Dissolution Behavior of SiO2 and Electrochemical Reduction of Dissolved SiO2 in Molten Chlorides
Authors: Ma Yuanjia , Yamamoto Takayuki , Yasuda Kouji , Nohira Toshiyuki 
Published: December 2020

1. Introduction

The global production of photovoltaic (PV) cells has increased by a factor of approximately 360, i.e., 0.285 GW year−1 to 102.4 GW year−1, from the year 2000 to 2018 [1,2]. Among the many types of solar cells, crystalline Si solar cell accounted for 96.9% of the worldwide production in 2018 [3]. Therefore, crystalline Si would be most likely to remain as the main stream of the PV industry in the long term.
Due to the high energy cost of the current production process of solar-grade silicon (SOG-Si), the next-generation production process for SOG-Si with low energy cost is required. For the past two decades, we have been studying the direct electrolytic reduction of solid SiO2 to Si in molten CaCl2 as a new production process of SOG-Si [4–6]. Recently, we have focused on electrochemical reduction of dissolved SiO2 in CaO-added molten salts [7].
In this study, we selected eutectic molten NaCl–CaCl2 as the electrolyte, because its melting point is comparatively lower than that of single CaCl2 melt, which enables the investigation in a wide temperature range. The structure and electrochemical reduction of dissolved SiO2 were investigated in molten NaCl–CaCl2–CaO.

2. Experimental

All experiments were conducted in a dry Ar atmosphere. The molten salts were prepared as follows: Firstly, NaCl and CaCl2 powders were mixed in a eutectic composition (NaCl:CaCl2 = 47.9:52.1 mol%), and then certain amounts of CaO (0–2.0 mol%) and SiO2 (0 or 1.0 mol%) powders were added to the molten eutectic mixture. For Raman spectroscopy, a platinum pan loaded with the mixed salt was placed in an air-tight high-temperature stage, and then heated to 1023 K.Then the structure of dissolved SiO2 was investigated by Raman spectroscopy. For electrochemical measurement, the mixed salts were loaded into a graphite crucible. A graphite plate was used as a working electrode. The counter and quasi-reference electrodes were a graphite rod and a Si rod, respectively. The potential was calibrated by the deposition potential of Na metal on a Mo wire. Samples obtained by the galvanostatic electrolysis were analyzed by XRD and SEM/EDX.

3. Result and discussion

Fig. 1 shows Raman spectra of molten (a) eutectic NaCl–CaCl2, (b) NaCl–CaCl2–2.0 mol% CaO, and (c) NaCl–CaCl2–2.0 mol% CaO–1.0 mol% SiO2 at 1023 K. Within the wave number range of 700–1200 cm−1, a band at 845 cm−1 is observed only for molten salt (c), whereas no apparent bands for molten salts (a) and (b). The structure of dissolved SiO2 in molten salt (c) is considered as SiO4
4− ion, considering that the main Raman bands for the stretch vibration of SiO4
4−, SiO3
2−, and Si2O5
2− in silicate melts have been reported at around 850, 950, and 1100 cm−1, respectively [8]. Since the SiO4
4− ion is regarded as the dissolved species of Ca2SiO4, this ion is consistent well with the ratio of added CaO/SiO2 (2.0 mol% and 1.0 mol%).
The galvanostatic electrolyses were also carried out at graphite plates at 50–200 mA cm−2 in molten NaCl–CaCl2–1.9 mol% CaO–0.97 mol% SiO2. From XRD and SEM/EDX analysis of the samples, deposition of Si was confirmed, which suggested the reduction of SiO4
4− ion as follows:
SiO4
4− + 4 e− → Si(s) + 4 O2− (1)
In the presentation, the effect of CaO concentration on the dissolution behavior of SiO2 and electrochemical reduction of dissolved SiO2 will be discussed.

Acknowledgement

The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto University.

R
eferences

[1] Photovoltaic Market 2017, RTS Corp., Tokyo, Japan (2017). [in Japanese]
[2] Industrial Rare Metal 2019, Arumu Publ. Co., Tokyo, Japan (2019). [in Japanese]
[3] Photovoltaic Market 2019, RTS Corp., Tokyo, Japan (2019). [in Japanese]
[4] T. Nohira, K. Yasuda, and Y. Ito, Nat. Mater., 2, 397 (2003).
[5] K. Yasuda, T. Nohira, R. Hagiwara, and Y. H. Ogata, Electrochim. Acta, 53, 106 (2007).
[6] T. Toba, K. Yasuda, T. Nohira, X. Yang, R. Hagiwara, K. Ichitsubo, K. Masuda, and T. Homma, Electrochemistry, 81, 559 (2013).
[7] Y. Ma, K. Yasuda, and T. Nohira, Abstracts of The 51st Symposium on Molten Salt Chemistry, p. 24-25, Sapporo, Japan (2019).
[8] B. O. Mysen, Structure and Properties of Silicate Melts, Elsevier, Amsterdam, Netherlands (1988).

Figure 1