Study Of Graphene Preparation And Application For Supercapacitor Electrodes

Due to its unique two-dimensional structures, graphene shows optical, mechanical, thermal, electrical and other properties, quickly being one of the hotspots of today’s scientific research fronts. Since graphene was first prepared by mechanical exfoliation in 2004, dozens of preparation methods occurs. But until now, none of the methods can efficiently prepare large-scale high-quality grphene with low cost, Each graphene preparation method has its own advantages and disadvantages, and graphene oxide reduction is considered as the one of the most promising approaches likely to achieve low cost, efficient, large-scale preparation of high quality. Now, the main problems of oxidation-reduction method are how to reduce the agglomeration of graphene after reduction and reduce the damage to graphene structure and improve the reduction effect. In this paper, we studied the preparation method of graphene by adjusting the surface potential of the graphene oxide solution, without any surfactant. Three different thermal expansion methods were used to prepare graphene and electrochemical properties of as-prepared graphene used as supercapacitor electrodes are studied. The main contents have the following three parts:(1) The influence of the oxidant amount and the oxidation temperature and time on the preparation effect of graphite oxide was studied. The optimal conditions for the preparation of graphite oxide is that the ratio of oxidant to graphite 3:1 and oxidation time 2 hours. Different amounts of hydrazine hydrate were added to the graphene oxide aqueous solution to obtain different pH values, and the graphene oxide aqueous solution were reduced by ultrasonic or magnetic stirring assisted thermal reduction. When the pH value of the graphene oxide is 11, good reduction effect is obtained. Stable well-dispersed graphene aqueous solution was prepared by hydrothermal reduction in autoclave. Scanning electron micro scopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD), optical microscopy(OM), Raman, atomic force microcopy(AFM), and Fourier transform infrared spectroscopy(FT-IR) were employed to characterize the prepared samples. The results showed tulle-like layered structure with folds on the surface, and the number of sample layers was less than five.(2) The graphene were prepared by low-temperature thermal expansion, microwave thermal expansion and arc thermal expansion graphite oxide respectively, and the samples were treated by ultrasonication. SEM, TEM, XRD, AFM and FT-IR analysis showed that the ultrasonic treatment is good for dispersion. The tulle-like layered graphene with folds on the surface, and layers less than five were obtained. FT-IR test showed that a few functional groups exist on its surface.(3)The graphene prepared by different thermal expansion methods were used as supercapacitor electrodes, and cyclic voltammetry measurement, galvanostatic charge-discharge measurement were employed to study its electrochemical performance. At a charge-discharge current density of 50mA/g, the specific capacitance of the graphene prepared by oxidation-reduction method with 1 mol/LNaOH was up to 155F/g. The specific capacitance of the graphene electrode prepared by low temperature thermal expansion was 181F/g at a charge-discharge current density of 100mA/g using 5mol/L NaOH electrolyte. The specific capacitance of the graphene electrodes prepared by 300℃ microwave thermal expansion was up to 265F/g at a charge/discharge current density of 50mA/g using 5mol/L NaOH electrolyte. In contrast, in 5mol/L NaOH electrolyte, at a charge-discharge current density of 50mA/g, the specific capacitance of the graphene electrode prepared by arc thermal expansion was up to 281F/g. All of the graphene prepared by oxidation-reduction method or different thermal expansion methods used as supercapacitor electrodes, had good charge-discharge efficiency and cycle life. The charge-discharge efficiency of graphene supercapacitor was gradually up to 99.5% after 100 cycle numbers, and remained unchanged in the later. After 1000 times constant current charge-discharge cycles, the specific capacitance of graphene remained almost unchanged.