Preparation And The Supercapacitor Properties Of ZIF-67 Derived Cobalt Composite Materials

Metal organic frame compounds(MOFs)are new porous materials which were made of metal ions or clusters connected with organic ligands through coordination bonds,hydrogen bonds,metal bonds or Van der Waals bonds.MOFs has many advantages,such as structural diversity,adjustable pore size,high specific surface area and rich active sites,which make them widely used in gas absorption and storage,catalysis,drug delivery and electronic information.In recent years,the application of MOFs in electrochemical energy storage fields such as lithium-ion batteries,fuel cells and supercapacitors has also been paid attention by researchers.However,because the intrinsic conductivity of MOF is limited,how to convert MOFs into MOFs derivatives with high conductivity and stable structure has become the key problem in the field of electrochemistry.It is found that the pyrolysis of MOFs at high temperature can be converted into metal/metal oxide and porous carbon composite.The composite not only retains the characteristics of high specific area and porosity of the MOFs,but also improves the conductivity of MOFs effectively,which lays a good foundation for the construction of electrode materials of high performance supercapacitor.In this paper,ZIF-67,a zeolite-like framework compound,was used as precursor.By adding melamine in the pyrolysis process,cobalt based/N-doped carbon nanotubes composite was designed and developed.The effects of raw material ratio,temperature,time and other process parameters were investigated,and the formation mechanism of N-doped carbon nanotubes was studied.The cobalt based/N-CNT composite was activated by KOH to further improve their electrochemical properties.By adding graphene oxide in the pyrolysis process,the cobalt based/N-CNT/r GO ternary composite system was constructed,and the structure and morphology of the composite were analyzed,and the mechanism of the property improvement was explored.The main contents of the study are as follows:(1)ZIF-67 precursor was synthesized from cobalt nitrate hexahydrate and 2-methylimidazole.The structure,morphology and pyrolysis behavior of ZIF-67 were characterized.Co/Co₃O₄/N-CNT composites were prepared by pyrolysis of ZIF-67 and melamine.The effects of pyrolysis temperature,raw material ratio and reaction time on the preparation were investigated.The structure,morphology and specific surface area of Co/Co₃O₄/N-CNT composites were characterized.The results show that Co/Co₃O₄/N-CNT composites are successfully prepared at 750?,the mass ratio of ZIF-67 to melamine is 1:5,and the holding time is 2 h.The results show that the specific capacitance of the composite is163.3 F/g when the current density is 0.5 A/g,and the retention rate of the specific capacitance is still 80.8%after 2500 cycles at the current density of 10A/g.(2)The Co/Co₃O₄/N-CNT composites were activated by KOH.The effects of different amount of KOH on the structure,morphology and specific surface area of Co/Co₃O₄/N-CNT composites were investigated.The results show that when the ratio of the composite and KOH is 1:2,the activation effect is the best,the specific surface area of the composite increases,and the electrochemical performance of the composite is also improved compared with the unactivated material.The specific capacitance is 266.1 F/g at the current density of 0.5 A/g,and the retention rate of the specific capacitance is still 84.5%after 2500 cycles at the current density of 10 A/g.(3)Co/Co₃O₄/N-CNT/r GO composites were prepared by adding GO into ZIF-67,mixing melamine to pyrolyze at high temperature.The effects of GO on the formation of Co/Co₃O₄/N-CNT/r GO composites were investigated.The structure,morphology and electrochemical properties of the composites were characterized.The results show that GO is reduced to r GO and Co₃O₄ nanoparticles are effectively dispersed in the pyrolysis process.The addition of GO does not change the structure of N-CNT,and the addition further improves the conductivity and specific surface area of Co/Co₃O₄/N-CNT.When the content of GO is 5%,the specific capacitance of the composite is 293.2 F/g,and the retention rate of the specific capacitance reaches 95.3%after 2500 cycles at the current density of 10A/g,showing excellent cycle stability.