Thermally Chargeable Supercapacitor with Charging and Discharging Cycles for Wearable and IoT Electronics

Abstract

Thermally chargeable supercapacitors are good candidates for energy harvesting and storage in wearable and internet-of-things (IoT) electronic devices. We report a Thermally chargeable supercapacitor (TCSC) which has good areal capacitance of 14.58 mF/cm^2 and charge storage capability in conjunction with a spiral bimetal coil and heat source to create an in-situ thermal cycling setup modeling real world application. The thermally chargeable supercapacitor (TCSC) has graphene oxide intercalated with sulfate ions (SGO) acting as electrolyte/separator and reduced sulfate graphene oxide (RSGO) electrodes fabricated by laser irradiation on a film of SGO over PET substrate using a 3D printer with laser diode assembly. The fabricated supercapacitor employs the soret effect as the transport mechanism, which results in high thermoelectric voltage. The TCSC showed improved capacitance and higher current output when electrode thickness and electrolyte concentration increased. Humid environments resulted in improved capacitance of the TCSC. The in situ thermal cycling setup was constructed by bending the TCSC with the help of aluminum foil as substrate. One end of the supercapacitor specimen placed over the bimetal coil with the attached end acting as the hot side and the end far away acting as the cold side. Thermal charging occurs when heat source is on causing the bimetal coil to detach from the surface and maintain the temperature gradient across the TCSC which leads to voltage generation. When heat source is off bimetal coil comes in contact with the heat source and temperature gradient drops close to zero. The TCSC produces a differential voltage of 10 mV for a temperature difference of 6K generated by programming the heat source close to human skin temperature of 34 degrees C. The in- situ thermal cycling setup which is reported here gives us clarity on how the TCSC performs under human skin temperature conditions.