High Electrochemical-performance Graphene’s Preparation, Characterization And Its Application In Supercapacitors

Graphite oxide with C:O atomic ratio of2:1has been prepared by improving the Hummers-Offeman method to exfoliate graphene oxide completely assisted by sonication with improvedoxidant dosage and reaction time. Specific capacitance (SC) of the graphene reached186F/g (10mV/s, with surfactant) and231F/g (10mV/s, without surfactant), respectively. A higher SC of270F/g was obtained by optimizing sonication-induced exfoliation that produced more defects ingraphene.Defects induced by sonication and graphene dimensions pre-determined by graphiteprecursors played important roles in the electrochemical performance of the graphene-basedsupercapacitors. Smaller and more defective graphene produced greater SC but it had poorconductivity and limited diffusion of electrolyte. By producing composites with graphene withdifferent contents of defects and/or different dimensions, the SC of graphene in supercapacitorswas made even greater with values reached316F/g and293F/g, respectively, at the scan rate of10mV/s. Therefore, composites of graphene with different characteristics has become a brand newand effective method to improve the electrochemical performance of graphene and the SC ofgraphene-based supercapacitors.An experimental method to overcome agglomeration of graphene has also been developed.Graphene oxide powders dried by a lyophilizer were reduced by hydrazine hydrate gas in ahydrothermal reactor. This method is effective because agglomeration does not occur whengraphene oxide with a large number of functional groups is dried by lyophilizer. Therefore, the SCof graphene electrode rose to323F/g (10mV/s) from270F/g for graphene that was prepared fromdispersion of reduced graphene oxide. Despite the great progress, scanning electron microscopyimages revealed that graphene sheets were still often stacked which should have made the diffusionof electrolyte in certain directions easier than in others. In order to enhance the diffusion ofelectrolyte we reduced further the graphene oxide that had been partly reduced in dispersion anddried by a lyophilizer. As result, the stacking is greatly reduced owing to the removal of functionalgroups in the partially reduced graphene powders and the SC of graphene reached highest value of338F/g at10mV/s.