Contributions of Both the Eastman Entropy of Transfer and Electric Double Layer to the Electromotive Force of Ionic Thermoelectric Supercapacitors

Recently, ionic thermoelectric supercapacitors have gained attention because of their high open circuit voltages, even for ions that are redox inactive. As a source of open circuit voltage (electromotive force), an asymmetry in electric double layers developed by the adsorption of ions at the electrode surfaces kept at different temperatures has previously been proposed. As another source, the Eastman entropy of transfer, which is related to the Soret coefficient, has been considered. Herein, we theoretically estimated the open circuit voltages generated in the Stern layer, the diffuse layer and by the Eastman entropy of transfer. The Grahame equation has been generalized to consider the temperature gradient in the diffuse layer. The ion coverage difference between the hot and cold electrodes and the open circuit voltage are obtained by solving self-consistent equations using the adsorption isotherm. The results are compared with experimental results using a metal electrode and a conductive polymer-based electrode. We show the possible origin of the high ionic Seebeck effect caused by the asymmetry in the coverages of adsorbed ions in terms of the various types of interface capacitance factor at the hot and cold electrodes.