| The supercapacitor,with rapid charge and discharge and long stability characteristics,can ease the intermittency problem in the electric generation of wind and solar energy.MXene nanosheets have excellent conductivity and redox activity,which can meet the needs of rapid charge and discharge characteristic,high capacity and long stability of supercapacitor.However,MXene nanosheets are prone to re-stacking in the process of preparing electrode materials,which leads to the decline of capacitance and rate performance.This study introduces interlayer molecule and constructs 3D structure,which restrains the re-stacking of MXene nanosheets.To further improve the energy density of the composites,MXene was compounded with high-capacity pseudocapacitance materials,greatly optimizing the electrochemical performance of MXene composite.(1)Using hexamethylenetetramine(HMT)as the interlayer molecule,MXene@HMT film was prepared by vacuum filtration,and N-MXene films were obtained by further carbonization.The HMT molecule has a spatial structure,which can inhibit the agglomeration of the MXene nanosheets at the nanoscale without destroying the integrity of the MXene film.The experimental results show that MXene@HMT can obtain better electrochemical performance after appropriate temperature carbonization.The N-MXene-300 exhibited a capacitance of 293 F g-1at a lower scan rate(10 m V s-1).In addition,the capacitance retention of N-MXene-300 was as high as 66.09%at 2000 m V s-1,which was significantly higher than that of pure MXene(29.38%).The asymmetric capacitor assembled by nitrogen-doped graphene and N-MXene-300,showing a high capacitance retention of 92.1%after 25,000cycles.(2)Three-dimensional(3D)hydrogel materials were prepared by mixing aluminum metal particles with MXene nanosheets,using trace graphene oxide(GO)as structural additive.Trace acid solution was used as the initiator to etch the blunted aluminum oxide layer,where the trivalent aluminum ions were released,disrupting the electrostatic repulsion in the solution and acting as a linker to assemble the nanosheets into a 3D structure.At the same time,Al can reduce GO to graphene,creasing the mechanical strength of the hydrogel.The MXene@r GO gel exhibited a maximum area capacitance of 4.33 F cm-2(10 m A cm-2)and maintained a high area capacitance of 1.76 F cm-2at 1000 m A cm-2,showing a high capacitance retention rate of 40.6%.When constructed as asymmetric capacitor with graphene,the capacitance of capacitor showed a high cyclic stability with a mere 8.37%decrease in 100,000 cycles test.(3)MXene nanosheets were used as the conductive agents to improve the conductivity of nickel-cobalt bimetallic hydroxide(Ni Co-LDH).In the preparation process,MXene nanosheets coated with ZIF-67 spherical structure were synthesized by electrostatic self-assembly of negatively-charged MXene and positively-charged metal-organic framework material(ZIF-67).The structure was converted into a 3D floral Ni Co-LDH/MXene structure during hydrothermal process.Due to the strong interaction between Ni Co-LDH and MXene,the stability of Ni Co-LDH was improved,thus optimizing the cycle stability of this material.At the current density of 1 A g-1,Ni Co-LDH/MXene exhibited a capacity of 635.7 C g-1.Due to the optimization of structure and conductivity,the material can still maintain a high capacity of 382.3 C g-1at 20 A g-1.Aasymmetric capacitor assembled with activated carbon shows an energy density of 44.6 Wh kg-1and can light the bulb for 20 minutes. |
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