Progress on the performances and applications of supercapacitors for thermoelectric conversion

Abstract

<p indent="0mm">The comprehensive utilization rate of energy in China is less than 40%, and most of the unused energy is lost in the form of heat energy. Improving the utilization rate of heat energy has become one of the urgent problems in the world. In the past few decades, immense efforts have been made to explore and develop alternative technologies to capture low-grade heat. Thermoelectric technology, as an effective way to realize low-grade heat recycling, provides a simple and environmentally friendly solution for the direct conversion of low-grade heat to electric energy. Among them, supercapacitors are favored by researchers because of their excellent electrochemical performances and convenient functions of thermoelectric conversion. Therefore, this paper summarizes the latest progress on the performances and applications of supercapacitors for thermoelectric conversion. First, based on the analysis of the research progress for thermoelectric conversion technology at home and abroad, we discuss the composition and types of supercapacitors, including the electrical double-layer capacitors, pseudocapacitors, and asymmetric hybrid capacitors. When thermoelectric conversion is carried out in supercapacitors, thermally-induced effects including thermocapillary effect and Soret effect will occur at the solid-liquid interface. The thermocapillary effect will change the pore size at the interface, which will affect the adsorption of ions. The occurrence of the Soret effect will cause migration of ions in the electrolyte, resulting in a potential difference. As for the characterization of the performance of the supercapacitor for thermoelectric conversion, related evaluation metrics are obtained by changing the temperature of electrochemical environments on the basis of other characterization parameters. Second, we introduce the progress of the performance of supercapacitors in thermoelectric conversion. The power density of the supercapacitors with thermoelectric conversion under temperature differences does not meet the ideal requirements, and the liquid type supercapacitors have the problem of easy leakage. In recent years, a large amount of research has been conducted to improve the electrode material and electrolyte. The electrolyte types are mainly based on ion concentration, cation/anion size and the addition of nanoparticles in electrolyte solution, while the selection of electrode materials is mainly based on metal materials with high work function and carbon materials with a large specific surface area. At the same time, major improvements have been made in selecting the appropriate electrolyte (solid electrolyte or gel electrolyte) to construct supercapacitors with high thermoelectric conversion performance. Afterwards, we review the progress on supercapacitors for thermoelectric conversion without temperature differences, which mainly starts from the presence or absence of external power sources. Moreover, the relationships between structural design and economic cost are highlighted through a comprehensive analysis of the literature. On one hand, the cost can be reduced by reducing occurrence of side reactions and removing the ion exchange membrane while the external power supply is connected. On the other hand, the external power supply can be removed to simplify the structure and reduce the cost. Third, we introduce the latest developments in the application of supercapacitors based on thermoelectric conversion in various fields. In the field of wearable electronic products, supercapacitors can be charged by using the body heat to extend the use time. In terms of cooling of electronic components, supercapacitors can be used as auxiliary equipment of other thermal management systems to enhance local heat transfer capabilities. As for power batteries, supercapacitors use the waste heat generated by the power system to perform thermoelectric conversion, which improves the electrical performance of the car to a certain extent and prevents a large amount of waste heat from accumulating and burning the battery. Finally, we discuss the major challenges faced by supercapacitors based on thermoelectric conversion, along with future prospects.