Semiconductor-Sensitized Thermal Cells Operated Under 100 °C

Abstract

Concept The semiconductor-sensitized thermal cell (STC) is a new method for converting heat directly into electric power using dye-sensitized solar cells (DSSC). In the STC, the dye of DSSC was replaced with a semiconductor to allow the system to operate using heat instead of light.1 The number of thermally excited charges in a semiconductor is determined by the Fermi-Dirac distribution2; for example, Ge generates a number of thermally excited charges that exceeds the number of photons in room light at 80 ˚C. STC is expected as a powerful energy-harvesting device for wide applications such as IoT devices, distribution power sources, etc. Previous Results As the STC demonstration, liquid display was operated at 80 ˚C and Bluetooth devices was operated in the heat of asphalt.3 The biggest issue of STC is the power restore phenomena “in” heat. Initially, it was thought that the STC would stop generating electricity when it reached a chemical equilibrium at the set temperature. However, when the distance between the electrodes is close, the discharge performance restores when the circuit switch is turned off. In this presentation, we will focus on this recovery and electrochemically investigate what kind of phenomenon causes the recovery. Methods and Results To examine the recovery phenomenon, we focused on the electrolyte thickness. The STCs consist of three layers, an electrolyte layer sandwiched with a semiconductor working electrode and a counter electrode. We fabricated 4 different Ge-STCs with different electrolyte thicknesses and examined their electrochemical properties. Fluorine-doped tin oxide (FTO) was used for the counter electrode. The CuCl, CuCl2 and LiCl in PEG600 was used for the electrolyte.4 The 0.78, 1, 2 and 3 μL of the electrolyte were used when the thicknesses of the electrolyte were 85, 114, 228 and 342 μm, respectively, to eliminate the difference in the contact area of the electrode/electrolyte. The obtained cells were named according to their corresponding electrode distances (Cell-85, -114, -228 and -342). As a result, the cell with the narrowest electrode spacing and the lowest electrolyte mass discharged the longest (Figure). In the presentation, we will report these phenomena and electrochemical impedance analysis in detail. References (a) Matsushita, S.; Araki, T.; Mei, B.; Sugawara, S.; Inagawa, Y.; Nishiyama, J.; Isobe, T.; Nakajima, A., A sensitized thermal cell recovered using heat. J. Mater. Chem. A 2019, 7, 18249-18256; (b) Matsushita, S.; Tsuruoka, A.; Kobayashi, E.; Isobe, T.; Nakajima, A., Redox reactions by thermally excited charge carriers: towards sensitized thermal cells. Mater. Horiz. 2017, 4, 649–656. Inagawa, Y.; Isobe, T.; Nakajima, A.; Matsushita, S., Ag2S-Sensitized Thermal Cell. J. Phys. Chem. C 2019, 123 (19), 12135-12141. Kohata, H.; Obinata, M.; Ikeda, T.; Sekiya, H.; Mei, B.; Wang, Y.; Mizukoshi, K.; Isobe, T.; Nakajima, A.; Matsushita, S., Power Generation at Room Temperature -How to Design of the Sensitized Thermal Cell-. Research Square 2021, 384614. (a) Ikeda, T.; Sekiya, H.; Kohata, H.; Isobe, T.; Nakajima, A.; Matsushita, S., Role of the ions in the Ge/(CuCl, CuCl2 and LiCl)/FTO-sensitized thermal cell. J. Electroanal. Chem. 2021, 895, 115413; (b) Matsushita, S.; Sugawara, S.; Ikeda, T.; Araki, T.; Sekiya, H.; Kohata, H.; Isobe, T.; Nakajima, A., Fermi Level Dependence of a Working Electrode on the Open Circuit Voltage in a Sensitized Thermal Cell. Chem. Lett. 2020, 49 (9), 1013-1016. Figure 1