Study On Assembly And Electrochemical Properties Of Transition Metal Oxides@3D Graphene Electrode Materials

Supercapacitors,also known as electrochemical supercapacitors,have the advantages of high power density and long cycle life.They are considered as the new generation of energy storage equipment and have shown excellent application prospects in many electronic devices and equipment.Compared with conventional two-dimensional(2D)graphene,3D graphene aerogel(GA)has the advantages such as the large surface area and excellent electrons,ionic conductivity and mechanical properties.And these advantages are helpful for its application in energy storage equipment.On one hand,graphene-based capacitor is mainly electric double layer capacitor and its electrochemical performance is limited due to that there is no redox reaction occurs.On the other hand,as a pseudo-capacitance material,the transition metal oxides could provide a high capacitance and energy density,but there are obvious shortcomings such as the poor cycle life.With the appropriate synthesis process and the control of the chemical compositions,the as-prepared transition metal oxide doped GA composites are expected to exhibit the advantages of GA and the oxides.Therefore,this kind of materials has become one of the research hotspots of energy storage equipment electrode materials.In this thesis,several kinds of transition metal oxides doped GA composites were prepared by using the directly hydrothermal synthesis or impregnation method combined with freeze-drying or calcination process.They were characterized by various techniques and their electrochemical performance as the supercapacitor electrode materials were investigated in details.The primary work was as follows:(1)Mn3O4-doped GA(Mn3O4@GA)composites were successfully prepared from manganese acetate and graphene oxide(GO)with polyethylene glycol 6000 and NaOH as surfacent and mineralizing agent by using an one-pot hydrothermal process combined with freeze-drying.The results showed that the Mn3O4 particles are uniformly dispersed into the 3D structure of GA.The specific capacity of the resulting composites was as high as 408.5 F g-1 at 0.5 A g-1.Meanwhile,the Mn3O4@GA composites electrode displayed excellent rate capability and cyclic stability.They retained a 96.81% of the initial specific capacitance after 2000 cycles at 1.0 A g-1.(2)Co3O4-doped GA(Co3O4@GA)composites were successfully prepared from cobalt nitrate and GO with urea as the active agent by a one-pot direct hydrothermal method combined with freeze-drying and calcination process.The results show that the resuling composites had a specific surface area of 463.2 m2 g-1.The spherical Co3O4 particles were uniformly dispersed into GA.The specific capacity of Co3O4@GA was as high as 673.4 F g-1 at 1 A g-1,which was much higher than that of Co3O4(296.1 F g-1).Meanwhile,the Co3O4@GA composites electrode displayed excellent rate capability and cyclic stability.The initial specific capacity only reduced 3.00% after 2000 cycles at 1.0 A g-1.(3)MnO2-doped GA(MnO2@GA)composites were successfully prepared from potassium permanganate and GO by a directly hydrothermal method in acid system combined with freeze-drying.The results show that the MnO2@GA composites had abundant pores and a large specific surface area(SBET = 196.6 m2 g-1).When the current density is 0.5 A g-1,the specific capacity of the MnO2@GA composites(379.7 F g-1)was significantly higher than that of the pure MnO2(101.3 F g-1).Furthermore,the composites showed better rate performance and cyclic stability.The specific capacity decreased about 3.87% after 2000 cycles at 0.5 A g-1.(4)GA was successfully prepared from GO by directly hydrothermal reduction combined with freeze-drying.Then,it was dispersed into the nickel nitrate ethanol solution.Finally,NiO nanoparticles were successfully doped into GA by the wet impregnation method combined calcination process to obtain the NiO@GA composites.The results showed that the composites had a larger specific surface area(SBET = 104.1 m2 g-1)than that of the pure NiO(SBET = 16.6 m2 g-1).The NiO@GA composites had a specific capacity of 893.1 F g-1 at 0.5 A g-1.Moreover,the composites exhibited excellent rate performance and cyclic stability.They retained a 97.5% of the initial specific capacitance after 2000 cycles at 0.5 A g-1,which was much higher than that of NiO(69.3%).(5)CuO@GA composites were prepared from copper nitrate by using a similar procedure to that of the aforementioned NiO@GA.The results showed that the rodlike CuO particles were uniformly dispersed into GA,and the resulting composites had a larger specific surface area(SBET = 100.3 m2 g-1)than that of the pure CuO(SBET = 0.6 m2 g-1).The specific capacity of CuO@GA was as high as 508.8 F g-1 at 0.5 A g-1 and this was much higher than that of pure CuO(180.0 F g-1).The composites also exhibited all-right rate performance and cyclic stability.They retained a 95.3% of their initial specific capacitance after 2000 cycles at 0.5 A g-1.