Desigh,Preparation And Capacitive Deionization Performance Of Ti₃C₂T_x Electrode Materials

With the continuous advancement of industrialization and urbanization,the shortage of freshwater resources has become increasingly serious.Desalination technology is an important way to solve the problem mentioned above.Capacitive deionization(CDI)is an electrochemical desalination technology developed during recent decades,which has received increasing attention due to its advantages of simple construction,low operation cost,low operating voltage(<2 V),and no secondary pollution,etc.However,the co-ion expulsion effect hinders the electrosorption capacity and charge efficiency of CDI.Ti₃C₂T_x is one of the most typical MXene,which possesses features of good hydrophilicity,good in-sheet conductivity,large intercalated pseudocapacitance,and good electrochemical stability.In addition,Ti₃C₂T_x has a large number of negatively charged functional groups(-OH,-O,and-F),making it very promising as a cation-selective cathode material to reduce the co-ion expulsion effect.In this dissertation,a series of modified Ti₃C₂T_x have been prepared by tuning the micro-nano structure of Ti₃C₂T_x,and their CDI performance has also been systematically studied.The main contents of the dissertation are as follows:(1)Construction of Na⁺intercalated Ti₃C₂T_x(Na⁺-Ti₃C₂T_x)material with large interlayer space and research on CDI performance.Na⁺-Ti₃C₂T_x was synthesized via etching Ti₃Al C₂ in the concentrated HF and following treatment in Na OH solution.The pre-intercalated Na⁺can effectively expand the interlayer spacing,increase the ion storage space,and reduce ion diffusion barriers.At the same time,the Na⁺-Ti₃C₂T_x is chemically bonded with a large number of functional groups such as-OH,-O,and-F.These functional groups endow it with strong electronegativity.The asymmetric CDI cell,in which Na⁺-Ti₃C₂T_x serves as a cation-selective cathode while the activated carbon(AC)serves as the anode,achieves a high electrosorption capacity of 12.19 mg g^-1 and a high charge efficiency of 0.826.(2)Construction of small particle size Ti₃C₂T_x material and research on CDI performance.Ti₃Al C₂(Ti₃Al C₂-MS)with a purity of 98.5%was prepared by the molten salt synthesis in 1200°C,where Ti C,Ti C,and Al powder were served as raw materials and KCl as molten salt.The liquid phase environment provided by molten salt at a high temperature can speed up the material transfer rate so that the synthesized temperature is reduced by 200°C compared with the traditional synthesis method.With the help of lower synthesis temperature and the blocking effect of molten salt between Ti₃Al C₂ crystals,the excessive growth of Ti₃Al C₂ crystals in the synthesis stage was stopped,and the small-size Ti₃Al C₂ crystals were obtained.The subsize and Na⁺intercalated Ti₃C₂T_x(Na⁺-Ti₃C₂T_x-MS)has been obtained by HF etching Ti₃Al C₂-MS and following Na OH treatment.The asymmetric CDI cell was assembled,where the Na⁺-Ti₃C₂T_x-MS acted as cathode and AC as the anode.This CDI cell shows a high electrosorption capacity of 14.8 mg g^-1 and a high charge efficiency of 0.81.The superior performance of Na⁺-Ti₃C₂T_x-MS can be attributed to the subsize particle and large interlayer space.Both two factors can effectively increase ion adsorption sites,shorten diffusion path lengths,and reduce diffusion barriers in the CDI process.(3)Construction of three-dimensional Ti₃C₂T_x(3D-Na⁺-Ti₃C₂T_x)and research on its CDI performance.First,the delaminated Ti₃C₂T_x(d-Ti₃C₂T_x)colloidal solution was prepared by Li F/HCl etching and subsequent sonication.Then,Na OH can effectively induce d-Ti₃C₂T_x nanosheets to flocculate and fabricate 3D porous architecture.The interlaced Ti₃C₂T_x layers form a 3D network structure and significantly enhances the conductivity.The conductive porous architecture speeds up the electrons transfer rate and reduces the ions diffusion barrier.The 3D-Na⁺-Ti₃C₂T_x electrode shows a high electrosorption capacity of 16.2 mg g^-1 even without adding conductive additives.