Preparation And Characterization Of Two-dimensional Sheet-like Porous Carbon Based On The Self-assembly Of Metal Organic Frameworks (MOFs)

Metal?oxide?and carbon materials show pseudocapacitor and electrochemical double layer capacitor respectively.They have been widely used as electrode materials for supercapacitors.Metal-organic frameworks?MOFs?are novel polymers produced by the introduction of functional metal centers into the polymer backbone or branches through chemical bonds.The porous structure based on metal-organic coordination compounds can be designed,regulated,and modified.At the same time,the metal source and the carbon source in the framework have a porous network structure intertwined with each other.The MOFs can be used as a self-sacrificing template.Porous metal?oxide?and porous carbon and their composite structures can be obtained by pyrolysis of MOFs.In this study,Co-based MOFs?Co-BDC?nanosheet precursors were prepared by solution diffusion method,and metal?oxide?-carbon composites Co@Carbon and Co₃O₄@Carbon were obtained by high-temperature pyrolysis of Co-BDC.The Co-BDC precursor was mixed with a polymer material and electrospun to obtain a cobalt-carbon nanofiber.The XRD,TEM,SEM,EDS,XPS,N₂ adsorption,TG,Raman spectroscopy and other methods were used to characterize the structure of the composites.In addition,the electrochemical properties of the composites were characterized.Symmetrical and asymmetrical supercapacitors were assembled using composite electrode materials,and their electrochemical energy storage properties were further studied.Specific work as follows:?1?Preparation and electrochemical performance of Co-MOFs-derived metal?oxide?-carbon composite electrode1.Preparation and characterization of Co-MOFs precursorsCobalt 1,4-benzenedicarboxylate?Co-BDC?was synthesized by using Co²⁺as metal cation and 1,4-benzenedicarboxylate?H₂BDC?as organic ligand and reacted statically at 35°C for 24 hours.The pink powdery solid was obtained by centrifugation,washed and dried.Microstructure tests show that Co-BDC crystals grow well under mild reaction conditions and have a regular two-dimensional?2D?sheet morphology.2.Preparation and characterization of Co@Carbon and Co₃O₄@Carbon composite electrodesBy controlling the reaction temperature and the flow rate of the atmosphere,Co-MOFs precursors were pyrolyzed in a nitrogen atmosphere to obtain Co@Carbon composites.The precursors of Co-MOFs were pyrolyzed in oxygen to obtain Co₃O₄@Carbon composites.Microstructure tests showed that the two types of MOFs have two-dimensional lamellar morphology,which is a composite structure of Co and Co₃O₄ particles dispersed in a thin carbon layer,while inheriting the porosity of the Co-BDC precursor with high specific surface area.3.Electrochemicalcharacterization ofCo@CarbonandCo₃O₄@Carbon composite electrodeElectrochemical tests such as cyclic voltammetry,galvanostatic charge and discharge,and electrochemical impedance spectroscopy were performed on the composite electrode.Due to the special composite structure of the electrode,the electron and ion transmission efficiency is improved.The electrode shows the better electrochemical performance in the aqueous electrolyte.The specific capacitances of Co@Carbon are estimated to be 109,100,90,81,73,and 52 F/g,respectively,at current densities of 0.25,0.5,1,2,3,and 7 A/g;The specific capacitance of Co₃O₄@Carbon is estimated to be 261,171,148,128 and 50 F/g,respectively,at current densities of 1,2,3,4 and 10 A/g.4.AsymmetricsupercapacitorassemblybasedonCo@Carbonand Co₃O₄@Carbon.Asymmetric supercapacitors?ASCs?were fabricated by utilizing the as-synthesized Co₃O₄@Carbon as the positive electrode and Co@Carbon as the negative electrode.The working voltage of the capacitor can be extended to 1.5 V in aqueous electrolyte.The ASCs achieve a high energy density of 8.8 Wh/kg?at a power density of 375 W/kg?and high cycling stability(¹00%after 1000 cycles at 10A g^-1).?2?Electrospinning of Co-MOFs precursor to prepare Carbon-Cobalt nanofibers and electrochemical propertiesA polyacrylonitrile hybrid emulsion loaded with different proportions of Co-BDC was prepared.The mixed emulsion was electrospun under high pressure to synthesize carbon-cobalt nanofibers.The microstructure and electrochemical properties of carbon-cobalt nanofibers were studied in depth.Studies have shown that as the Co loading increases,the specific capacitance of the nanofiber electrode gradually increases.