Fabrication Of Three-dimensional Networked Cobalt/Nickel Composites Based On PNT@ZIF-67 And Study On The Properties Of Supercapacitors

In order to cope with the negative impact of traditional energy on the environment,countries around the world are vigorously developing new energy technologies,therefore,higher requirements are placed on energy storage systems.Among many energy storage devices,supercapacitors exhibit extremely high competitive advantages due to their advantages such as fast charging and discharging speed,high power density,long service life and low maintenance cost.Nonetheless,currently available supercapacitors all have the problem of low energy density,which greatly limits their application scenarios.Therefore,the technical problem that needs to be broken through in the field of supercapacitors is to increase its energy density as much as possible while retaining its advantages of ultra-high power density and extremely long service life.It can be seen from the calculation formula of the energy density of the supercapacitor(E=1/2CV2)that the energy density of the supercapacitor device should be improved from two aspects of increasing the specific capacitance of the electrode material and expanding the working voltage of the device.This article mainly starts from optimizing the spatial structure of electrode materials to promote the diffusion of electrolyte ions and improve the conductivity of materials to obtain electrode materials with good electrochemical performance,so as to promote the energy density of the supercapacitors composed of them.In addition,the prepared electrode materials and commercial activated carbon(AC)are used to form Asymmetric supercapacitors(ASC)devices to expand the operating voltage range of the overall device and improve the energy density of supercapacitors based on the above two improvements.(1)Polypyrrole nanotubes(PNT)were synthesized by the template method,and then the diamond-shaped dodecahedral cobalt-based zeolite imidazole coordination polymer(ZIF-67)was combined with the one-dimensional tubular structure of PNT by chemical deposition to obtain the PNT@ZIF-67 precursor.Nitrogen-doped carbon nanotube-based cobalt metal composite material(N-CNT@Co/C)was obtained by high-temperature carbonization in N2 atmosphere.One-dimensional nitrogen-doped carbon nanotubes(N-CNT)connect three-dimensional Co/C nanoparticles in series to form a networked structure,showing good cycle stability and conductivity,and its specific capacitance reaches 215.3 F g-1 and the current density is 1 A g-1,when the current density reaches 10 A g-1,there is still 75.49%of the capacitance retained,which is significantly improved compared to N-CNT and Co/C,and has good rate performance.It shows that the structural design of MOF materials with special spatial structure can help to improve the electrochemical performance of electrode materials,which provides a more practical idea for the structural design of supercapacitor electrode materials.(2)Using PNT@ZIF-67 as a precursor,the nitrogen-doped carbon nanotube-based cobalt oxide material(N-CNT@Co3O4/C)was obtained after two high-temperature treatments.The needle-liked nickel hydroxide(Ni(OH)2)was grown on the surface by a simple hydrothermal method to obtain a three-dimensional networked N-CNT@Co3O4/C@Ni(OH)2 composite material which specific capacitance is as high as 1344.4 F g-1(current density is 1 A g-1).In addition,the N-CNT@Co3O4/C@Ni(OH)2//AC ASC device composed of this material and AC has an energy density of 57.7 Wh kg’1 at a power density of 850 W kg-1,which is expected to become the electrode material of the applied supercapacitor.(3)Using PNT@ZIF-67 as a sacrificial template,through a simple solvothermal reaction and etching,a PNT@NiCo-LDH(Layered Double Hydroxides,LDH)nanocage composite material with a three-dimensional network structure was prepared,and its mass specific capacitance is 1448.2 F g-1 at a current density of 1 A g-1.After calculation,its electrochemical performance is 31.87%higher than that of the NiCo-LDH nanocage without PNT,indicating that structural optimization of the electrode material can significantly improve its electrochemical performance.In addition,PNT@NiCo-LDH and NiCo-LDH and commercial AC were assembled into ASC devices(PNT@NiCo-LDH//AC and NiCo-LDH//AC),and the results showed that PNT@NiCo-LDH//AC showed better electrochemical performance,which also has great application value in the field of supercapacitors.