The Role of Granule Size on the Kinetics of Electrochemical Reduction of SiO2 Granules in Molten CaCl2

Abstract

As a fundamental study to develop a new process for producing solar-grade silicon, the effect of granule size on the kinetics of the electrochemical reduction of SiO 2 granules in molten CaCl 2 was investigated. SiO 2 granules with different size ranges were electrolyzed in molten CaCl 2 at 1123 K (850 °C). The reduction kinetics was evaluated on the basis of the growth rate of the reduced Si layer and the behavior of the current during electrolysis. The results indicated that finer SiO 2 granules are more favorable for a high reduction rate because the contact resistance between the bottom Si plate and the reduced Si particles is small and the diffusion of O 2ions in CaCl 2 inside the porous Si shell is easy. Electrolysis using SiO 2 granules less than 0.1 mm in size maintained a current density of no less than 0.4 A cm-2 within 20 min, indicating that the electrochemical reduction of fine SiO 2 granules in molten CaCl 2 has the potential of becoming a high-yield production process for solargrade silicon.

Electrochemical Preparation of Water-Reactive Silicon with Potential Applications in Hydrogen Generation

“…Once forming a core (SiO 2 )-shell (Si) structure for a SiO 2 particle, the reaction front would relocate from the particle surface to the inner part, and the reduction rate would decrease because of potential drop and more difficult diffusion of O 2− ions. [39][40][41] In case of incomplete reduction, pure Si can be recovered via H 2 O rinse to remove residual CaCl 2 and subsequent HF acid rinse to remove unreacted SiO 2 . Figure 5 illustrates the process of a single SiO 2 particle transforming from precursor to product.…”

Silicon Electrochemistry in Molten Salts

“…To that end, dependence of the reduction rate on SiO 2 particle size has been studied. Granules were used as the precursors with the dimensions from 0.1 mm to 2.0 mm . It was found that finer granules were more favorable for the reaction rate due to lower contact resistance with the Si substrate.…”

“…Granules were used as the precursors with the dimensions from 0.1 mm to 2.0 mm. 41 It was found that finer granules were more favorable for the reaction rate due to lower contact resistance with the Si substrate. Porosity of the produced structure facilitated diffusion of produced oxygen (9).…”

Purity and Minority Carrier Lifetime in Silicon Produced by Direct Electrolytic Reduction of SiO₂ in Molten CaCl₂

“…From this starting point, we proposed the direct electrolytic reduction of solid SiO 2 in molten CaCl 2 at the relatively low temperature of 1123 K. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] In contrast to traditional molten salt electrolysis, the cathodic reduction of SiO 2 proceeds through a solid-state reaction. We performed a series of studies in which we investigated the reduction of SiO 2 plates, 10,11 sintered SiO 2 pellets, 12 and stratified SiO 2 granules, [22][23][24][25][26] and clarified the reaction mechanisms involved. The purities of the obtained Si powders were also measured, as shown in Table 1.…”