ZrO₂ Solid Electrolyte Cell Aided Investigation On Electrodeposition Of Si In Na₃AlF₆-based Melt

The traditional metallurgical industry is an energy-intensive and highly polluting industry.With the continuous deepening of research and development of green renewable energy,clean and efficient new electrochemical metallurgy techniques will play an increasingly important role.Silicon is the most basic functional material in the information,new energy and new material industries.However,the current high cost of high-purity silicon production limits the rapid development of these industries.The electrodeposition process in melt can directly prepare elemental silicon and alloys from silicon oxides.It has attracted wide attention due to the advantages of simple operation,short process,low energy consumption and low environmental pollution.The Na₃AlF₆system is expected to be an ideal electrolyte system for green preparing Si due to its high solubility to SiO₂which is abundant in nature and its successful application in aluminum electrolysis.In this paper,a novel three-electrode electrolytic cell with Pt,O₂（air）│YSZ as a reference and auxiliary electrode was constructed by Y₂O₃stabilized ZrO₂solid electrolyte（YSZ）tube.Based on the volatility of Na₃AlF₆-based melt and the stability test of Pt,O₂（air）│YSZ reference electrode,cyclic voltammetry,chrono-amperometry and other electrochemical tests were performed by using the Ir wire as the working electrode in the Na₃AlF₆-based melt system at 1323K.Combining with thermodynamic theoretical calculations,scanning electron microscope observations and Energy spectrum analysis,the electrodeposition law of metal（alloy）in Na₃AlF₆-based melt on the cathode and the electrochemical reduction behavior of Si（Ⅳ）in the Na₃AlF₆-based melt were studied under completely carbon-free conditions,and the corrosion mechanism of melt on YSZ tube were also investigated.The main conclusions are as follows:（1）In the test of the volatility of melt,it is found that the volatilization rate of Na₃AlF₆-5%SiO₂melt per hour is 1.8%,and it becomes 0.9%after adding 5%Al₂O₃,indicating that the addition of Al₂O₃can significantly inhibit the volatilization of melt.In the open-circuit potential test of Na₃AlF₆-based melt for more than 2 hours,it is found that the Pt,O₂（air）│YSZ reference electrode has good reversibility,stability and reproducibility,and a fast response,which possesses the characteristics of the reference electrode.（2）In the Na₃AlF₆-5%SiO₂melt,it is found that there are two reduction peaks of complex Si（IV）.The reductions of two complex Si（IV）ions are irreversible processes with a single-step of4-electron transfer.The first reduction peak is not obvious on the cyclic voltammetry curve.The electrode reaction process of the second complex Si（IV）is controlled by diffusion,and the reduction product Si is insoluble.The reduction peak potential on the cyclic voltammetry curve is between-1.6 and-1.7 V,the charge transfer coefficientαis between 0.21 and 0.39,and the Si（IV）diffusion coefficient is on the order of 10^-7cm²·s^-1.The deposition potentials of Al,Na（Zr）are negative than-1.8 V,and increase negatively in turn.Intermetallic compound particles of Si₄Zr₅are found to generate during potentiostatic electrolysis at-1.8 V or-2.0 V,whose generation potential is between-1.7 V and-1.8 V.The deposited Si,Al,and Na metals are mainly derived from oxygen containing compounds produced by the Na₃AlF₆-SiO₂melt itself but Zr metal from the ZrO₂of the corrosion of YSZ tubes by the melt.The precipitation potentials of related metals（or intermetallic compound）relative to Pt,O₂（air）∣YSZ RE are in good agreement with thermodynamic calculations.（3）In the Na₃AlF₆-5%SiO₂-5%Al₂O₃melt,Si can be obtained only in the form of Si-Ir alloy on the electrode Ir,and the reduction peaks of two complex Si（IV）are also found.The reductions of two complex Si（IV）ions are irreversible processes with a single-step of 4-electron transfer,in which the electrode reaction process of the first complex Si（IV）is controlled by diffusion,and its reduction peak on the cyclic voltammetry curve is between-1.5～-1.7 V,and the charge transfer coefficientαis in the range of 0.34 to 0.72,and the Si（IV）diffusion coefficient is on the order of 10^-7cm²·s^-1.It is found that intermetallic compound particles of Si₄Zr₅are generated during potentiostatic electrolysis at-2.0 V,and not product related to Al and Na.（4）It is found that the YSZ tube is mainly corroded from three aspects in cryolite-based melt.One is the chemical reaction of melt to YSZ tube（the stabilizer Y₂O₃in cubic phase ZrO₂is chemically leached by melt）,which causes ZrO₂to transition from cubic to monoclinic phase,accompanied by volume expansion and embrittlement of YSZ tube.The second is the physical dissolution of the inner wall of the YSZ tube in melt.The third is the peeling of ZrO₂particles from the inner wall of the YSZ tube into the melt.Adding 5%SiO₂to pure Na₃AlF₆can reduce the corrosion of YSZ tube,while adding 5%Al₂O₃can increase the corrosion of YSZ tube by melt.Applying an appropriate negative potential can reduce the corrosion of the YSZ tube by the melt,but a too much negative potential will cause electrochemical reduction of the ZrO₂,which will aggravate the corrosion of the YSZ tube.Based on the above corrosion mechanism,the countermeasures to reduce or completely prevent the corrosion of the YSZ tube by cryolite based melt in the subsequent research are proposed.