Construction And Electrochemical Properties Of Three-Dimensional Carbon Nanotubes/Graphene Composite Structures

Carbon materials, a kind of important electrode materials, are widely used in electrochemicalenergy devices such as fuel cells, supercapacitors and batteries due to their good electricalconductivity and stability, low price and environmental benignity. The molecular and crystalstructure of carbon materials play key roles in their properties and dominate their application inelectrodes. Carbon nanotubes （CNTs） and graphene are the typical one-and two-dimensionalnanomaterias, respectively. Compared with other type of carobn structures, these two materialsown better conductivity and stability. Moreover, their electronic structure and chemical propertiescould be modified and controlled by nitrogen or boron atom doping. Hence CNTs and grapheneare regarded as ideal electrode materials. Their specific surface area, however, would dramaticallydecrease when they are used in the form of disorder powder due to the stacking and covering ineach other. For example, stacking graphene powder loses80%specific surface area. To maintainthe features of individual nanostructure for CNTs and graphene, this thesis is focusing on thepreparation of three dimensional （3D） CNTs/graphene composite structures by growing CNTs onthe two dimensional graphene planar surfaces, playing their synergetic roles to promote theirapplication in fuel cells and supercapacitors. The main contents are summarized as follows:1．With the flake graphite powder as raw material, graphite oxide （GO） was synthesized by themodified Hummers method. The large number of oxygen-containing functional groups on thesurface and the edge of GO can be used to adsorb divalent Ni ions, which were reduced byhydrazine hydrate to obtain reduced GO （rGO） supported Ni complex. Then3D NCNTs/graphene（NCNTs/G） was prepared by chemical vapor deposition （CVD） using pyridine and rGO supportedNi as the corresponding precursors. The total N content in the as-prepared NCNTs/G wasmeasured to be6.6at.%. In alkaline medium,3D NCNTs/G catalyst presents better performancefor oxygen reducion reaction （ORR） than3D CNTs/G catalyst. The transferred electron number ofORR on the3D NCNTs/G catalyst is about3.5, indicating its potential application in fuel cells.This work also provides a strategy for the design and prepartion of3D metal-free ORR catalysts.2．We have developed a simple route to synthesize3D CNTs/G/NF seamless hybrids bysequentially growing CNTs and graphene on Ni foam （NF） substrate with ethanol through twosteps of CVD. Then MnO₂nanoparticles were immobilized onto CNTs to formMnO₂/CNTs/G/NF hybrids. This hybrid structure integrates the highly conductive CNT core and the large pseudocapacitance of MnO₂shell. MnO₂/CNTs forest is sufficiently connected with theNi foam substrate through the bridge of graphene interlayer. These3D hybrid foams coulddirectly act as supercapacitor electrodes without post-transfer or further treatment.