Study On Preparation And Electrochemical Properties Of ZIF-67 And Its Composites

As a new and efficient energy storage device,supercapacitors have the advantages of high power density,long service life,high security and fast short time storage and release.As a result,supercapacitors have wide application prospects in portable consumer electronics,electric vehicles,and wind and solar energy.The performance of electrode materials is the main factor affecting the performance of supercapacitors.Currently,the most studied materials are carbon-based materials,transition metal oxide and conductive polymer materials.In recent years,metal-organic frameworks?MOFs?are developed as a new type of porous materials due to their large specific surface area,porosity and regulatory pore,function and special optical and electrical properties.Zeolite imidazole frameworks?ZIFs?are typical MOFs materials,imidazoles derivatives as ligand link and become a kind of porous material similar to that of the zeolite framework structure,besides the advantages of MOFs.And its stability is similar to the zeolite,so in the high performance of super capacitor has great development potential.In this paper,ZIF-67,ZIF-67@Mn-ZIF,Co₃O₄ and Co₃O₄@Mn O₂ were synthesized by hydrothermal method.Then X-ray diffraction?XRD?,scanning electron microscope?SEM?,transmission electron microscope?TEM?and X-ray photoelectron spectroscopy?XPS?were used to represent their structure characterization and morphology.Finally,to evaluate the electrochemical properties of Co₃O₄ and Co₃O₄@Mn O₂ electrodes,the cyclic voltammetry?CV?and galvanostatic charge-discharge measurements using a standard threeelectrode system were carried out in a 1M Li OH aqueous electrolyte.The main works of this paper are summarized as follows:?1?The ZIF-67 samples were prepared by the hydrothermal method,and the structure and morphology of the samples were characterized by XRD,SEM,TEM and XPS.The samples were found to be more homogeneous and could be prepared in large quantities.Co₃O₄ samples were gotten through sintering ZIF-67,which is characterized through XRD,SEM,TEM on the structure and morphology characterization.The result show that the samples are consistent with the standard card and relatively uniform,the surface of the Co₃O₄ samples is rough;?2?In order to evaluate the specific capacitance of Co₃O₄ samples,the cyclic voltammetry?CV?measurement were carried out in the scanning rate of 1,2,5,10,20,50,and 100m V/s respectively.The specific capacitances of different scanning rates are 156,154,146,130,103,65,and 46F/g respectively.When the current density is increased from1A/g to 10A/g,the capacitance is reduced from 187F/g to 45F/g,which is about 25%of the capacitance.After the 2000 cycle,the retention rate of the capacitor is around 80%,indicating that the electrochemical properties of the sample need to be improved.?3?Based on the above conclusions,the ZIF-67@Mn-ZIF and Co₃O₄@Mn O₂ were preparated in the same method.The structures of two samples were characterized through the XRD,which is showed that there is no obvious difference between the composite samples and the single samples,namely no obvious Mn element characteristic peak.XPS spectra of samples showed that there is Mn elements in composite sample;Through SEM and TEM,and the sample was found to be homogeneous and the morphology presented a polyhedral shape.However,the sintered samples may have collapsed due to uneven temperature in the sintering process.?4?In order to evaluate the specific capacitance of Co₃O₄@Mn O₂samples,the cyclic voltammetry?CV?measurement were carried out in the scanning rate of 1,2,5,10,20,50,and 100m V/s respectively under electrochemical workstation.The specific capacitance of those scanning rates are 337,334,314,282,239,164,109 F/g,respectively,which showed the better the performance compared with the single material;When the current density is increased from 0.5A/g to 10A/g,the capacitance is reduced from 413F/g to 168F/g.When the current density is increased by 20 times,the retention rate of the capacitor is still about40%.After the 2000 cycle,the retention rate of the capacitor was around 110%,indicating that the electrochemical performance of the sample was much higher than that of the single sample.