Synthesis,Morphology Control And Electrochemical Performance Of Two-dimensional Ti₃C₂Tx Mxene

Supercapacitors have become a class of promising energy storage devices due to their high-power output,fast charge-discharge,and ultra-long cycle life.The electrode materials are the core component of the supercapacitors and determine the capacitance and cycling performance of devices.MXenes,a family of two-dimensional（2D）transition metal carbides/nitrides with the general formula M_n+1X_nT_x（n=1–4,M stands for transition metal,X represents carbon and/or nitrogen,T_x represents surface terminations）,show promising potential in the field of energy storage owing to their 2D lamellar structure,metal-like conductivity,tunable surface groups and intercalation pseudocapacitive charge storage mechanisms.However,the lengthened ion pathway in restacked 2D MXenes greatly limits the electrochemical performance of dense film electrodes.Optimizing the preparation process of Ti₃C₂T_x and regulating the morphology of Ti₃C₂T_x are used to improve the electrochemical performance of the film electrode in this work,and the specific contents are as follows:（1）MXenes are synthesized based on Ti₃Al C₂,Al-Ti₃Al C₂ MAX phases and Li F/HCl,HF/HCl etchants,respectively,and the effects of different synthesis routes on their morphology and electrochemical performance are investigated.In order to facilitate the distinction,the MXenes synthesized based on the Ti₃Al C₂ phase and Li F/HCl and HF/HCl etchants are denoted as Ti₃C₂T_x@Li F/HCl and Ti₃C₂T_x@HF/HCl,respectively;and the MXene synthesized based on the Al-Ti₃Al C₂ phase and HF/HCl etchant is denoted as Al-Ti₃C₂T_x@HF/HCl.Among them,the Al-Ti₃C₂T_x phase isn’t successfully etched by Li F/HCl owing to its high crystallinity.Ti₃C₂T_x@Li F/HCl,Ti₃C₂T_x@HF/HCl and Al-Ti₃C₂T_x@HF/HCl films are prepared by vacuum-assisted filtration.The structure and morphology of the films are characterized by XRD,SEM and XPS,and the Ti₃C₂T_x@Li F/HCl film exhibits the relatively largest interlayer voids.The nanosheet sizes are observed by AFM,and the Ti₃C₂T_x@Li F/HCl nanosheet size is the relatively smallest.The square resistance（R_s）of the films is tested by the four-probe method,and the R_s of the Al-Ti₃C₂T_x@HF/HCl film is relatively the smallest,showing the most excellent electrical conductivity.Through electrochemical tests,Ti₃C₂T_x@Li F/HCl,Ti₃C₂T_x@HF/HCl,and Al-Ti₃C₂T_x@HF/HCl film electrodes show the specific capacitances of 394,342,and 399.3F g^–1 at 1 A g^–1 as well as the capacitance retention rates of 61.7%,55.6%and 63.6%at 10A g^–1,respectively.Compared with Ti₃C₂T_x@HF/HCl,Ti₃C₂T_x@Li F/HCl and Al-Ti₃C₂T_x@HF/HCl film electrodes show higher specific capacitance and rate capability,which are attributed to:（1）The larger interlayer voids and smaller nanosheet sizes of Ti₃C₂T_x@Li F/HCl improve the electrolyte ion transport efficiency;（2）the higher conductivity of Al-Ti₃C₂T_x@HF/HCl facilitates charge transport.Therefore,increasing the interlayer voids,reducing the size of nanosheets,and increasing the electrical conductivity are effective strategies to improve the electrochemical performance of MXene film electrodes.（2）Ti₃C₂T_x with high conductivity is synthesized by etching the Al-Ti₃Al C₂ phase with HF/HCl,and an easy,effective and controllable vacuum hydrothermal method is used to further regulate the morphologies and compositions of Ti₃C₂T_x.First,the effects of different hydrothermal temperatures（120,150,180°C）and times（6,12,18 h）on the electrochemical properties of Ti₃C₂T_x films are explored.Compared with the pristine Ti₃C₂T_x,the electrochemical properties of the Ti₃C₂T_x film electrode with hydrothermal treatment are significantly improved,and the Ti₃C₂T_x@150℃/12 h film electrode exhibits the best specific capacitance and rate performance.The structure and morphology of the films are analyzed by XRD,SEM,etc.Compared with the pristine Ti₃C₂T_x film,the Ti₃C₂T_x@150°C/12h film shows obviously enlarged interlayer voids and comparable electrical conductivity,resulting in significantly improved electrochemical performance.The effects of different loadings(1.38,3.07 and 6.44 mg cm^–2)on the electrochemical performance of Ti₃C₂T_x@150°C/12h film electrodes are investigated.Among them,the Ti₃C₂T_x(1.38 mg cm^–2)film electrode shows the best specific capacitance and rate performance with an excellent specific capacitance of 498.3 F g^–1 at 1 A g^–1 of current density,which is a 53.5%improvement of the specific capacitance over the pristine Ti₃C₂T_x film electrode(324.7 F g^–1)with the same loading.Moreover,the Ti₃C₂T_x(1.38mg cm^–2)film electrode still shows excellent capacitance retention（99.2%）after 5 000cycles at 10 A g^–1.Excitingly,the Ti₃C₂T_x(3.07 mg cm^–2)film electrode achieves an ultra-high volumetric capacitance of 1650 F cm^–3 due to its high packing density.Furthermore,the symmetric supercapacitor based on Ti₃C₂T_x(1.38 mg cm^–2)film electrode also shows high specific capacitance(73.3 F g^–1)and energy density(3.7 Wh kg^–1)at 1 A g^–1.Therefore,vacuum hydrothermal treatment is an effective strategy to improve the electrochemical performance of Ti₃C₂T_x film electrodes and may also show universality in other MXenes.