Preparation And Electrochemical Properties Of Ti/V And Ti/Nb MXene Solid Solutions

With the depletion of fossil energy and the increasingly serious problem of global warming,the development of clean energy has been paid more and more attention,among which efficient energy storage technology has become the key to break through the dilemma.Supercapacitors are becoming the preferred energy storage solution in many fields due to their excellent power density and long cycle life.MXene,a new family of two-dimensional materials composed of transition metal carbides/nitrides,has become a hot topic in the research of electrode materials for supercapacitors due to their high specific capacitance,excellent electrical conductivity and excellent mechanical flexibility.In this paper,bimetallic MXenes were synthesized by the preparation and etching of Ti/V and Ti/Nb MAX solid solutions,and their electrochemical performance was regulated,which greatly improved the capacitive performance of monometallic MXene.The main results are as follows:（1）The(Ti_1-yV_y)₃AlC₂ MAX solid solutions with different V content（y=0,10%,20%,33%）were prepared by using titanium,vanadium,titanium carbide,aluminum powders through high temperature solid phase reaction,from which the corresponding(Ti_1-yV_y)₃C₂T_x MXene solid solutions were prepared by etching and intercalation.XRD patterns show that the layer spacing of MXene solid solution decreases with the increase of V atom content.The electrochemical test of all samples shows that the REDOX peak of solid solution in cyclic voltammetry becomes weaker with the increase of V content,indicating that the contribution of pseudocapacitance decreases with the increase of V content.The specific capacitance of Ti₃C₂T_x,(Ti_0.9V_0.1)₃C₂T_x,(Ti_0.8V_0.2)₃C₂T_x and(Ti_0.67V_0.33)₃C₂T_x at 1 A g^-1 are 250.2 F g^-1,402.6F g^-1,509 F g^-1 and 457.1 F g^-1,respectively.The capacitance of(Ti_0.8V_0.2)₃C₂T_x is 2.03 times that of Ti₃C₂T_x,indicating that introducing V atom into M site of Ti₃C₂T_x MXene improves its electrochemical performance.When the current density increases from 1 A g^-1 to 100 A g^-1,the capacitance changes from 457.1 F g^-1 to 289.7 F g^-1,corresponding to a capacitance retention as high as 65.1%.The capacitance retentions of(Ti_0.8V_0.2)₃C₂T_x and(Ti_0.67V_0.33)₃C₂T_x after charging and discharging at 10 A g^-1 for 10000 times is only 6.4%and 19.4%,while that of(Ti_0.9V_0.1)₃C₂T_x is as high as 81.2%.（2）Nb atoms were introduced into the raw materials for preparing Ti₃AlC₂ to form(Ti_1-yNb_y)₃AlC₂ solid solution,where y=0,10%,20%,33%,from which(Ti_1-yNb_y)₃C₂T_x MXene solid solutions ware prepared by etching and intercalation.The layer spacings of all Ti/Nb MXene solid solutions decreased.Moreover,the F content on the surface of Ti/Nb MXene solid solution decreases with the increase of Nb atom,which has a positive effect on their capacitive properties.The specific capacitance of(Ti_0.9Nb_0.1)₃C₂T_x,(Ti_0.8Nb_0.2)₃C₂T_x and(Ti_0.67Nb_0.33)₃C₂T_x at 1 A g^-1 is 322.7 F g^-1,444.4 F g^-1 and 480.4 F g^-1,respectively.Among them,(Ti_0.67Nb_0.33)₃C₂T_x has the largest specific capacitance,which is 1.92 times that of Ti₃C₂T_x.When the charge/discharge rate increased from 0.5 A g^-1 to 100 A g^-1,its specific capacitance changes from 579.7 F g^-1 to 322.2 F g^-1,corresponding to a capacitance retention rate of 55.6%.After 10000 times of charging and discharging at 10 A g^-1,(Ti_0.9Nb_0.1)₃C₂T_x and(Ti_0.8Nb_0.2)₃C₂T_x retained 67.3%and 76.7%of their capacitance,respectively,while(Ti_0.67Nb_0.33)₃C₂T_x retained only 13%.（3）Composite hydrogels were prepared by mixing positive precrosslinked chitosan with negative Ti₃C₂T_x MXene with weight ratios of 1:5,1:6,1:7,1:8 and 1:9 with the aid of hydrogen bonding and electrostatic action.After freeze-drying and compression,Ti₃C₂T_x@CS composite aerogel film samples with a large number of holes on the surface and inside were obtained.The interlayer spacings and internal resistances of the composites increase,while the specific capacitances decrease with the increase of CS content.The rate performance of all the composites is significantly improved compared with that of Ti₃C₂T_x.With a CS/MXene mass ratio of 1:8,the Ti₃C₂T_x@CS-8 electrode has the best comprehensive electrochemical performance with the layer spacing of 1.35 nm.At 1 A g^-1,the specific capacitance is 288.9 F g^-1.When the charge/discharge rate increased from 0.5 A g^-1 to 100 A g^-1,the capacitance is222.2 F g^-1,and the capacitance retention rate is 72.1%.Inside the aerogels,abundant pores and increased layer spacing accelerate ion transport,and larger surface area provides more active sites for electrochemical reactions,thus improving the rate performance of MXene.Similarly,the mass ratio of chitosan and(Ti_0.8Nb_0.2)₃C₂T_x was 1:8 to prepare composite aerogel.When the current density increased from 0.5 A g^-1 to 100 A g^-1,the specific capacitance changed from401.6 F g^-1 to 211.1 F g^-1,and the corresponding capacitance retention rate was 52.6%,which was significantly higher than that of vacuum filtered(Ti_0.8Nb_0.2)₃C₂T_x membrane samples.In addition,the(Ti_0.8Nb_0.2)₃C₂T_x@chitosan aerogel sample has excellent cycling performance,and the capacitance retention rate reaches 83.7%after 10000 charges and discharges.