The Synthesis Of Two Dimensional Transition Metal Carbide Based Composite And Studies Of Their Electrochemical Performance

Supercapacitor,as a new energy storage device,have been attracting considerable interest in the energy storage field due to its high power density and great cycling performance.However,the energy density of supercapacitor is limited by its energy storage mechanism and the low working voltage.Fabricating new electrode materials with high specific capacitance and good rate performance is a effective approach to improve the electrochemical performance of supercapacitor.A new big family of two-dimensional graphene-like transition metal carbides and/or nitrides labeled as MXenes have been regarded as promising candidates for developing high performance supercapacitor electrodes owing to their good electric conductivity and rich hydrophilic surface groups.However,one of the main challenges to exploit the full potential of MXenes to store energy is to prevent MXene nanosheet restacking that prevents full utilization of surface area and blocks electrolyte access.The surface groups also weaken the electric conductivity of MXene.In order to solve these problems,in this work we combined MXene with other materials and exfoliation multi-layer MXene to few-layer MXene nanosheets to fabricate MXene-based electrode with high electrochemical performance.Furthermore,we also discussed the role of the surface groups on MXene in its electrochemical behavior.On this basis,this work focuses on the preparation process of the flexible Ti₃C₂T_x MXene-based electrode,systematically characterizing the physical phase and morphology of the prepared electrode materials,and deeply studying their electrochemical performance and application of the assembled flexible supercapacitors in portable devices.The main conclusions are as follows:?1?Coplanar asymmetric supercapacitors are assembled with Ti₃C₂T_x anode and Co-Al-LDH cathode by modified two-step screen-printing process.The areal energy densities of assembled flexible all-solid-state asymmetric supercapacitors reached 8.84?Wh cm^-2,which is higher than that of symmetric supercapacitors(3.38?Wh cm^-2).After 10,000 cycles of charging and discharging,the specific capacitance of assembled flexible all-solid-state asymmetric supercapacitors maintained 92%.In addition,the coplanar asymmetric supercapacitors were integrated with the force sensing resistors as portable power source units to fabricate lightweight and inexpensive integrated force sensors,which were used to detect applied pressure variation.Notably,the two-step screen-printing method can also be extended to other MXenes and various positive electrode materials for fabrication of coplanar asymmetric supercapacitors,which promotes the development of MXene-based supercapacitors and their integration with other portable multifunctional electronic systems.?2?Binder-free Ti₃C₂T_x film electrodes on nickel foam with adjustable mass loading were successfully fabricated via a modified electrophoretic depositionn technique.Using the few-layer MXene nanosheets,the restacking of Ti₃C₂T_x nanoflakes can be eliminated.The Ti₃C₂T_x film with large surface area and ideal pore size distribution?around 4 nm?would exhibit improved electrochemical capacitance due to the increase of ion-accessible surface area and pseudocapacitive contribution.The fabricated electrode possessed improved specific capacitance up to 140 F g^-1 with excellent cycling performance and good rate performances in 1 M KOH electrolyte.?3?Flexible conductive rGO/Ti₃C₂T_x films were produced through vacuum-assisted filtration of the mixed suspension followed by thermal reduction.The rGO conductive networks tightly link the separated layers of Ti₃C₂T_x and connect the separated Ti₃C₂T_x particles,improving the conductivity of whole electrode.The volumetric capacitance and gravimetric capacitance of the rGO/Ti₃C₂T_x film in 6 M KOH can reach 370 F cm^-3 and 405 F g^-1,respectively,owing to the synergistic effect between rGO and Ti₃C₂T_x.The assembled all-solid-state supercapacitor exhibited a high energy density of 63 mW h cm^-3 at a power density of 0.06 W cm^-3.Moreover,flexible all-solid-state supercapacitors based on rGO/Ti₃C₂T_x films were integrated with thin-film solar cells by the co-cathode method to energy conversion and storage devices.?4?Flexible freestanding rGO/Ti₂CT_r films were fabricated by vacuum-assisted filtration followed by thermal reduction.The rGO/Ti₂CT_r hybrid films present improved reversible capacity up to 700 mAh g^-1 at 0.1 A g^-1,high coulombic efficiency,excellent cycling stability and rate performance.Similarly,rGO/Ti₃C₂T_r films also showed an improved electrochemical performance for Li-ion storage.?5?Few-layer Ti₃C₂T_x nanosheets were filtrated to fabricate Ti₃C₂T_x paper.Various surface treatment methods were used to control the surface functional groups of MXene,and the electrochemical properties of MXene paper with different functional groups were analyzed.The electrochemical activity of each functional group was obtained.The method in this work to improve the electrochemical performance of Ti₃C₂T_x also works for other MXenes,for example Nb₂CT_x?V₂CT_x and Ti₂NT_x.