Structure Design,Controllable Preparation Of Zeolitic Imidazolate Frameworks,and Supercapacitor Properties Of Its Derivatives

Due to the high stability,high specific surface area and high porosity,zeolitic imidazolate frameworks?ZIFs?are believed to be ideal precursors for porous double hydroxides,porous metal oxides and porous carbon materials,which are widely applied in energy field.Therefore,this work is devoted to the novel structure design and controllable preparation of ZIFs,which are also used as precursors for the novel porous metallic layered double hydroxides and nitrogen-doped hierarchically porous carbon.At the same time,the supercapacitor properties of these derivatives as robust electrode materials is studied.All results are shown as follows.1)Novel Co-containing zeolitic imidazolate framework with a leaflike morphology?ZIF-L-Co?was synthesized in aqueous media and proved to be the transient phase in the formation of ZIF-67.The phase and morphology of the products could be tailored by changing the concentration and molar ratio of the reagents.This study provides a new insight into the crystal growth of ZIFs.2)A facile route for synthesizing porous nickel-cobalt layered double hydroxide nanoflake array?Ni-Co LDH-NFA?on nickel foam is provided.The ZIF-L?Co?nanoflake array?ZIF-NFA?is first grown on nickel foam,which serves as a sacrificial template to synthesize Ni-Co LDH-NFA when it reacts with nickel nitrate at room temperature.The as-prepared Ni-Co LDH-NFA could be directly used as battery-type electrodes without polymer binder and exhibits outstanding specific capacity of 894 C g^-1at a current density of 2 A g^-1.In addition,an assembled asymmetric supercapacitor device also exhibits excellent specific energy density of 48.6 Wh kg^-1at a specific power density of 1700 W kg^-1.Even at a high power density of 17 kW kg^-1,the device could still remain an energy density of 18.5 Wh kg^-1.3)Uniform multi-layered core-shell crystals such as ZIF-67@ZIF-8@ZIF-67 and ZIF-8@ZIF-67@ZIF-8 were simply prepared for the first time by adopting the condition for the preparation of relatively larger sized crystals to fabricate the shells on the surface of relatively smaller sized seeds most likely via epitaxial growth without any auxiliary condition.Meanwhile,the size of core and the thickness of shell were also easily tailored by using different sized seeds and varying the molar feeding ratio of Zn²⁺/Co²⁺respectively during synthesis.4)This work proposes a multi-layered zeolitic imidazolate frameworks engaged self-templated synthesis of nitrogen-doped hollow porous carbon dodecahedrons with various architectures of single-shell?N-SS-HPCDs?,double-shell?N-DS-HPCDs?,yolk-shell?N-YS-HPCDs?,and yolk-double-shell?N-YDS-HPCDs?.Owing to the high surface area,high nitrogen doping amount,graphitic structure,hierarchically micro/mesoporous structure,and the unique yolk-double-shell hollow structure,the prepared N-YDS-HPCDs electrode exhibits a distinguished capacitance of 346 F g^-1,excellent stability?about 93%capacitance retention after 10 000 cycles?,and a high energy density of 11.64 Wh kg^-1at a power density of 250 W kg^-1.5)Heterogeneous core-shell ZIF-L?Zn?@ZIF-67 nanoleaves are controllably synthesized in aqueous media at room temperature in the light of our previously reported strategy.After direct carbonization under nitrogen atmosphere,hierarchically porous amorphous carbon@graphite carbon core-shell nanoleaves with nitrogen-doping and carbon nanotubes?N-C@GC/CNTs?are obtained.The morphology,microstructure and evolution process of N-C@GC/CNTs are investigated by scanning electron microscopy,transmission electron microscopy,adsorption/desorption isotherm etc.systematically.And optimized N-C@GC/CNTs is employed to construct supercapacitors.The results indicate that N-C@GC/CNTs exhibits a distinguished capacitance(252.1 F g^-1)and excellent stability?about 91.2%capacitance retention after 10000 cycles?as supercapacitor electrode due to hierarchical pore structure,high nitrogen content and superior electrical conductivity resulted from graphitic structure and carbon nanotubes.