Synthesis And Electrochemical Performance Of MXene-based Composites

In recent years,with the aggravation of serious environmental problems and the exhaustion of fossil energy,it is urgent to develop efficient,clean energy storage and conversion devices.Supercapacitors have higher power density and faster charging speed than lithium-ion batteries,which has attracted extensive attention from researchers.As a new type of two-dimensional nanomaterial,graphene-like layered transition metal carbide Ti3C2 MXenes has shown great application potential in the field of supercapacitor electrode materials.However,due to the easy stacking of Ti3C2 sheets,the mass specific capacitance of pure Ti3C2 as the supercapacitors electrode material is not high.For the above problems,in order to further improve the electrochemical performance of Ti3C2,different synthesis methods and synergistic effects are adopted in this paper.The organ-like Ti3C2 and ultrathin Ti3C2 are compounded with conductive polymer PPy,derived carbon and metal oxide Co3O4 to prepare electrode materials for supercapacitors with high performance.The specific research content includes the following four parts:(1)Organ-like Ti3C2/PPy nanoparticle composites with different mass ratios were prepared via low-temperature in-situ chemical oxidation in HCl solution.Ti3C2/PPy-2 composites have demonstrated good electrochemical performance as electrode materials for supercapacitors.The organ-like Ti3C2/PPy nanocomposites exhibits the highest specific capacitance of 184.36 F g-1 at 2 mV s-1,which is about 37.67%higher than that of pure Ti3C2;and keeps excellent cycling stability almost 83.33%capacitance retention after 4000 charging-discharging cycles at 1 A g-1.The synergistic effect of two materials with different energy storage mechanism materials improves the electrochemical performance of nanocomposite Ti3C2/PPy-2.(2)In order to further improve the mass specific capacitance of PPy/Ti3C2 composites,polypyrrole nanospheres/ultrathin Ti3C2-MXene(PPy/Ti3C2)heterostructure nanocomposite with excellent electrochemical properties were prepared via one-step low temperature in-situ polymerization method in this chapter.When the amount of Ti3C2 reaches 70 mg,it is most beneficial to form a unique 3D structure.In the three-electrode test system,the specific capacitance of the PPy/Ti3C2-70(S2)heterostructure nanocomposite was as high as 458 F g-1 at the scanning rate of 2 mV s-1,which is about 2.47 times increased compared with pure Ti3C2.And S2 shows good cycling stability.The S2 symmetric supercapacitor achieved high energy density of 21.61 Wh kg-1 at the power density of 499.94 W kg-1.Meanwhile,after 4000 charge and discharge tests the initial capacitance retention of 73.68%at 1 A g-1.The excellent electrochemical performance of S2 is attributed to the outstanding hydrophilicity of Ti3C2,which enables its surface terminal functional groups to be fully dispersed in the aqueous electrolyte and exhibits a strong ?-conjugation and electrostatic forces with the PPy framework in close combination.In addition,the large specific surface area of Ti3C2 provides more nucleophilic active sites.The combination with PPy prevents them from agglomerating and stacking spontaneously,which makes S2 has smaller inherent resistance and charge transfer resistance.(3)In order to further improve the cycling stability of Ti3C2-based composites,the two-step methods were adopted to synthesize ultrathin Ti3C2/PAP(PPy and PANI copolymer)hollow sphere by low-temperature chemical bath firstly,then pyrolysis ultrathin Ti3C2/PAP hollow sphere by heat treatment,and finally synthesizes ultrathin Ti3C2 nanosheet/N-C hollow sphere nanocomposite(pyrolyzed PAP hollow sphere,namely nitrogen-doped C hollow sphere,referred to as N-C hollow sphere),and serve as electrode material for supercapacitor.The experimental results showed that the N-doped hollow C spheres were successfully supported on the surface of the wrinkled ultrathin Ti3C2 and significantly improved the electrochemical performance of the composite Ti3C2/N-C electrode.In the three-electrode test system,the specific capacitance of the Ti3C2/N-C nanocomposite was as high as 205.09 F g-1 at the scanning rate of 2 mV s-1,which is approximately 1.77 times higher than that of pure Ti3C2.Moreover,after 4000 cycles charge and discharge tests at current density of 1 A g-1,the initial capacitance retention can still reach 93.97%and showing good rate performance and cycling stability.The Ti3C2/N-C symmetric supercapacitor achieved high energy density of 16.39 Wh kg-1 at the power density of 400.05 W kg-1.Even when the power density is 3998.68 W kg-1,the energy density is still 6.72 Wh kg-1.Meanwhile,after 4000 charge-discharge tests the initial capacitance retention of 78.26%.(4)Transition metal oxides are another important classes of pseudocapacitive electrodes.The ultrathin Ti3C2 is used as an excellent conductive matrix to combine with high theoretical specific capacitance Co3O4 to prepare ultrathin Ti3C2/flower-like Co3O4 electrode material,which can be further used in the study of electrochemical performance of supercapacitors.The ultrathin Ti3C2/flower-like Co3O4 composites was prepared by hydrothermal method and heat treatment with cobalt source of cobalt nitrate hexahydrate,precipitant of urea and ammonium fluoride.The results showed that Co3O4 nanoflowers with small size and uniform morphology were successfully loaded on the surface and interlayer of the wrinkled ultrathin Ti3C2,and exhibited good electrochemical performance.The specific capacitance of the Ti3C2/Co3O4 nanocomposite is 297.98 F g-1 at the scanning rate of 2 mV s-1,which is approximately 2.64 times higher than that of pure Ti3C2.Moreover,after 4000 cycles charge and discharge tests at current density of 1 A g-1,the initial capacitance retention can still reach 84.18%and show good cycling stability.