Electrochemical Fluorination And Passivation Mechanism Of Graphite Anode In Fluoride Molten Salts System

Hall-Heroult molten salt electrolysis is still the popular process for primary aluminum production nowadays,and carbon electrodes are employed as anode and cathode for electrolysis in cryolite-alumina molten salt at 950～970 ℃.However,when the alumina content in electrolyte is low,anode effect will take place during the electrolysis process which results in many disadvantages such as,wastage of power energy,electrolyte temperature going up,aluminum loss increase,and cells service life reduction.Especially,the high global warming potentials gases perfluorocarbons（PFCs）,CF₄ and C₂F₆,which are significantly contributed to the global greenhouse effect,are generated during anode effect.The phenomenon and harm of anode effect are already widely known,but the mechanism of anode effect occurring in aluminum electrolytic cells is still unclear,and the anode effect factor,which is the frequency of anode effect,has been one of the most important parameter of modern aluminum electrolysis technology.Therefore,this topic aims to understand the mechanism of anode effect taking place during the aluminum electrolysis process,and is devoted to study the electrochemical fluorination and passivation behaviors of graphite electrode in some fluoride molten salts with various liquidus temperatures and different fluorine-contained anions.In the work,the cyclic voltammetry,chronopotentiometry,and electrochemical impedance spectroscopy techniques were employed to study the electrochemical reactions of graphite electrode in LiF^-KF,KF^-KBF₄,and NaF^-AIF₃-Al₂O₃ molten salt systems,and the gas chromatography and X-ray photoelectron spectroscopy（XPS）were used for qualitative analysis of the anodic reaction products.In addition,to ascertain the thermal characteristic of graphite fluoride compound,the non-isothermal kinetics of the thermal decomposition of graphite fluoride（CF）n was studied using differential scanning calorimetry-thermogravimetric analysis（DSC-TGA）.In LiF^-KF melt,F^-anion discharges on graphite electrode,and reacts with carbon under low potentials（from ca.0.15 Vvs.Pt-QRE）forming solid carbon fluoride compounds CF_x.The electrochemical fluorination reaction of graphite electrode can be described as:xF^-+C→Fx+xe-The result of Ip/v1/2-v curve obtained from voltammograms recorded at various scan rates indicated that mechanism of the electrode reaction is a simple totally irreversible electron transfer process.According to the analysis of XPS,the CF_x was determined to be formed in the electrode surface,whereas the results of gas chromatography showed that CF₄ and C₂F₆ are not formed during the electrode reaction process.The electron transfer coefficient a of the electrode reaction was calculated to be 0.27 according to the cross calculation by cyclic voltammetry and chronopotentiometry.In addition,an opposite electro-wetting effect was observed during anodic cyclic voltammetry via see-through cell.This phenomenon is mainly caused by the repelling between CF_x layer and F^-in the electrolyte under the action of electric field,because the F^-anion intercalated into CF_x layer or the F atom in ionic C-F bond accepts electron made CF_x layer show negative charge,which would repel the negative charges in electrolyte;when the electrode potential is not high enough or the electric field force is removed,this phenomenon would disappear.What is more,the CF_x layer formed in the graphite surface would make the electrode be passivated in LiF^-KF melt.The main reason for the electrode system passivated is that the CF.x layer blocked the mass transfer of F^-.Under higher potentials（from ca.3.7 V vs.Pt-QRE）,F^-discharges on CF_x layer forming F2,and the electrons are transferred to electrode by ionic C-F bonds in the CF_x layer.In KF^-KBF₄ melt,except for the reaction between F^-and carbon to form CF_x,some fluorine-contained complex anion group BnF₃n+1-would also react with carbon at slightly higher potentials（ca.1.9 V vs.Pt-QRE）to form CF_x,The complex anion groups may be BF₄-or dissociation product of BF₄-.The electrode reactions can be described as:C+xF^-→CF_x+xe-C+xBnF₃n+1-→nxBF_<sup>3+CF_x+xe-In the molten salt system,the electrode reaction mechanism of F^-reacting with carbon is totally irreversible electron transfer process coupling with a preceding chemical reaction.The preceding chemical is dissociation reaction of BF₄-and can be described as:nBF₄-←→BnF₃n+1-+（n-1）F^-In addition,the results of XPS and gas chromatography showed that the products of F^-and BnF₃n+1-reacting with carbon is only CF_x.Moreover,according to the cross calculation by cyclic voltammetry and chronopotentiometry,the electron transfer coefficient a of the electrode reaction was calculated also to be 0.27 in this molten salt.In NaF^-AlF₃-Al₂O₃ melt with low alumina content,the anodic process of graphite includes four electrode reactions,and they are the reaction for the formation of CO2(at 1.4 V vs.Al/Al³⁺),CF₄(at 2.47 V vs.Al/A1³⁺),CF_x(at 2.8 V vs.AI/AI³⁺),and F2(at 4.13 V vs.Al/A1³⁺),respectively.For the electrode reaction of CO2 formation,its mechanism was determined to be a totally irreversible electron transfer process coupling with a post chemical reaction.This electrode process includes two charge transfer steps,and forms intermediate product CxO.The electrochemical impedance spectra indicated that electron transfer resistance of the first electron transfer step is more great than that of the second step（i.e.Rat1>>ct2）,and both of Rct1 and Rct2 are reduced following polarized potential increasing,espectially,the Rct1 decrease faster.It explained that the process of CO2 formation in low alumina content contained electrolyte is controlled by the first electron transfer step,and this step is significantly influenced by the change of overpotential.In this molten salt system,F^-anion reacts with carbon to form CF₄ at slightly higher potentials.This electrode reaction includes two successive electrochemical reactions,and two electrons are transferred in each electrochemical steps.The first electrochemical reaction generates an intermediate product CxF2 which is a type of solid carbon fluoride compound.In addition,complex anion group AlF63-will react with carbon at more high potentials and directly forms solid carbon fluoride compounds CF_x which will decompose immediately at such high temperature,therefore,the electrode reaction mechanism of CF_x formation in this system is a totally irreversible electron transfer process coupling with a post chemical reaction.The electrode reaction based on four-charge transfer by one step,ultimately,forms a dense CF_x layer graphite fluoride（CF）,.The dense CF_x layer will passivate graphite electrode because of the blocking of mass transfer.The results of non-isothermal kinetics analysis indicated that thermal decomposition of graphite fluoride（CF）n is achieved by one reaction step,and the onset temperature is 800 K.The kinetics of（CF）n decomposition depend on the extent of conversion,and is divided into four stages.In each stage,the mechanism function is different.When 0.01<a<0.15,the two dimensional nucleation and growth can be used to describe the process,and the mechanism function is f（α）=2（1-α）[-ln（1-α）]1/2;when 0.2<a<0.65,the one dimensiona/nucleation and growth can be employed to describe the process,and the mechanism function is f（α）/-α;when 0.7<a<0.85,the process is described using a two-parameter Sestak-Berggren equation,which is expressed asj（α）= am（1-α）n（m=-0.294,n=1.428）;finally,at where 0.9<a<0.99,the one dimensional nucleation and growth is suited for the process,of which the mechanism function is f（a）= 1-α.The results of gas chromatography and XRD showed that the decomposition products are CF₄,C₂F₆,and amorphous carbon,and the ratio of CF₄/C₂F₆ is 12.The decomposition reaction can be expressed as:（CF）n→C*+XCF₄↑T+YC₂F₆ T X/Y≈12 where C*is amorphous carbon.