Low grade thermal energy harvester using graphene-based thermocells

Graphene-based thermocells for the conversion of low grade waste heat into electrical energy are investigated. A maximum current and power density of 0.63 A m(-2) and 0.19 W m(-2) respectively for 1 mV s(-1) at a temperature gradient of 50 degrees C is obtained. The maximum energy conversion efficiency relative to Carnot efficiency is 1.57% which is 1.1 times higher than the literature data. A constant ohmic overpotential of 0.07 V is calculated from equivalent circuit analysis. This low value of ohmic overpotential indicates higher ionic conductivity in the electrolyte medium. The mass transfer overpotential is low and is calculated as 0.2133 V for all load variations, indicating constant redox behaviour and an increased energy conversion efficiency of the device. The double layer capacitance is estimated as 3.72 F at a very low load (ca. 1 mV s(-1)) and 0.32 F at a very high load (ca. 100 mV s(-1)) thereby demonstrating the functioning ability of the device at higher loads. The Seebeck coefficient for a graphene electrode is evaluated to be 0.0117 V K-1 and is in satisfactory agreement with the literature value of 0.02 V K-1 for carbon-based devices.