Study On The Transition Mechanism And Controllability Of Si(?) In The Preparation Of Electrowining Silicon By Molten Salt Electrodeposition

The demand for energy is growing with the development of human society,which restricts and threatens the sustainable development of human society.It is crucial to develop and spread clean and renewable energy to address energy and environmental issues.Among various types of sustainable renewable energy,photovoltaic power generation has been developed as one of the promising opts for for an energy-conservation and a low-carbon future.High purity silicon,known as the preferred base material for photovoltaic cells,plays an essential role in the manufacture of photovoltaic cells.This paper first investigated the impact of CaO-SiO2 on the basic properties of the CaCl2-CaF2 system.The graphite crucible,liquid magnesium and CaO-SiO2 were served anode,cathode and raw material,respectively,and Si(?)was electrodeposited to prepare the electrode-silicon in CaCl2-CaF2 melt.Then,we employed the three electrode system,and this work concerned a study on investigating the electrochemical behaviors of silicon in molten CaCl2-CaF2-CaO-SiO2,by means of linear scan voltammetry,cyclic voltammogram square wave voltammetry,chronoamperometry etc.Furthermore,the morphology and structure of Si in alloy samples conducted on the different electrodes were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM)with energy dispersive spectroscopy(EDS)and inductively coupled plasma atomic emission spectrometer(ICP-AES).In the study of the basic physicochemical properties of the electrolysis system,the molten CaCl2-CaF2 salt was used as the conductive medium,and both the ratio and the experimental temperature also were determined.The physicochemical properties,such as lose weight rate and electrical conductivity,were measured,and studied the effect which added different content of CaO-SiO2 in CaCl2-CaF2 system.The effect of temperature on the lose weight rate of CaCl2-CaF2 melt was investigated.With the increase of temperature,the loss weight rate increases gradually and the average loss weight rate is 0.024 g/?.The density of the molten salt decreases with increasing temperature,while adding CaO-SiO2 can increase the density of CaCl2-CaF2 system slightly.The liquidus temperature of CaCl2-CaF2 system was 768?comfirmed through the cooling curve method,and increase in CaO-SiO2 content results in the increase of the liquidus temperature.The liquidus temperature of the molten salts increased by 17.5?for every 2wt.%CaO-SiO2.The temperature dependence of electrical conductivities for all kind of melt systems are linear increasing with the increase of temperature,but decreases due to the increase of the proportion of CaO-SiO2 in the melt.Finally,a suitable composition ratio was determined.The measured liquidus temperature,density and conductivity of 66 wt.%CaCl2-33 wt.%CaF2-1.4wt.%CaO-0.6wt%SiO2 system were 785°C,2.25 g/cm3,1.92 S/cm,respectively.On the basis of the thermodynamic analysis,it was feasible for the preparation of Mg-Si alloy by electrodeposition method,and the process parameters such as back EMF and current efficiency were studied.In the CaCl2-CaF2 system or CaCl2-CaF2-CaO-SiO2 system,the back EMF exhibits a gradual increase over the increase of current intensity.An increase of every 2wt.%CaO-SiO2 in the bath results in 0.26 V back EMF decrease.The back EMF tends to decrease linearly with increasing the temperature.As the temperature increased by 20?,the back EMF decreases by about 36 m V.With the electrolysis time prolonged,the back EMF increased.According to the change of the back electromotive force with electrolysis time,the feeding period measured is 43 min.When the current density is in the range of 0.05~0.1 A·cm-2,the electrode process entered the Tafel region controlled by electrochemical polarization.In 0.1~0.3 A·cm-2 range,the electrode reaction process was changed from electrochemical polarization control to concentration polarization control.The current efficiency increases with the increase of electrolysis temperature,and maximum current efficiency obtained is 73.8% at 920?(810?~960?/30?).The current efficiency increases first and then decreases with increasing current.The current efficiency reaches the maximum value of 71.2% at 4 A(2~7A/1A).The maximum current efficiency is 75.3% by electrolysis for 120 min(60~210min/30min).After that,the current efficiency decreases gradually.Under the condition of tungsten electrode and Pt electrode as working electrode and reference electrode,respectively,the electrode reaction process and discharge mechanism of Si(IV)were investigated using various electrochemical methods in CaCl2-CaF2-CaO-SiO2 salt at 1023 K.Cyclic voltammetry shows that the onset reduction potentials of Si(I?)and Ca(?)are-0.63 V and-2.52 V,respectively,and the generated intermediate product CaC2 is observed as well.A series of typical cyclic voltammograms with different scan rates in CaCl2-CaF2-CaO-SiO2 melt show that the cathodic/anodic reaction of Si(?)ions is diffusion controlled and reversible at lower scan rate.Then the diffusion coefficient of Si(?)ions is calculated as 3.22×10-5 cm2/s,and the diffusion activation energy for diffusion is found to be 4.425 k J/mol using Arrhenius equation.The cathodic reduction of Si(?)ions to metallic silicon using binary CaCl2-CaF2 system with addition of CaO-SiO2 is a single diffusion-controlled process and involved in four electrons.Chronoamperometry shows that the silicon deposition involves an instantaneous nucleation with diffusion-controlled nuclei growth.Nucleation polarization is not observed during the electrodeposition of Si.The cathodic peak potential value of of Si metal deposition is about-0.9 V(vs.Pt wire)in the molten CaCl2-CaF2-CaO-SiO2 melt at 1023 K,and the intermediate product Mo Si2 occurs at the same time.Then a series of typical cyclic voltammograms with different scan rates show that the electrochemical reaction of Si(?)/Si is diffusion controlled and reversible at lower scan rate.The diffusion coefficient of Si(?)is determined to be 8.13×10-5 cm2/s via linear voltammetry in the CaCl2-CaF2-CaO-SiO2 salt at 1023 K.The electrodeposition of silicon proceeds through a single reduction wave under diffusion control by square wave voltammetry,and the reversal chronopotentiometry further confirms this conclusion.The chronoamperometry curves under the applied potential of-0.81 V,-0.83 V,-0.85 V and-0.87 V were studied.According to the nucleation mechanism,the nucleation process of Si(?)follows the instantaneous nucleation process.Compared with the classical dimensionless model,it is determined that the nucleation of Si(?)is an instantaneous nucleation process again.Moreover,we find that when t/tmax is less than 3,the nucleation model of Si(?)is closer to the instantaneous nucleation.As observed from the XRD pattern,the collected deposit was identified to be MoSi2 and Si.