Preparation And Electrochemical Properties Of Graphene Supported CoR_xFe_2-xO₄（R=Ce,Nb,La） Composites

In the face of the huge energy crisis and the requirements of green chemistry,the development of cheap supercapacitor materials is one of the hot targets in the development of clean energy.In this paper,the goal is to prepare electrode materials for high-capacity iron-based supercapacitors.On the basis of CoFe₂O₄ research on the preparation of monodisperse different dispersants polyvinylpyrrolidone（PVP）by solvothermal method,rare earth doping and graphene loading are used in two steps The graphene-loaded CoR_xFe_2-xO₄composite material was prepared,and the morphology,crystal structure,interaction between the components and electrochemical performance of CoFe₂O₄ in the material were studied.The specific research contents and results are as follows:Ethylene glycol with less polarity is used as solvent,ammonium acetate（NH₄Ac）is used as slow-release precipitant,and PVP is added as dispersant to obtain different dispersant CoFe₂O₄ nanoparticles with smaller particle size.The influence of CoFe₂O₄ morphology and crystal structure on its electrochemical performance was studied by SEM,XRD,FTIR and electrochemical workstation.PVP further reduces the polarity of the solvent.The carbonized PVP is dispersed on the surface of CoFe₂O₄.CoFe₂O₄ changes from bulk to spherical.The grain size decreases from 15.55 nm to 3.58 nm.The specific surface area increases from 26.10m²/g to 70.98 m²/g.The larger specific surface is conducive to the combination of the electrode and the electrolyte,which increases its capacitance from 262.5F/g to 355F/g.La³⁺,Ce³⁺,Nd³⁺were selected to make modified CoFe₂O₄ by partially replacing Fe³⁺in the octahedral lattice.The effect of rare earth elements and their contents on the structure and electrochemical properties of the material was characterized by XRD and FTIR.The results showthat a homogeneous compound that Ce³⁺and La³⁺enter into the Fe-O octahedral lattice and partially replace Fe³⁺formed.As the ion radius of Ce³⁺and La³⁺are much larger than Fe³⁺,a weaker Fe-O bond aobtained,and the binding force of Ce-Fe is stronger than that of La-Fe;however Nb³⁺cannot enter into the octahedral lattice,the crystal structure of CoFe₂O₄ does not change whith Nb droping,which formed a heterogeneous compound.The capacitance of CoCe_0.08Fe_1.92O₄ was 525 F/g,which was larger than another electrode materials.Increasing the content of Ce leads alower capacitance.Graphene（RGO）was prepared by the modified Hummer,and then was employed as the carrier to prepare the CoR_xFe_2-xO₄/RGO composite.The effects of graphene content on the structure and electrochemical properties of the materials were characterized by XRD,FTIR and Raman analysis.The results show that under high temperature and high pressure,graphene was folded,and ferrite nanoparticles grow along graphene forming a fluffy porous structure.Three dimensional electrolyte transport channel and two dimensional electron transport network improve the conductivity of materials.The presence of graphene can further improve the capacitance of the material,the current response value in the CV increased,and gradually transits to rectangle;the discharge capacity of GCPL increases,when the mass content of graphene is 20%,20%,25%,the maximum capacitance of the CoCe_0.08Fe_1.92O₄/20%RGO、CoLa_0.16Fe_1.84O₄/20%RGO and CoNv_0.16Fe_1.84O₄/25%RGO were704.5 F/g,687.5 F/g,607.5 F/g,respectively.The excellent combination of Ce-Fe and the excellent conductivity of graphene make CoCe_0.08Fe_1.92O₄/20%RGO the maximum capacitance.