Study of graphene oxide cathode in direct thermal charing cell for harvesting low-grade heat

Abstract

Energy crisis is an important social issue in the modern world and sustainable energy development is in surge. Meanwhile, enormous amount of waste heat, which is also a form of energy, is produced during industrial and commercial activities. Low-grade waste heat contributes 60% to the total amount of waste heat around the globe. Energy generation from waste heat is considered as a possible solution to solve the above problems sustainably. Despite the rapid development of thermoelectric devices that convert heat into electricity, the high working temperature is far beyond the low-grade-heat range. Recently, researchers have put substantial efforts on developing electrochemical systems that can efficiently convert low-grade waste heat into electricity. However, these technologies face a huge challenge in practical application because most systems require spatial or temporal temperature differences to operate. High operation costs are involved to put these electrochemical systems into real practice.

In this study, Direct Thermal Charging Cell (DTCC) is proposed to harvest low-grade waste heat in isothermal condition. Graphene oxide in DTCC cathode plays the crucial role in converting heat into electricity. Pseudocapacitive voltage of graphene oxide can be thermally induced and a temperature coefficient of 3.1 mV/K is achieved in this study. The mechanism of the thermal response in graphene oxide electrode is investigated in this study. Graphene oxide electrode is characterized by elemental analysis, x-ray photoelectron spectroscopy and work function measurement. The positive relationship between thermal response and oxygen content of graphene oxide is revealed.

Mechanism of DTCC is also discussed in this study. DTCC can be thermally charged and then discharged electrically at isothermal condition. No temperature difference has to be applied across electrodes and no thermal cycle is needed. DTCC can achieve an energy conversion efficiency of 3.32%, which is equivalent to 25.3% of the Carnot efficiency. The thermal charging process and electrochemical-capacitor-like discharging process of DTCC are investigated. X-ray photoelectron spectroscopy and work function measurement was done to characterize the redox reactions in graphene oxide cathode and polyaniline anode during discharge. Electrochemical measurements of DTCC are used to explore its working mechanism.