Syntheses Of Metal Oxide (Hydroxide)/Graphene Oxide Composites And Their Applications In Electric Energy Storage

With the rapid development of the world economy,the demand of energy resourses is increasing greatly.Non-renewable fossil fuels,such as coal,petroleum are causing more and more serious environmental pollution during the process of production and use.So to find appropriate alternative energy has become an investigation topic in recent years.Due to high storage capacity,good stability and environmental friendliness,supercapacitor is considered to be an ideal energy storage device.Among the common electrode materials for supercapacitors,transition metal oxides or hydroxides have attracted a great deal of attention because of their excellent pseudocapacitance characteristic as well as large reserve and low cost.But poor conductivity and low cyclic stability limits their practical application.Through doping highly conductive and stable carbon based materials such as carbon nanotubes and graphene,above defects are expected to be overcome.In addition,the preparation process of the electrode is also an important influence factor for the electrochemical performance of the resulting electrfode,including the conductivity,stability of the electrode and the utilization of the active components.In this thesis,several kinds of novel composites were prepared based on nickel,cobalt and graphene oxide and their structures,morphologies and the application performances were studied in detail.1.?-Nickel hydroxide/graphene oxide composite material was prepared by a solvothermal method.It was found that the synthesized material showed a loose lamellar structure.As an electrode material for supercapacitor,it showed excellent electrochemical properties.In 6 M KOH solution,the material possessed the highest specific capacitance of 1298.6 F·g-1 at the current density of 0.2 A·g-1,and when it was continually charged and discharged 1000 cycles at current density of 1 A·g-1,81.6 % of the initial capacitance was retained.2.A novel nickel-cobalt hydroxide/graphene oxide composite with pompon-like and dense pore morphology was prepared by one-step solvothermal method and studied as the electrode material for supercapacitor.The results showed that addition of graphene oxide effectively improved the specific capacitance and cyclic stability of the resulting composite.When 10 mg of graphene oxide was added,the prepared composite displayed the optimal electrochemical properties.The specific capacitance was as high as 2050.6 F·g-1 at 1 A·g-1 and 84 % of the initial specific capacitance was retained after 1000 cycles at a current density of 5 A·g-1.An asymmetric capacitor assembled by the composite material and activated carbon also showed excellent electrochemical performance.In a wide potential window of 0~1.5 V,its specific capacitance reached 330.3 F·g-1 at 0.5 A·g-1,the power density and energy density were 375.3 W·kg-1 and 103.2 Wh·kg-1,respectively.In addition,the specific capacitance only decreased 12.5 % after 4000 cycles at 10 A·g-1.?-Nickel-cobalt hydroxide/graphene oxide composites with 3D flower-like structure consisted of nanosheets were also in situ developed on nickel foam substrate by a hydrothermal procedure.Because no adhesive was added,the resulting electrode possessed better conductivity.In addition,insertion of acetate ions expanded the layer spacing of the nanosheets,which was beneficial to the electroyte ion diffusion.As a result,the obtained electrode showed better electrochemical properties than that prepared by the transitional slurry coating method with teflon as adhesive.At the current density of 1 A·g-1,the specific capacitance was 1329.8 F·g-1,and when the current density was increased 10 times,the specific capacitance rention rate was 72.6 %.It is noteworthy that at a high current de nsity of 10 A·g-1,91.6 % of the initial capacitance was maintained after 1000 cycles.3.Nickel oxide-cobalt oxide/graphene oxide composites were synthesized successfully by a hydrothermal procedure and following high-temperature carbonization treatment.When used as the electrode material for supercapacitor,the prepared composites showed high specific capacitance,good rate performance and satisfactory cycling stability.The highest specific capacitance was 460.2 F·g-1 at 1 A·g-1,and when the current density was increased to 50 times,the specific capacitance still kept 65 %.At the current density of 5 A·g-1,the specific capacitance only lost 10.9 % after 4000 cycles.The composites showed satisfactory cycling stability.