Study On Electrochemical Erosion Mechanisms Of Graphite Cathode In Aluminum Reduction Cells

In the context of“carbon peaking”and“carbon neutrality”,the most attractive problem in the aluminum reduction industry is to reduce energy consumption.One of the effective ways is applying the graphite cathode blocks to the aluminum reduction cells.However,the erosion of the graphite cathode limits the lifetime of the aluminum reduction cells.Therefore,it is significant to clarify the mechanism of cathode erosion,which would advance the cathode design,improve the lifetime of aluminum reduction cells and reduce energy consumption.Currently,there is no sufficient research work on the electrochemical erosion of the cathode.Studies on graphite cathode erosion have focused on the physical damage caused by sodium penetration leading to the cathode expansion.In this work,the Al-C diffusion coupled electrolysis experiments were performed to investigate the influence of current density and electrolysis time on the formation rate and microstructure of aluminum carbide.The ionic structure and composition of the electrolyte were investigated based on thermodynamic calculations,quantum chemical calculations,and high-temperature in situ Raman spectroscopic analysis.The adsorption properties of major clusters in electrolytes and the formation mechanism of Aluminum carbide on the graphite surface were studied by using first-principles calculations.The main works and conclusions of this research are as follows:The effects of current density and electrolysis time on formation of aluminum carbide were clarified by Al-C diffusion coupled electrolysis experiment,in which the thickness of Al₄C₃ layer was quantitatively analyzed.The results showed that:（1）The electrolysis voltage increased by 0.05 V to 0.35 V,and the average thickness of the Al₄C₃layer increased from 4.7μm to 11.9μm when current density varied from 0.2 A/cm² to1.6 A/cm².The thickness of the Al₄C₃ layer and the current density conformed to the function of y=4.85-0.96i+3.34i² under the electrolysis for 3.0 hours.（2）When the electrolysis time was extended from 1.0 hour to 5.0 hours with the current density of 1.0A/cm²,the average thickness of Al₄C₃ layer increased from 3.13μm to 8.37μm,which causes the electrolysis voltage increasing by 0.08 V to 0.22 V.The relationship between electrolysis time and thickness of Al₄C₃ layer was defined by y=0.31+2.99t-0.27t²,in which the thickness tends to be 8.5μm with extending electrolysis time.The effect of CR on the ionic composition of molten Na F-Al F₃,Na F-Al F₃-Al₂O₃,and Na F-Al F₃-Al₄C₃ systems was illustrated based on thermodynamic theory.Quantum chemical calculation was performed to obtain the structure properties and Raman spectra of ionic clusters in molten salts.A scheme was proposed to simulate the high temperature Raman spectra of molten salts,which was validated by experimental high temperature in situ Raman spectroscopic analysis.The results show that:（1）The Na F-Al F3 system contains 8 ions(F^-,Al F₄^-,Al F₅^2-,Al F₆^3-,Al₂F₇^-,Al₂F₉^3-,Al₂F₁₀^4-,and Al₃F₁₄^5-),which was used to calculate the electric conductivity of molten salts.In addition,as CR=2～3,Na₃Al F₆,Na₅Al₃F₁₄,Na₂Al F₅ and Na Al F₄ are the main fluoro-aluminate clusters in molten Na F-Al F₃ system with the maximum concentrations of 25.92%,22.14%,18.08%and 11.33%,respectively.（2）When the CR range is from 1.0 to 3.0,it is confirmed that Na₂Al₂O₂F₄ and Na₂Al₂OF₆ are the main products of Al₂O₃ dissolution in molten Na F-Al F₃ salts with the maximum concentrations up to 23.11%and 36.59%,respectively.While,as the CR is greater than 3.0,the maximum concentrations of Na₆Al₂OF₁₀,Na₄Al₂O₂F₆,Na₂Al₃O₄F₃,Na₂Al OF₃,and Na₃Al₃O₄F₄ are 4.03%,5.80%,8.22,8.02%and 8.24%in molten Na F-Al F₃-Al₂O₃ salts.（3）Na₃Al₃CF₈ was proved the main product of the Al₄C₃ dissociation in the molten Na F-Al F₃ salt.Its characteristic Raman peaks locate at 445 cm^-1 and 715 cm^-1.First-principles calculation was performed to investigate the adsorption properties of the primary atoms and clusters on the graphite surface,which clarified that the chemisorption is formed on the graphite surface.The results show that:（1）The order of adsorption strength of a single atom on the graphite surface is F>Al>Na,with average adsorption energies of-2.375 e V,-1.300 e V,and-0.892 e V,respectively.The adsorption sites cause adsorption energy varying by 0.1 e V.（2）The adsorption strength of Al F_nclusters on the graphite surface is in order of Al F₆>Al F₅>Al F₄,with average adsorption energies of-4.671 e V,-4.241 e V,and-3.840 e V.The effect of adsorption sites on adsorption energy of Al F_nclusters could be ignored,which is about 0.05 e V.（3）The adsorption strength of Na_xAl_yF_x+3y clusters on graphite surface is in the order of Na₂Al F₅>Na₃Al F₆>Na Al F₄,with the average adsorption energy of-0.800 e V,-0.734 e V,and-0.692e V,respectively.（4）Based on analysis of electronic structure,for adsorption of a single atom on the graphite surface,electrons transfer from Na or Al atom to C atom of graphite layers,while F atom will get electrons from the carbon atom.For adsorption of Al F_n and Na_xAl_yF_x+3y clusters on the graphite surface,chemical bonds are formed between Na-C and between F-C.There are no direct bonds or strong interactions between aluminum and carbon atoms.Therefore,defluorination reactions are required to expose the aluminum atoms,which leads to the formation of Al-C bonds.Otherwise,the adsorption of fluoro-aluminates on the cathode will not produce aluminum carbide directly.The adsorption model of fluoro-aluminates on the graphite cathode was established to investigate the defluorination reaction of ionic clusters based on the first principles theory and clarify the formation mechanism of aluminum carbide on the graphite cathode.The results show that:（1）The control step in the progressive defluorination reaction of a single Na₃Al F₆ cluster is*Al F₃→*Al F₂+F^-,which causes the energy barrier of 4.74e V.（2）The defluorination via polymerization is easier than the progressive defluorination of single Na₃Al F₆,in which only 7.5 e V（polymerization by Al-Al）and 8.2 e V（polymerization by Al-F-Al）are needed to overcome the energy barrier from*2Al F₃defluorination to*2Al F₂.（3）When the equilibrium potential of-4.08 V is applied,the energy barrier of the defluorination reaction is significantly reduced.Only 0.71 V and1.55 V are required to overcome the energy barrier of the control step in the defluorination reaction via polymerization by Al-Al interaction and Al-F-Al linkage,respectively.（4）When the ratio of aluminum to carbon on the graphite surface is Al_nC₁（2≤n≤6）and Al_nC₂（3≤n≤5）,which leads to the forming of the specified fragments of Al₄C₃.In summary,the electrochemical erosion mechanism of graphite cathodes was investigated systematically in this work,which elucidated the effect of cryolite ration on the microstructure and composition of molten salts,and developed a new method for investigating the ionic structure of molten salts by combining high temperature Raman spectroscopy and quantum chemical calculations,clarified the electrochemical reaction mechanism on the cathode surface,and revealed the formation mechanism of aluminum carbide on the graphite cathode.This work will provide a scientific basis and theoretical guidance for the future optimization of cryolite electrolyte,the development of new cathode materials,and the improvement of the service life of aluminum electrolytic cells.