Study On Preparation Of Three-dimensional Graphene By Chemical Vapor Deposition And Its Structure And Property

Graphene has good application prospects in the field of electronics and energy storage devices because of its high specific surface area,rich active sites and excellent conductivity.However,graphene suffers from the problems of agglomeration and low edge utilization rate.Therefor,three-dimensional graphene had solved these problems in some extent,but there are still troubless such as large aperture.In this thesis,vertical graphene sheets on the electrospun carbon nanofibers were prepared,obtaining novel three-dimensional graphene materials.Firstly,a small-size graphene structure was formed on the electrospun carbon nanofibers by carbonization in ammonia,where the parameters including temperature,flow rate of ammonia and time were minvestigated;Secondly,a larger size vertical graphene sheets on the nanocarbon fibers etched by ammonia were further grown by chemical vapor deposition,where the parameters of temperature,flow rate of methane and hydrogen,and time were minvestigated.We improve the performance of the supercapacitor and aluminum ion battery using the novel three-dimensional graphene materials as electrodes.The main research work and results are as follows:The radially oriented small-size graphene sheets were grown on the surface of nanocarbon fibers by controlling the carbonization parameters,where the parameters including temperature,flow rate of ammonia and time were vestigated and the corresponding electrochemical properties were measured simultaneously.With temperature increasing,the graphene sheets on the surface of fibers were gradually formed,while the diameter of the fibers decreased,and a good graphene structure was obtained at 1100 ℃.The effect of NH3 flux on the structure of graphene was investigated at 1100 ℃.As the flow rate of NH3 increased,the diameter of the fibers decreased,the graphene structure gradually formed on the surface of fibers,and the density of graphene increased.At1100 ℃,it is favorable to the formation of graphene sheets and refine the fibers diameter with the increase of flow rate of NH3.When the flow rate of NH3 is 180 mL/min,a greater graphene structure with uniform distribution and high surface density was obtained.The effect of carbonization time was investigated at1100 ℃ with NH3 flow rate of 180 mL/min.As the carbonization time increased,the fibers diameter decreased and the density of graphene on the surface of fibers increased gradually.A greater graphene structure with uniform distribution andhigh areal density was obtained at 2 h of carbonization,while a small-sized graphene sheets with a radial orientation was formed at the same time.The electrochemical workstation and the LANHE battery test system were used for testing.The results show that the prepared nanocarbon fibers with graphene nanostructured have excellent electrochemical performance: the electrode specific capacitance of 375 F/g was obtained in three-electrodes mode;in organic symmetry supercapacitor,the specific capacitance of device reaches 80 F/g and the energy density reaches 69.4 Wh/kg;the initial specific capacity of the battery reaches 78 mAh/g and the energy density reaches 42.6 Wh/kg and the power density reaches 1704 W/kg in aluminum ion battery,the specific capacity remains at 72 mAh/g after 5000 cycles with the coulombic efficiency over 99%.The dependence of the growth behavior and structure of the radially oriented graphene sheets on the process parameters such as gas flow rates and growth time were investigated.Simultaneously,the corresponding electrochemical properties were measured.With increasing the growth temperature,the graphene sheets increased gradually and a well-structured graphene sheets was obtained at1200 ℃.The effect of CH4 flow rate on the structure of graphene was investigated at 1200 ℃.As the flow rate of CH4 decreased,the diameter of the fiber decreased,the number of graphene sheets on the surface of the fiber decreased,and the density of graphene increased.When the flow rate of CH4 is18 mL/min,the well-structured graphene with uniform distribution and high surface density was obtained.The effect of growth time on the height of graphene sheet was investigated at 1100 ℃ with a CH4 flow of 18 mL/min.The height of the graphene sheets on the surface of fibers increased with time extend.After 12 h growing,few layers of graphene sheets with uniform distribution was obtained and interconnected porous structures were formed.The height of graphene sheets are nearly 100 nm.The electrochemical workstation and the LANHE battery test system were used for testing.The results show that the prepared three-dimensional graphene fibers have excellent electrochemical performance.The initial specific capacity of the battery reaches 160 mAh/g and the energy density reaches 66.9 Wh/kg and the power density reaches 2007 W/kg for aluminum ion battery.The specific capacity remains at 128 mAh/g after 3000 cycles with the coulombic efficiency over 98%.