Synthesis And Electrochemical Performance Of Two Dimensional Titanium Carbide MXene（Ti₃C₂T_x）Based Composites

Two-dimensional transition metal carbides（MXene）are a new family of graphene-like two-dimensional（2D）materials,which have unique layered structures and high electron mobility.Therefore,these materials show good application prospects in the field of energy storage and energy conversion.At present,about 20 different MXenes have been successfully synthesized,among which the transition metal titanium carbide Ti₃C₂T_x is most widely explored.Ti₃C₂T_x MXene is considered as a favorable candidate for high energy electrode materials due to its higher metal conductivity and unique hydrophilicity than graphene.Nevertheless,compared to the prosperity of graphene,the development of MXene composite system is still in an infant stage.Therefore,in this work,a series of MXene-based composite,such as MnO₂/Ti₃C₂T_x,NiSe₂/Ti₃C₂T_x and Ti₃C₂T_x/Co（OH）₂,were designed and synthesized.The relationship between the structure and electrochemical activity of the electrode materials was researched in detail,and the charge storage mechanism of the composite materials was also investigated.The main contents are as follows:（1）A novel MnO₂/Ti₃C₂T_x MXene composite as electrode materials for supercapacitors.MnO₂ with high specific capacity(the theoretical value of up to 1370F g^-1),is a kind of great potential capacitor electrode material,but it often suffers in poor conductivity,which limits practical applications according to industry needs.Therefor,we designed a novel MnO₂/Ti₃C₂T_x MXene composite which is prepared by coupling one-dimensional MnO₂ nano needle and two-dimensional Ti₃C₂T_x MXene.This novel structure can be combine the shorter ionic conduction path and higher electrochemical activity of one-dimensional MnO₂ with the high electrical conductivity of MXene,and thus can lead a strong synergistic effect between Ti₃C₂T_x MXene and MnO₂.The as-prepared MnO₂/Ti₃C₂T_x MXene with high performance can be used as the flexible supercapacitor device which can reach the energy density of 0.7μWh cm^-22 at the power density of 80.0μW cm^-2.Compared with the flexible SCs employing the other MnO₂ or carbonaceous materials,the as-prepared MnO₂/Ti₃C₂T_x composites flexible supercapacitor exhibits much better electrochemical performance.In particular,the capacitance retention can exceed 100%after 1000 cycles under 0.2 mA cm^-2,indicating that MnO₂/Ti₃C₂T_x composite flexible supercapacitor has excellent electrochemical stability.Moreover,the flexible supercapacitor also has good capacitive behavior and high flexibility,demonstrating that the prepared MnO₂/Ti₃C₂T_x nanocomposite is a very promising electrode material for flexible supercapacitors device.（2）Ultrathin Ti₃C₂T_x（MXene）nanosheets wrapped NiSe₂ octahedral crystal for enhanced electrochemical performance.Metal selenides,such as NiSe₂,have exhibited great potentials as multifunctional materials for energy storage and conversation.However,the pure selenides can be oxidized and corroded during electrochemical process.Therefore,in this work,a novel NiSe₂/Ti₃C₂T_x hybrid is fabricated by wrapping NiSe₂ octahedral crystal with ultrathin Ti₃C₂T_x MXene nanosheet.Firstly,NiSe₂ is stabilized by Ti₃C₂T_x MXene sheet wrapping on the surface,and their outstanding electrical properties are secure.Secondly,the strong interfacial interaction between NiSe₂ octahedral crystal and Ti₃C₂T_x MXene can provide enhanced electrochemichal performance.The NiSe₂/Ti₃C₂T_x hybrid exhibits a high specific capacitance of 531.2 F g^-1 at 1 A g^-1 for supercapacitor,and low overpotential of 200mV at 10 mA g^-1,small Tafel slope of 37.7 mV dec^-1for hydrogen evolution reaction（HER）.Moreover,the hybrid also shows the good cycle stability both in surpercapcitore and HER test.And these results highlight the promising potentials in NiSe₂/Ti₃C₂T_x hybrid for multifunctional applications such as energy storage and conversion.（3）Synthesis of MXene/Co（OH）₂ 2D-2D heterojunction materials and the mechanism of tunable pseudocapacitance of Co（OH）₂ by MXene.MXene combined with metal oxide carbon materials and conductive polymers,can significantly improve the electrochemical performance of these materials.However,how MXene regulates the pseudocapacitance of materials has not been studied in depth.Therefore,in this work,Co（OH）₂ as a modle,which is a typical pseudocapacitance material,was hybrided with Ti₃C₂T_x MXene to obtain a layered 2D/2D heterojunction Ti₃C₂T_x/Co（OH）₂ hybrid to shed light on the pseudocapacitive energy storage mechanism of MXene-based electrode materials,.and thus can provide theoretical support for the design of MXene-based composites.As a result,the 2D/2D heterojunction of optimized Ti₃C₂T_x/Co（OH）₂ hybrid shows significant improvement on the performance of supercapacitor.The total pseudocapacitance of the Ti₃C₂T_x/Co（OH）₂ hybrid is 2 times more than that of bare Co（OH）₂.Meanwile,as confirmed by quantitative kinetic analysis,comparaed with that of bare layered Co（OH）₂,the intercalation pseudocapacitive contribution increases from 35.3%to 56.4%after formation of 2D/2D heterojunction of Ti₃C₂T_x/Co（OH）₂ hybrid,which can assure the high rate capability and excleent cycle stability of the hybrid due to fast ion diffusion in the bulk,Meanwile,pseudocapacitance can be tailored by the content of Ti₃C₂T_x MXene,and thus can provide a new approach to design other MXene-base electrode materials with high rate capacbily and stability peformence for supercapaciors.