63−/Fe(CN)64−) redox potential, providing a thermoelectric coefficient of ∼1.4 mV/K. A highly porous carbon nanotube (CNT) sheet, which is wrapped onto a thin platinum (Pt) wire, was used as an electrode for the redox reaction. The electrode performance was examined by comparing the output powers from the thermocells using a bare Pt wire and CNT sheet wound electrodes. The CNT sheet electrode showed a higher output power from 8.5 to 15.6 μW, and the short-circuit current density (jsc) was increased ∼1.8 times compared to that of the Pt wire electrode. The performance of the CNT sheet based thermocell was examined according to the winding number of the CNT sheet, the temperature difference between the two electrodes and the operating temperature. The series connection of the thermocells, to demonstrate the voltage and power scaling, was also examined with an understanding of the primary internal resistance that limits the output electrical power.