Reserch On Controllable Preparation And Supecapacitors Performance Of ZIF-67 Derived Hollow Nanomaterials

Metal organic frameworks?MOFs?are porous material assembled by organic ligands and metal centers.Due to its high specific surface area,high dispersion of metal centers and adjustable porosity,it is often used as a template or precursor for preparing electrode materials.At present,there are two main treatments using MOFs as the precursor to prepare nanomaterial:one is calcination.That is the strategy of preparing porous nanoparticles and in situ carbon-doped nanomaterials by annealing MOFs under different atmosphere conditions to destroy the organic ligands used for bridging action.The second is etching.That is a processing strategy that the organic ligand anions is replaced by inorganic anions with relatively strong coordination ability,and finally realizes phase transition.Compared with calcination,etching treatment conditions are more mild and controllable,and the obtained nanomaterials usually have higher specific surface area and more reactive sites,so they show more excellent electrochemical properties.Among many MOFs materials,based on the advantages of uniform morphology of zeolite imidazoles metal organic framework?ZIF-67?and simple synthesis method,it is the most commonly precursor for preparing electrode materials.However,due to the chemical stability is poor of ZIF-67,easily damaged in the process of etching,therefore,there are few reports on the etching of ZIF-67 materials.Here,we successfully prepared a series of ZIF-67 derived high-performance supercapacitor electrode materials via using the strategy of coating ZIF-67 surface with bimetallic hydroxide protective shell and then etching.Mainly research contents are as follows:1.Synthesis of ZIF-67-derived Co_xNi_y?PO₄?_z and research on supercapacitor performance:Bimetal phosphates,especially those with hollow structures,have been recognized as excellent supercapacitor electode materials.However,research on hollow bimetal phosphates is rare relatively because of the difficulty in controlling the preparation process.In this work,the first non-spherical hollow bimetal?Co/Ni?phosphate nanocage material?ZIF-67-LDH-CNP-110?was successfully prepared by means of shell coating and controlled etching.The specific capacitance of ZIF-67-LDH-CNP-110 is 1616 F g^-1 at 1 A g^-1,and 80.32%of the initial value when the high current density is 10 A g^-1,showing excellent rate performance.In addition,the ZIF-67-LDH-CNP-110//AC hybrid supercapacitor presents a energy density of 33.29 Wh kg^-1 at a power density of 0.15 kW kg^-1.2.Synthesis of ZIF-67-derived?-Co/Ni?OH?₂@Co₃O₄-70 and research on supercapacitor performance:It has been reported that composite transition metal oxides can significantly improve the comprehensive supercapacitors performance of transition metal hydroxide.In this work,a series of Co₃O₄ embedded?-Co/Ni?OH?₂hollow nanocages are successfully constructed by controllable NaH₂PO₂ etching and in-situ O₂ oxidation process using ZIF-67 as template.Composite nanocages can combine the advantages of two components:?-Co/Ni?OH?₂ components provide rich electrolyte diffusion channels and sufficient reactivity sites,and Co₃O₄ components provide well conductivity and excellent cyclic stability.Therefore,The optimized?-Co/Ni?OH?₂@Co₃O₄-70 shows ultra-high specific capacity of 1000 F g^-1(1 A g^-1),excellent rate performance(maintaining 74%of the initial specific capacitance,1 A g^-1to 10 A g^-1)and significantly enhanced cyclic stability?maintaining 72.34%of the 8000charge-discharge cycles?.In addition,the assembled?-Co/Ni?OH?₂@Co₃O₄-70//AC hybrid supercapacitor exhibits a high energy density of 23.88 Wh kg^-1 at a power density of 0.075 kW kg^-1.3.Synthesis of ZIF-67-derived?-Co/Ni?OH?2@CQDs and research on supercapacitor performance:On the basis of the above two research systems,in order to further enhance the conductivity and cyclic stability of the material,we introduce highly conductive carbon quantum dots into the reaction system.Firstly,a series of carbon quantum dots?CQDs?embedded ZIF-67 precursors are synthesized?ZIF-67@CQDs-X,X=1.25,2.50,5.00,7.50?.Then,the CQDs doped?-Co/Ni?OH?₂ hollow nanocages are successfully constructed by a facile and controllable chemical etching process??-Co/Ni?OH?₂@CQDs-X,X=1.25,2.50,5.00,7.50?.The optimized?-Co/Ni?OH?₂@CQDs-2.50 electrode delivers a high specific surface area(277.99 m₂ g^-1).Therefore,?-Co/Ni?OH?₂@CQDs-2.50 electrode presents a high specific capacitance(1400 F g^-1,1 A g^-1).Meantime,due to the high conductivity of CQDs in situ composite,?-Co/Ni?OH?₂@CQDs-2.50 exhibits superior rate performance(1120 F g^-1,10 A g^-1)and excellent cycling lifespan?retaining 79.93%of initial capacitance after 10000 cycles?.In addition,the prepared Co/Ni?OH?₂@CQDs-2.50//PPD/rGO device exhibits an energy density of 57.29 W h kg^-1 at the power density of 0.375 kW kg^-1.4.Synthesis of ZIF-67-derived cobalt nickel sulfide and research on supercapacitor performance:On the basis of the above system,we replaced the coated core shell with SiO₂,and obtained cobalt-nickel silicate with hollow structure by means of depositional and dissolution equilibrium principle,and then obtained cobalt-nickel sulfide with hollow morphology by hydrothermal vulcanization process.The hollow morphology not only provides more active sites for the electrochemical reaction,but also facilitates the migration of electrolyte ions and promotes the electrochemical reaction kinetics.At the current density is 1 A g^-1,its specific capacity is as high as 734 F g^-1,and when the current density is expanded by 10 times,its specific capacity remains 70.03%,indicating that the hollow cobalt-nickel sulfide has a good ratio performance.Based on this preliminary exploration,cobalt-nickel sulfide with hollow structure will be a promising supercapacitor material.