The reaction mechanism of electrolytic reduction of SiO 2 at a liquid Zn cathode in molten CaCl was investigated with the aim of establishing a new production process of solar-grade Si. Three types of Zn/SiO 2 contacting electrodes were prepared depending on the objectives. Cyclic voltammetry suggested two reduction mechanisms of SiO 2 at a Zn electrode. One is a direct electrolytic reduction that proceeds at potentials more negative than 1.55 V vs. Ca 2+ /Ca. The other is an indirect reduction by liquid Ca-Zn alloy at potentials more negative than 0.85 V. The both reduction mechanisms were confirmed to proceed at 0.60 V by electrolysis and immersion experiments. Impurity analysis by ICP-AES was conducted for the Si prepared by potentiostatic electrolysis at 0.60 V, and confirmed that the concentrations of the metal elements and P were lower than the target levels for primary Si before directional solidification process.
Electrolytic production process for solar-grade Si utilizing liquid Si-Zn alloy cathode in molten CaCl2 has been proposed. Toward the establishment of the process, the behavior of liquid Zn metal was investigated in molten CaCl2 at 1123 K. The evaporation of Zn metal was largely suppressed by an immersion into molten salt, which enables the use of Zn electrode in spite of the high vapor pressure of Zn. The cyclic voltammetry suggested the reduction of SiO2 at 1.45 V vs. Ca 2+ /Ca on a Zn cathode. After the potentiostatic electrolysis at 0.9 V, Si particles with diameters of 2-30 µm were precipitated in the solidified Zn matrix by the slow cooling process of the produced liquid Si-Zn alloy. The alloying rate between solid Si and liquid Zn was measured as 4.56 μm s −1 , and it linearly decreased with the Si content in liquid Zn phase.
Central Research Institute of Electric Power Industry (CRIEPI) has operated four types of SOFC stacks, which are developed by Kyocera, NGK SPARK PLUG (NTK), NGK INSULATORS (NGK), and Denso in a NEDO’s project since FY2018 to FY2019 in order to clarify the technical issues for high-efficiency and toughness of SOFC stacks or make a concept for the high-efficient stacks. Each SOFC stack has been operated with the high fuel utilization (Uf = 80%) as a severe condition for high-efficiency for more than 3,000 hours, and degradation factors for SOFC durability have been analyzed by using the electrode polarization model which we have developed. Analysis results until now showed that dominant degradation factor was increase of cathode overvoltage or IR loss as same as that of the SOFC stacks which were modified for improving long-term durability in the previous term.
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