Study On Preparation And Electrochemical Performance Of Graphene/MnO₂ For Supercapacitors

In this paper,graphene/MnO₂ composites were prepared by hydrothermal method and chemical co-precipitation method:on the basis of the reduction of the graphene oxide raw material by hydrothermal method to obtain graphene,graphene/MnO₂ and CNT/graphene/MnO₂ was obtained by chemical co-precipitation.The physical and chemical properties of graphene and its composites were characterized by FTIR,Raman,XRD,FESEM and N₂ adsorption etc.The properties were characterized by methods of electrochemical tests such as cyclic voltammetry,galvanostatic charge-discharge and electrochemical impedance spectra.The effects of hydrothermal temperature,graphene contents and carbon nanotube contents on the properties of the composites were studied.The results showed that:After hydrothermal reduction,the surface functional group content gradually decreased with the gradual increase of hydrothermal temperature,the degree of reduction for graphene gradually increased,and the layer distance decreased from 0.92nm to 0.3568 nm.In the composite materials,MnO₂ is all?-MnO₂ of birnessite.The morphology changes with the content of graphene.When the mass ratio of graphene and MnO₂ is 1:0.5,MnO₂ is almost invisible.When the mass ratio of graphene and MnO₂ decreases to 1:1,the morphology of MnO₂ is nanowire.With the mass ratio of graphene to MnO₂ further decreases to 1:2,the morphology of MnO₂ starts to aggregate and form a few particles.When the mass ratio of graphene and MnO₂ is 1:3,the morphology of MnO₂ begins to agglomerate and stack,and some of them accumulate to form spherical MnO₂.When the mass ratio of graphene and MnO₂decreases to 1:4,the MnO₂ has a serious agglomeration phenomenon,and it piles up to form large globular particles.Carbon nanotubes interspersed in spherical MnO₂ and connect with graphene to provide more efficient electrical channels.The specific capacitance of Graphene/MnO₂ composite electrodes are increase firstly and then decrease in 0.25 A/g,0.5 A/g and 5 mV/s test conditions with hydrothermal temperature rise,and the specific capacitance of graphene/MnO₂electrodes prepared at different hydrothermal temperature under the current density of0.25 A/g respectively were 125 F/g,164.06 F/g,184.69 F/g,143.13 F/g.By contrast,when the hydrothermal temperature is 160?,its capacitance is the largest,and the charge transfer resistance is minimum.As the graphene content of graphene/MnO₂decreased,the electrode specific capacitance increased firstly and then decreased,the value was reached maximum when the mass ratio of graphene and MnO₂ was 1:2.The specific capacitance of composite electrodes with different graphene content at scanning rate of 5 mV/s respectively were 103.34 F/g,109.16 F/g,127.95 F/g and123.45 F/g.As graphene content decreased,the charge transfer resistance of the composite electrodes gradually increased.As the CNT contents of CNT/graphene/MnO₂ increased,the BET specific surface area of the composites increased firstly and then decreased.When the CNT contents of composites was 10wt%,the pores are mainly mesoporous with the highest porosity and the specific surface area as high as 125.02 m²/g.As the contents of CNT increases,the specific capacitance of the composite electrodes at the current density of 0.5 A/g,1.0A/g and the scanning rate of 5 mV/s is increased firstly and then decreased.The specific capacitance of the composite electrodes at scanning rate of 5 mV/s respectively were 127.95 F/g,147.45 F/g,137.70 F/g and 127.70 F/g,and the specific capacitance retention reaches up to 86.77%at 20%.