In-Situ Observation of Redox Reactions in Ge-Sensitised Thermal Cells

Abstract

Semiconductor-sensitised thermal cells (STC) are a new type of thermal energy conversion system that makes effective use of waste heat even at low-temperature. This system generates electricity via the redox reactions of electrolyte ions caused by thermally-excited carriers in the semiconductor electrode. However, there has been no direct proof of the redox reactions. In this manuscript, these reactions were directly observed via the change in colour of copper electrolyte ions, depending on their valence. This observation indicated that the oxidation reaction occurred at the semiconductor electrode and the reduction reaction occurred at the counter electrode, as we expected. These results will lead to the clarification of the principles and improvement of the STC performance.

Room-temperature power generation of Ge-sensitized thermal cells with interdigitated array electrodes

“…The redox ability of thermally excited carriers in semiconductors has been demonstrated using -FeSi2 [9] and Ge. [20] The analogy of STC and DSSC has been shown by organic-perovskite- [10,21] and Ag2S- [11] sensitized cells since those semiconductor-sensitized cells could generate electricity from light and heat, respectively. The doping-amount control in the electron-transfer layer of the semiconductor-electrode showed that the open circuit voltage (Voc) of STC is the difference between the Fermi-level of the semiconductor electrode and the electrolyte-ion redox level.…”

“…Ge-STC, which has been well-researched in previous reports [12,13,20,22,24], was selected to develop such an IDE system. In those earlier studies, the Ge/n-Si wafer electrode was the semiconductor electrode, and a polymer electrolyte (CuCl, CuCl2, LiCl mixed in polyethylene glycol (PEG) 600) was used.…”

“…[24] ・The change in colour of copper electrolyte ions from brown (Cu 2+ ) to colourless (Cu + ) due to redox reactions using thermally excited electrons of the Ge electrode was observed. [20] ・During several hundred nA discharging, the voltage was maintained at ~0.1 V until 40 h and dropped to zero after 70 h. [12] No Ge elution was detected via ICP emission spectroscopy after the power generation. [22] ・Longer recovery time led to longer power generation time, up to a specific value (from hundreds of seconds to several days, depending on the battery and ambient temperature).…”

Counter electrode dependence of germanium-sensitized thermal cells

“…1). 19,20) The structure of this device is straightforwardwedged between the semiconductor and counter electrodes (CEs) is an electrolyte. The power generation stops when the chemical reactions reach thermal equilibrium at the set temperature.…”

“…As a result, the STCs can be discharged again. 6,14) Germanium (Ge), 6,11,13,14,20) β-FeSi 2 , 10,19) CuFeS 2 , 12) Ag 2 S, 7) and organic perovskites 8,9) have been investigated as semiconductor electrodes for generating thermally excited charges in STCs. On the other hand, the counterpart, the CE materials, have not been discussed.…”