Preparation And Electrochemical Performance Of Graphene-RuO₂Composites Used In Supercapacitors

Graphene sheet is a single layer of carbon atoms arranged in a honeycomb oflattice structure, and has unique electrical, thermal, mechanical and opticalperformance. Graphene, with developed specific surface area (2630m2/g), is not onlyan ideal electrode materials for electric double layer capacitors, but also an perfectcandidate used as carrier. The ruthenium oxide could be highly dispersed on thesurface of graphene, and the prepared RuO2/GS electrode composites have caughtmore and more attention. Besides, two-dimensional layer structure makes thegraphene very suitable for extending pores. Considering that the activated graphenehas extensively pore structure and more ion channels, so its capacitance is higher thanunactivated graphene.In this paper, an improved Hummers method was applied to prepare the highlyoxidized graphite. Then an instantly high temperature expansion process was used toremove most of the oxygen containing groups on the surface of graphite oxide, andthe optimal expansion temperature was800oC which was determined by a series ofparallel experiments. The as-prepared graphene sheet is about4~6layers detected byatomic force microscope, and presents the specific capacitance of168.9F/g with acurrent density of1A/g.However, graphene-based supercapacitors suffer from graphene aggregationbecause of van der Waals interactions between neighboring sheets, which can reduceeffective specific surface area. So RuO2particles acted as holders betweenneighboring graphene sheets are badly in need of keeping neighboring sheetsseparated for better charge-discharge performance. In this paper, the RuO2particleswere loaded on the surface of the paper-like graphene sheets by hydrothermal reaction.The influence of the annealing temperatures and ruthenium content on theperformance of ruthenium oxide/graphene composites was analyzed. XRD, Romanspectra, cyclic voltammetry and galvanostatic charge-discharge were performed toresearch the structure and capacitor performance of graphene and RuO2/GScomposites. It is concluded that the composites show the highest initial specificcapacitance and best cyclic performance when the annealing temperature is150oC.There is an obvious increase of the specific capacitance of the RuO2/GS with the increase of Ru content when the mass ratio is below40%, and the specific capacitanceof RuO2/GS (Ru:40wt%) composites achieved551F/g at1A/g. At last, ZnCl2wasused as activation reagent for graphene activation, and the appropriate dosage ofZnCl2was detected. TEM, cyclic voltammetry and galvanostatic charge-dischargewere performed to research the structure and capacitor performance of activatedgraphene. The activated graphene has the highest specific capacity of187.7F/g whenthe mass ratio between graphene and ZnCl2is1:6. Moreover, the capacitance retentionof the activated graphene remain95%after2000cycles at1A/g, while unactivatedgraphene is only87%with the same treatment.