Planar-type thermally chargeable supercapacitor without an effective heat sink and performance variations with layer thickness and operation conditions

Thermally chargeable supercapacitor (TCSC) is a good candidate for simultaneous energy harvesting and storage in wearable and internet-of-things (IoT) electronic devices. Here we report planar-type TCSC made of graphene oxide layers intercalated with sulfate ions (SGO) acting as electrolytes/separators and reduced SGO layers (rSGO) as electrodes. The planar type configuration has advantage in creating a large temperature difference but the amount of charge or current is limited due to the relatively small cross sectional area. In addition, this type of thermal energy harvesters often suffer from a small temperature difference due to the large thermal resistance of ambient air when heat is not rigorously removed by a heat sink or/and forced convection. Here, we tested the performance of TCSC without an effective heat sink when the thickness of the SGO layer was increased along with different concentration of sulfuric acid and humidity. It was found that thicker SGO and higher humidity resulted in higher capacitance. The thermopower of TCSC was measured to be high (4.53 mV/K) under 50% relative humidity environment, and time-dependent energy harvesting by thermal charging and then energy usage by electrical discharging have been demonstrated. Temperature distributions in TCSC mounted on a forearm were simulated when the convective heat transfer coefficients on TCSC and skin as well as heat conduction through TCSC are altered. Under higher (or lower) convective heat transfer conditions considering the thermal contact resistance between TCSC and skin, it is advantageous to have higher (or lower) heat conduction through TCSC for larger temperature gradients across TCSC. Temperature distribution in TCSC was also experimentally tested, demonstrating that it is feasible to maintain a temperature difference of ~4 °C across TCSC and an output voltage of ~20 mV. The experimental outcomes could provide insight for harvesting thermal energy for wearable and distributed electronics without an effective heat sink in practice.