Study On Molten Salt Electrolysis Of(Cr/V)₂AlC And Capacitance Behavior Of Its Derived Two-dimensional V₂CT_X

The MAX phase is a ternary transition metal carbonitride,it has the excellent properties of both metal and ceramic,because of its unique combination of intermetallic covalent bonds.At the same time,the MAX phase is also a precursor for the preparation of two-dimensional MXene materials.Because of its graphene-like layered structure and abundant functional groups bonded on the surface,it has been widely studied and applied in the fields of energy storage and catalysis.At present,the preparation methods of MAX phase mostly use pure metals to sinter at high temperature and high pressure to prepare MAX phase block,and MAX phase powder is obtained by ball milling.The preparation process is long,the cost is high,and the product purity is low.This paper started from the preparation of Cr₂AlC by molten salt electrolysis,the process of deoxidation and phase evolution of metal oxides during the electrolysis process were studied,and the optimal ratio of metal oxides and carbon had been determined.On this basis,V₂AlC was prepared by molten salt electrolysis,and high-performance supercapacitor electrode material V₂CT_X was obtained through etching and alkalization.The preparation of Cr₂AlC and V₂AlC by molten salt electrolysis has the advantages of short process flow,low cost,and high product purity.In addition,Cr₂AlC shows good oxygen resistance at high temperature.The main conclusions drawn in this article are as follows:(1)Cr₂AlC was synthesized in molten Ca Cl₂ at 850°C and the electrolysis voltage of 3.1 V,with the precursor of Cr₂O₃/Al₂O₃/C.The influence of the ratio of raw materials on the purity of the electrolysis product was studied by the controlled variable method,and it was determined that the purity of the electrolysis product obtained when Cr₂O₃:Al₂O₃:C=1:0.75:1 was the highest.Through the phase analysis of the electrolysis products at different time nodes of the electrolysis process,and combined with thermodynamic calculations,it is determined that the formation of Cr₂AlC follows the following sequence:Cr₂O₃?Cr+C?Cr_mC_n+Al?Cr₂AlC.(2)V₂AlC was prepared through the molten salt electrolysis process,using the ratio of V₂O₃:Al₂O₃:C=1:0.75:1.The purity of the product prepared by electrochemical reduction in molten salt is higher,and the particle size is much smaller than that of V₂AlC prepared by sintering and ball milling,which provides a good reaction kinetic basis for its subsequent applications.V₂AlC was treated with hydrofluoric acid to selectively etch the aluminum atomic layer to obtain two-dimensional V₂CT_X MXene,and the prepared V₂CT_X presents an accordion-like two-dimensional layered structure.(3)The V₂CT_X layer spacing was further expanded,and the content of surface-F functional groups was reduced after KOH treatment.As an electrode material for water-based super-capacitors,V₂CT_X exhibited excellent performance in electrochemical performance tests.The electric capacity in 1M H₂SO₄;1M KOH;1M Li₂SO₄ electrolyte can reach 695 F/g,478 F/g and 410 F/g respectively,and 453 F/g at the scanning rate of 100 mv/s in 1M H₂SO₄,showing good high rate performance,much higher than the MXene material obtained by traditional preparation methods.It also exhibited a long cycle life in acidic,alkaline and neutral electrolytes.The capacity retention rates after 10,000 cycles were 81.9%,97.2%and 106.6%,respectively.