(Invited) Sensitized "Thermal" Cell

Abstract

Efficient and low-cost thermal energy-harvesting systems are needed to solve the global energy crisis. If we were able to extract electrical energy directly from the geothermal sources, then the night would be safe regardless of developed or underdeveloped countries; there is no problem in the treatment of radiation waste. Heat, called low quality energy, would become a great renewable energy. As such a dream cell, we suggested sensitized thermal cells (STCs), that is a new green power generation system operate by heating without a cooling temperature part (Japanese Patent Application No. 2015-175037). The structure of STC imitates the dye-sensitized solar cells (DSSCs). In DSSCs, electrons in dyes are photoexcited by light irradiation, and the excited electrons are injected into the conduction band of an electron transport material. The transported electrons reduced oxidant ions in electrolyte, and the reductant ions were oxidized by the photoexcited holes, and we can get photo-excited current. If thermally excited electrons in the semiconductor can generate the ion redox reactions, a novel heat-to-electron conversion system functioned without a temperature difference, named a “sensitized thermal cell”, can be obtained. This concept was firstly demonstrated using β-FeSi2 as the semiconductor material, n-Si as the electron transfer material, and a cupper ion conductor (CUSICON) as the electrolyte (1). The cell exhibited an open circuit voltage of 0.67 V and a short circuit current of 9.1 μA cm-3 at 600 °C. The acquisition voltage continuously maintained this value for at least 35 h in nitrogen. The redox reactions of the electrolyte ions due to thermal excited charge carriers were demonstrated using XPS. To convince the STC's principle, we also focused on an organic perovskite. This material has been used in a sensitized solar cell, and recent calculations show that it can generate several thermally excited charges at temperatures over 60 °C. Thus, if this perovskite cell can generate electric power with both light irradiation and thermal excitation, the STC concept would be perfectly proven, and the device succeeds thanks to the careful analysis of the perovskite’s unstableness (2). The battery characteristics of the cell were convinced both light irradiation and thermal excitation above the phase transition temperature of the organic perovskite. And very recently, it was revelaed that Ag2S also could act as the sensitizer both by light and heat (3). These results clearly prove the concept of an STC. In this presentation, we will also report the latest results of the STCs, especially focused on the termination process. [References] (1) S. Matsushita, A. Tsuruoka, E. Kobayashi, T. Isobe and A. Nakajima, Mater. Horiz., 2017, 4, 649–656.(2) S. Matsushita, S. Sugawara, T. Isobe and A. Nakajima, ACS Appl. Energy Mater., 2019, 2, 13-18.(3) Y. Inagawa, T. Isobe, A. Nakajima, and S. Matsushita, J. Phys. Chem. C, in press. DOI: 10.1021/acs.jpcc.9b01922