Preparation Of Graphene/Polypyrrole/Ferricyanide Films And Nitrogen-doped Graphene/Cobaltosic Oxide Composite As Electrode Materials For Supercapacitors

Supercapacitors have attracted much attention because of their long cycle life, simple principle, and high dynamic of charge propagation, which provide a promising option for the increasing demand of energy devices in the 21 st century. One factor that restricts the development of supercapacitors is the property of electrode materials. The electrode materials include carbon materials, conducting polymers and metal oxides, but there are some disadvantages for the single material. Graphene is one of the carbon materials, with two-dimensional structure, its large surface area and high conductivity, has become an attractive material for supercapacitors. Polypyrrole（Ppy） is a typical conductive polymer, owing to its better environmental friendliness and good capacitive performance. However, its poor cyclic stability and mechanical property limit the applications of Ppy. As a kind of metal oxide, the theory of specific capacitance value of cobaltosic oxide can be up to 3560 F/g, but it has some disadvantages such as large resistance value or short cycle life. These defects can be improved by loading Ppy on the graphene or loading cobaltosic oxide on the nitrogen-doped graphene which could be a promising composite material.（1）Electroactive composite films of graphene/polypyrrole/ferricyanide(RGO/Ppy/FCN^3-) were synthesized on platinum substrates modified with RGO using unipolar pulse electropolymerization method. The composite films were characterized by Fourier transform infrared spectroscopy（FT-IR）, scanning electron microscopy（SEM） and energy dispersive X-ray spectroscopy（EDS）. Galvanostatic charge/discharge, electrochemical impedance spectroscope（EIS） and cyclic voltammetry（CV） methods were used to study the electrochemical capacitive performance and the cycling stability of the RGO/Ppy/FCN^3- films. Results showed that the films were the composite with three-dimensional porous spheres. The specific capacitance of the films was 337.5 F/g at a current density of 2 A/g. Meanwhile, the capacity of RGO/Ppy/FCN^3- films stayed about 70 % after 2000 cycles. Therefore, the composite films exhibit outstanding capacitance performance and are excellent electrode materials for supercapacitors.（2）A simple and scalable method has been developed for the control over the morphologies of Co₃O₄, which directly grows on amine modified graphene（NH₂-Gs）, by changing the weight ratio between the ammonia and graphene oxide. It shows that the relative NH3 content could distinctly alter the elemental composition and the relative percentages of functional groups in NH₂-Gs, and the Co₃O₄ particles on the NH₂-Gs with a relatively higher amount of amine groups exhibit a relatively small size. Among the synthesized samples, Co₃O₄/NH₂-G（2）（the weight ratio of ammonia to GO is 5） exhibits the highest capacitive performance, 2108.4 F/g at 1A/g and 1356.7F/g at 15 A/g, which is much higher than that of Co₃O₄ based materials reported previously. The electrode also has a satisfactory cycling performance with capacity retention of 64 % after 800 cycles at 5 A/g. The enhanced electrochemical performances of Co₃O₄/NH₂-G（2） are ascribed to the higher specific surface area, the smaller Co₃O₄ particle sizes, the strong interaction between Co₃O₄ and NH₂-G（2）, and the higher conductivity. The excellent electrochemical performance makes the resultant composite materials to be a promising candidate for application in supercapacitors.（3）Various methods have been developed for the control over the morphologies of Co₃O₄, which directly grows on various N-modified graphene. It shows that the capacitive performance of various N-modified graphene is different. Among the synthesized samples, the weight ratio of ammonia N of the Co₃O₄/N-RGO 550 oC sample is higher than other samples. The weight of ammonia N is the key factor for the capacitive performance of the materials. Co₃O₄/N-RGO 550 oC exhibits the highest capacitive performance, 3850.9 F/g at 1 A/g and 1966.7 F/g at 15 A/g, which is much higher than the other materials. The electrode also has a satisfactory cycling performance with capacity retention of 91 % after 800 cycles at 5 A/g.The excellent electrochemical performance makes the resultant composite materials to be a promising candidate for application in supercapacitors.