Preparation And Electrochemical Performance Of Ti₃C₂T_x MXene Composite Supercapacitor Electrode

With the development of renewable energies and the consumption of traditional fossil fuels,energy storage has become an exigent issue for the sustainable development of human society.Supercapacitors are an important source of power that can meet the growing demand for energy conversion and storage because they provide fast charging and discharging processes,relatively low cost,long cycle life and high power performance.Recently,Two-dimensional（2D）transition metal carbides and nitrides（MXenes）have attracted a lot of attention.Ti₃C₂T_x,as a typical member of the MXenes family,is considered as a promising electrode material for supercapacitors due to its large layer spacing,ultra-high conductivity and rich surface chemistry.In order to further improve the electrochemical performance of Ti₃C₂T_xand solve the layer-stacking problem of layered Ti₃C₂T_xmaterials.In this paper,N doping method and 3D ice template method were used to deal with the above problems,respectively.And the role of Ti₃C₂T_xas conductive substrate in the composite material was studied.Firstly,the monolayer Ti₃C₂T_xwas N-doped with hydrazine hydrate as a nitrogen source,and its electrochemical performance was tested.In 1 M H₂SO₄aqueous solution,The N-Ti₃C₂T_xfilm electrode presents a higher rate performance（60%）than the Ti₃C₂T_xfilm electrode（42%）when the scan rate increased from 2 to 100 m V s^–1.Notably,at the same current density,the specific capacitance of the N-Ti₃C₂T_xfilm electrode is significantly superior than that of the Ti₃C₂T_xfilm electrode.At 2 m V s^–1,the specific capacitance of the N-Ti₃C₂T_xfilm electrode is 340 F g^–1,which is more than that of the Ti₃C₂T_xfilm electrode(234 F g^–1).The outstanding electrochemical performance of the N-Ti₃C₂T_xfilm electrode could be ascribed to the following effects:N-doping could improve the conductivity and promote the transport of electrons;N-doping could provide additional pseudocapacitance;hydrazine hydrate as a nitrogen source provided a reductive environment to avoid oxidation of the Ti₃C₂T_xfilm and decreased the amount of–OH and–F surface groups.Excellent electrochemical properties show that the N-Ti₃C₂T_xfilm is a promising advanced SC electrode material.In order to solve the two-dimensional stacking problem of MXene layers,a two-step in situ method was proposed to prepare independent Ag nanoparticles modified 3D Ti₃C₂T_xfilm.Ti₃C₂T_xwas used as a reducing agent to reduce Ag NO₃solution by–OH group,and the Ag NPs obtained could firmly anchor the surface of Ti₃C₂T_x.Then,3D Ti₃C₂T_xfilm was prepared by vacuum freeze-drying method.In the freeze-drying process,small ice crystals disappeared from the Ti₃C₂T_xinterlayer,leaving gaps to construct a 3D network.Through scanning electron microscopy test,the thickness of 3D-Ti₃C₂T_x/Ag is 10 times of the original,effectively preventing the layer stacking phenomenon,and the specific capacitance of 3D-Ti₃C₂T_x/Ag electrode at 2 m V s^-1is 356 F g^–1,the capacitance retention rate is 94.7%after 40,000 cycles at the current density of 10 A g^–1,showing good electrochemical performance.In addition,due to the metal-like conductivity of Ti₃C₂T_x,the electrochemical properties of Ti₃C₂T_xas conductive substrate and Co S nanowire composite were studied and explored Ti₃C₂T_xfunction and optimal ratio.Meanwhile,Co S nanowires with high capacitive activity can be used as interlayer spacer to inhibit the stacking of Ti₃C₂T_xnanosheets.As a results,at current density of 1 A g^–1,the specific capacitance of Co S/Ti₃C₂T_x-5 electrode is increased by 528 F g^–1.At the current density of 10 A g^–1,the ultra-high capacitance retention is 99.3%after 20 000 cycles.The method of Ti₃C₂T_xas conductive additive to improve electrochemical performance can be used as a reference for other materials such as sulfide and selenide.