Study On Tuning The Composition And Microstructure Of Two-dimensional Titanium Carbide Ti₃C₂T_x And Their Effects On Its Electrochemical Performance As Supercapacitor Electrode Materials

The excessive use of fossil fuels and emission of greenhouse gases into the environment have become one of the major challenges nowadays.Renewable energy and to use renewable energy safely,conveniently and more efficiently is the key to solve this problem.Supercapacitors,as one of the major categories of energy storing devices other than rechargeable batteries,play an important role in solving the issue.Because of its excellent electric conductivity,hydrophilicity and comparable energy density,Mxenes,the two-dimensional transition metal carbides/nitrides,has become a promising candidate as supercpacitors’electrode materials.In this dissertation,Ti₃C₂T_x Mxene has been used as the electrode materials in supercapacitors and under thoroughly investigation with rGO added or external pressure applied to study its electrochemical performance change as chemical composition changes or microstructure changes.The details of the two research work are as follows:1.Mxenes and rGO,both as two-dimensioanl materials,have exhibited promising properties in many application fields.In this paper,Mxene/rGO sponge materials are prepared by a simple method,and exhibit high specific capacitance of 80.0 F g^-1 at a scan rate of 2 m V s^-1 and long lifetime(almost no obvious capaci-tance decay over 10,000 cycles at a current density of 1 A g^-1).Moreover,the effects of rGO ratio on the material properties such as microstructure,d-spacing,electron transport and ion diffusion,and its electrochemical performance are also investigated.This work should provide some insights into optimizing the electrochemical properties of supercapacitors using two-dimensional Mxene/rGO-based composite materials as electrodes.2.With the rapid development of micro-,wearable,foldable and deformable electronic devices,such as foldable smart phones,ultra-thin TVs,wearable smart watches and etc.,flexible energy storage deveices such as supercapacitors are in great demand.In this work,flexible supercapacitors based on Ti₃C₂T_x free-standing film are fabricated and its electrochemical performance with applied external pressure has been systematically investigated.Based on the experimental results,a process model was established to explain the structural change of film and ion diffusion movement during charge/discharge process with increasing external pressure.Briefly speaking,increase of applied pressure can promote the electrolytic ions diffusing into the interlayer spacing to contribute to the energy storing process in the way of the surface capacitive contribution,thus improving the capacitance.However,as the pressure increases to a certain value,it will reduce the interlayer spacing,which will lead to the reduction of ions that can be accommodated in the interlayer space,thus the capacitance decreases.This work will provide guidance on designing flexible superacapacitor devices of high performance with 2D Mxenes or other 2D nanomaterials as electrodes for use under pressure conditions.Due to the combination of many excellent properties,MXenes has a promising application prospect in energy materials,especially in supercapacitors.MXenes has higher energy density and specific capacitanc e compared to traditional carbon materials（graphene,carbon nanotubes and carbon fibers）,while MXenes also exhibit extremely high electrical conductivity and long cycle life compared to conductive polymers,metal oxides and hydroxides.Although MXenes are chemically active and easy to be oxidized,the oxidation could be minimized through using inert gas protection and other methods during the synthesis and the device fabrication process.Our work of studying the relationship between material composition/structure and its electrochemical performance should provide useful guidance on designing Ti₃C₂T_x based supercapacitors with superior properties.