Regulation of Electrode–Electrolyte Interactions for Improved Heat Recovery of a Thermo-Induced Electric Double-Layer Capacitor

Abstract

Low-grade heat recovery through ion redistribution (i.e., Soret effect) in electric double-layer capacitors is a recent emergence for the conversion of thermal energy into electrical energy. A popular approach to improving the performance of such devices is optimizing the composition of the electrolyte. However, in addition to the thermodiffusion behavior of electrolyte ions, the electrode–electrolyte interactions also play a vital role in determining the ionic Seebeck coefficient, although it has not been clearly investigated. In this work, the crucial role of electrode–electrolyte interactions in thermoelectric conversion efficiency is demonstrated by employing vertical graphene as a base material for surface modification. After the surface modification via ozone treatment, oxygen-containing functional groups are enriched on graphene sheets. After improving the interfacial interactions between vertical graphene electrode and electrolyte, the ionic Seebeck coefficient is largely enhanced by 4.6-fold (up to 1 mV K–1) while the capacitance is improved by 1.29 times. Density functional theory calculations reveal that such a high thermal response originates from the stronger ion–electrode interaction and the higher charge transfer efficiency via oxygen-containing functional groups. In addition, the energy stored in supercapacitors can be extracted by discharging through a resistor. The findings of this study can help us to better understand the role of electrode–electrolyte interactions in increasing the ionic Seebeck coefficient, providing a new strategy for improved heat-to-electric conversion.