Synthesis And Supercapactive Behaviors Of Reduced Graphene Oxide Based Composite Materials

Supercapacitors, as a new power device has been paid a wide attentions because of its excellent performances including high power density, long life cycle and environmental safety. However, its lower energy-density, compared with the lithium ion battery, is a constraint in the large scale application. Therefore, now the main research is focusing on looking for electrode materials with high-performance. Graphene, as a new type of carbon materials, has widely used in supercapacitors owing to its huge specific surface area and high specific capacitance. However, the strong inter-sheet van der waals forces make graphene suffer from agglomeration or restacking. Therefore, the design and synthesis of functionalized graphene and graphene-based composites are deemed as an effective way, by which the application of graphene can be widened greatly.In this work, methyl green（MG） was attached on the graphene oxide through noncovalently functionalization. As a result, the reduced graphene oxide was modified. In addition, the graphene was used as supporting materials to hybridize with Bi₂S₃ for obtaining the Bi₂S₃/RGO composites. Moreover, the supercapactive behaviors of MnCo₂O₄ were also studied. The as-prepared samples were analyzed and investigated by conventional techniques such as（XRD, SEM and TEM） and electrochemical measurements. The main is as follows:1. The water-soluble methyl green（MG） was selected to decorate reduced graphene oxide（RGO） through non-covalent functionalization for enhancing supercapacitor performance. The fast redox reactions from MG could generate additional pseudocapacitance, which endows RGO higher capacitances. The result shows that the MG-RGO composite（with the 5:4 mass ratio of MG: RGO） gives a maximum value of 341 F g^-1 at 1 A g^-1 within the potential range from-0.25 to 0.75 V. Moreover, even at the discharge current density of 20 A g^-1, the specific capacitance of the MG-RGO composite electrode was still as high as 246 F g^-1, which indicate an excellent rate capability. Furthermore, the MG-RGO composite exhibits a long cycle life（12% capacitance decay after 5000 cycles）. In addition, the excellent electrochemical performance of the MG-RGO composite is owing to a strong positive synergistic effect between MG and RGO. At the same time, it also shows that non-covalent functionalization of graphene with small organic molecules is deemed as a favourable method for achieving the high-performance electrode materials.2. MnCo₂O₄ nanoflowers were successfully synthesized by a mild reflux method following thermal treatments. The morphology and microstructure of the as-obtained samples were investigated by field emission scanning electron microscopy（FE-SEM）, transmission electron microscopy（TEM）, X-ray diffraction（XRD）, and fourier transform infrared（FT-IR） spectroscopy. Meanwhile, their electrochemical performances were examined by cyclic voltammetry（CV）, galvanostatic charge/discharge techniques. The experimental results indicated that the optimal reflux condition is a key factor for obtaining product with high capacitance. The MnCo₂O₄ which was fabricated at the volume ratio of ethanol to water up to 1: 1 achieved the specific capacitance of 314 F g^-1. Moreover, the MnCo₂O₄ material exhibited an excellent cycling ability that after 2000 cycles at a current density of 2 A g^-1, the capacitance retentions are 90.4%.3. Bi₂S₃/RGO composites were prepared through a solvothermal route, and meanwhile the supercapactive behaviors of the composites were studied by means of cyclic voltammogram（CV） and galvanostatic charge-discharge. On the one hand, RGO sheets as an excellent substrate endows the composites materials with high electrical conductivity. On the other hand, the growth of Bi₂S₃ on the RGO can appropriately decrease the restacking of RGO sheets. The capacitance performance depends on the mass ratio of Bi₂S₃ and RGO. The optimal ratio is given by means of the parallel experiments. Meanwhile, the electrochemical performance of the single electrode was measured. The composite gives a maximum value of 400 F g^-1 at 1 A g^-1 within the potential range from-1 to 0 V.