Modification And Electrochemical Properties Of Graphene Electrode Materials

The graphite oxide?GO?,obtained by modified Hummers method using graphite powders as raw materials,was reduced to prepare the graphene?RGO?through the chemical method and hydrothermal method,respectively.According to X-ray diffraction?XRD?and field emission scanning electron microscopy?FESEM?analysis,the degree of reduction of the graphene prepared by chemical process?H-RGO?was superior to that of the graphene obtained by hydrothermal process?T-RGO?.Compared to the T-RGO sample,the H-RGO sample displayed the morphology of more disorder,fluffy,transparency and wrinkles.From nitrogen sorption isotherms and four-point probe,the surface area and conductivity of the H-RGO sample exceeded that of the T-RGO sample.Electrochemical properties of the as-prepared samples were evaluated by cyclic voltammetry?CV?and galvanostatic cyclic?GC?measurements and electrochemical impedance spectroscopy?EIS?.The results showed that the H-RGO electrod exhibited the excellent rate performance and cycling stability,and an obvious increase in specific capacitance that is almost 55%higher than the T-RGO electrode at the current density of 0.5 A?g^-1.The residual capacitances of the H-RGO and T-RGO electrodes were 47%and 12%while the current density was up to 10A?g^-1,respectively.The N-doped graphene materials?NGs?were prepared by hydrothermal process via microwave-assisted using urea as a dopant.The graphene structures of the NG samples were determined by XRD.It was found that the oxygen functional group on the samples surface did not be reduced absolutely.According to FESEM,the NG samples displayed the similar morphologies of 3D structure with transparency,layer and wrinkles.From XPS,the N element was incorporated into the RGO structure by the form of pyrrole N and graphite N.The conductivity and electrochemical performance of the NG samples were determined by four-point probe,CV and GC measurements and EIS.The results showed that the conductivities of the NG samples increased first and then decreased with the increasing N dopant levels.It was found that the conductivity and specific capacitance reached the maximum while the N content was 11.07 wt%,which the maximum values were 826.4S?m^-1and 293.3 F?g^-1(at current density of 0.1 A?g^-1),respectively.And the sample also revealed the higher surface area(303.2 m²?g^-1)than other samples.Through changing the reaction time and temperature,it can be seen that the specific capacitance decreased with the longer time and lower temperature,which attributed to the restack between the graphene sheets and the reduced surface area.Furthermore,the rate performance and cycle stability of the NG samples were improved obviously.The proposed mechanism for improved conductivity and capacitive performance due to the nitrogen functional groups on RGO was discussed.The N and S co-doped graphene materials?NSGs?were prepared by hydrothermal process using thiourea as a dopant.The graphene structures of the NSG samples were determined by XRD.It was found that the oxygen functional group on the samples surface did not be reduced absolutely.According to FESEM,the NSG samples displayed the similar morphologies with a certain transparency,layer and wrinkles.From XPS and four-point probe,N element was incorporated into the RGO structure by the form of pyridine N and oxide N.However,S element was introduced into the RGO matrix by the form of thiophene S and sulphone.The conductivities of the NSG samples were enhanced dramaticlly due to the N and S co-doped.According to the Raman and nitrogen sorption isotherms analysis,the degrees of graphitization of the NSG samples were gradually decreased and the defect in the samples was enlarged with the introduced N and S functional groups.Compared to the RGO sample,the surface areas of NSG samples were decreased obviously.The electrochemical performance of the NSG samples were determined by CV and GC measurements and EIS.The results showed that the specific capacitance of the RGO sample was superior to that of NSG samples at low current density.But,the capacitive performance of the NSG samples exceeded that of the RGO sample with the increasing current density to 1 A?g^-1.The specific capacitance of the NSG samples decreased with the increasing N and S co-dopant levels.Through changing the reaction time and temperature,it can be seen that the longer time and lower temperature were beneficial to improve the specific capacitance of the NSG samples.Furthermore,the rate performance and cycle stability of the NSG samples were improved obviously.The proposed mechanism for improved conductivity and capacitive performance due to the nitrogen and sulfur functional groups on RGO was discussed.