The Electrodes Distance Dependance of Battery Performance in Sensitized Thermal Cell
Authors: Kohata Haruki , Isobe Toshihiro , Nakajima Akira , Matsushita Sachiko 
Published: December 2020

1. Introduction

It is essential for modern society to develop a stable energy source with a low environmental load. In this situation, we focus on thermal energy, which we can find anywhere around the world, including geothermal. Sensitized thermal cells (STCs) are new thermal energy conversion system for generating electrical power from heat.[1][2] The STC imitates a dye-sensitized solar cell (DSSC). In this system, the redox reaction in the electrolyte is caused by the thermally excited charges of the semiconductor instead of the photoexcited charges of the dye as in the DSSC. In STCs, the power generation end was confirmed, and surprisingly found that the power generation characteristics can be restored by heat.[3] As a factor of this restore, we hypothesized that electrolyte ions are diffused to the electrode/electrolyte interface to compensate for the insufficient reaction ions during the open-circuit state. In this study, the relationship between the ion diffusion and the battery performance was evaluated by changing the distance between electrodes.
2. Materials and Methods

The STCs were manufactured in a glove box under Ar atmosphere. The electrolyte was prepared by mixing CuCl, CuCl2, LiCl, polyetylene glycol (PEG) for 10 minutes. The concentration was LiCl (0.60 mmol / PEG(g)) and Cu ion (1.0 mmol / PEG(g), Cu+ : Cu2+ = 1 : 1). The n-Si and Ge was selected as an electronic transport layer and the semiconductor, respectively. An n-Si substrate (1.5 cm × 2.5 cm) on which Ge was deposited was used as the working electrode. An FTO glass substrate used as the counter electrode, on which the double-sided insulating tape (114 μm thickness) with a hole of 6 mm diameter was attached. The electrolyte was dropped into the hole and the n-Si/Ge substrate was used to cover the hole to form a cell (the Ge side touched the electrolyte). This time, (number of tapes : electrolyte volume [μL]) was changed to (1 : 1, called cell 1), (2 : 2, cell 2), (3 : 3, cell 3). Also, we fabricated other cell used a thinner insulating tape [0.85 μm thickness] (1 : 0.78, thin cell). For these 4 cells, I-V curve measurements and the long-term operation, were performed.
3. Results and Discussio
ns

The I-V curves for each cell at 80 ºC are shown in Fig. 1. It was found that the open circuit voltage (Voc) was almost constant regardless of the distance between the electrodes. This result supports our consideration that the Voc of STC is caused by the difference between the redox level of the ions and the Fermi level of the working electrode. Also, the shape of I-V curve suggests that the rate-determination step was the diffusion ions transfer. These results suggest that ion diffusion is a major factor in the performance of STCs in this system.
The long-term operation was performed at 80 oC. After 200 nA discharge was performed, the Voc measurement was immediately started while keeping the cell at 80 oC for several hours. In cell 1 and thin cell, the Vocwas recovered to the starting value before discharging. On the other hand, in cell 2 and cell 3, the Voc wasn’t recovered. A similar tendency of Voc recovery was found when 200 nA discharge was performed again (Fig. 2, number 2), and no long power generation was observed in cell 2 and cell 3. Therefore, it was suggested that the smaller the distance between the electrodes is, more suitable for repeated discharge. This result may be related to the ion transfer in the electrolyte during recovery (i.e., open circuit state).Nernst equation (eq 1) shows the relationship between voltage and reactive ions.
E = Eo + RT/nF(ln(aox/ared)) (eq 1)
It is expected that the reaction ion deficiency state near the electrode was eliminated by the ion diffusion, and Voc of cell 1 and thin cell were recovered. These results strongly suggested that the STC recovery is closely related to reactive ions transfer in the electrolyte.
4. Conclusion

Here, we investigated the electrodes distance dependence of battery performance. It was suggested that the smaller the distance between the electrodes is, more suitable for repeated discharge. This can be one of the guidelines for the practical application of STCs.
5. Reference

[1] S. Matsushita, et al., Mater. Horiz, 4, 649-656, 2017.
[2] Y. Inagawa, et al., J. Phys Chem C, 123, 19, 12145-12141, 2019.
[3] S. Matsushita, et al., J. Mater. Chem. A, 7, 18249-18256, 2019.
6.
Acknowledgement

This work was financially supported by Sanoh industrial Co., and Tohnic corporation.

Figure 1