Kinetic Studies on Direct Electrolytic Reduction of SiO2 Granules in Molten CaCl2
Authors: Zhong Ming , Yang Xiao , Yasuda Kouji , Nohira Toshiyuki , Homma Takayuki 
Published: February 2020

1. Introduction

The photovoltaic (PV) industry develops rapidly in recent years. As the main raw material of solar cells, solar-grade silicon (SOG-Si; 6N purity) is dominantly produced by the Siemens process which is based on hydrogen reduction and thermal decomposition of trichlorosilane (SiHCl3). The disadvantages of the Siemens process such as low productivity, low reaction efficiency and high energy consumption could seriously restrict the mass dissemination of solar cells in the future. Therefore, the development of a new production process of SOG-Si is urgently needed.
Our group has been investigating a new process to produce SOG-Si via the combination of molten salt electrolysis and directional solidification in recent years.[1,2] Solid SiO2 granules can be directly reduced to Si by electrolysis in molten CaCl2. In this paper, the kinetics of direct electrolytic reduction of SiO2 granules in molten CaCl2 was investigated. The effects of the SiO2 granule size and cathodic potential on the reaction rate were clarified. The improvement of kinetics by using SiO2-Si mixtures as the raw material was confirmed.

2. Experimental

The experiment was conducted in molten CaCl2 at 1123 K in an Al2O3 crucible. Before melting, the CaCl2 was dried well under vacuum to remove the residual moisture. The working electrode was an Al2O3 tube with SiO2 granules inside and with a graphite plate at the bottom. The counter electrode was a glassy carbon rod. The reference electrode was an Ag+/Ag electrode prepared by immersing a silver wire into molten CaCl2containing 0.5 mol% AgCl in a mullite tube.
Approximately 0.1 g of SiO2 granules with four size ranges (<0.1 mm, 0.10-0.25 mm, 0.5-1.0 mm, and 1.0-2.0 mm) were electrolyzed at different cathodic potentials (0.6 to 1.2 V vs. Ca2+/Ca). In some cases, SiO2-Si mixtures with different composition ratios were used as the raw material. The reduction kinetics was evaluated on the basis of the growth of the reduced Si layer and the current behavior during electrolysis. The post-electrolysis samples were observed by SEM and analyzed by EDX.

3. Results and Discussion

The observation of the cross-sections of the post-electrolysis samples indicates that the reduced Si layer grows from the bottom to the top. The reduction occurs via two routes: along the surface of SiO2 granules and from the surface to the core, to form a core (SiO2)-shell (Si) structure.[3]
Smaller SiO2 granules are favorable for faster reduction because the contact resistance between the bottom graphite plate and the reduced Si particles is small and the diffusion of O2- ions in CaCl2inside the porous Si shells is easy.[4] A more negative cathodic potential is also favorable for faster reduction due to the fact that electron transfer is involved in the rate-determining step of the overall process.
Figure 1 shows the photographs of the cross-sections of the working electrodes using SiO2-Si mixtures with different composition ratios as the raw material after electrolysis at 0.6 V for 20 min. The thickness of the reduced layer apparently increases with the increase of Si amount in the raw material. Since Si has a high electrical conductivity at 1123 K, new electron transfer paths are created and thus the reaction area is larger by using SiO2-Si mixtures compared with using only SiO2granules as the raw material. Therefore, the reduction kinetics is improved by the addition of Si.

Acknowledgement

This study was partly supported by JST-CREST and Grants-in-Aid for Scientific Research A from the Japan Society for the Promotion of Science (JSPS).

References

[1] T. Nohira, K. Yasuda and Y. Ito, Nature Materials, 2, 397 (2003).
[2] T. Toba, K. Yasuda, T. Nohira, X. Yang, R. Hagiwara, K. Ichitsubo, K. Masuda, and T. Homma, Electrochemistry, 81, 559 (2013).
[3] X. Yang, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, Metall. Mater. Trans. B, 45, 1337 (2014).
[4] X. Yang, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, Metall. Mater. Trans. B, 47, 788 (2016).

Caption list

Figure 1 Photographs of the cross sections of the working electrodes after electrolysis at 0.6 V vs Ca2+/Ca for 20 min in molten CaCl2 at 1123 K. SiO2 size range: <0.10 mm. Si size range: 0.10-0.25 mm.

Figure 1