| Graphene is a promising electrode material for electrical energy storage and conversion due to its high specific surface area,excellent electrical properties and high chemical stability.However,the absence of band gap in pristine graphene and the stack owing to the strong van der Waals forces always lead to the low electrochemical performance,which greatly impedes its application in practice.Recently,it is found that the electrochemical performances of graphene-based materials can be significantly enhanced by introducing defects or doping,constructing three-dimensional composites and combining with other components.However,the current electrochemical performances of graphene based materials are far from their practical application.Therefore,it is urgent to clarify the relationship between structure-composition-property-efficiency and strengthen the electrochemical performances of graphene based electrode materials.Based on the above mentioned,we have carried out the following three works in this thesis:Firstly,in order to solve the problems that the high-defect density graphene cannot be synthesized with high temperature chemical vapor deposition(CVD)process and the defect morphology cannot be controlled by etching technology,we devise a novel Cu foil pretreatment approach to induce the C atoms involved in Cu bulk to segregate onto Cu surface and establish synergetic C-Cu complex catalyst.The existence of C-Cu on the surface of Cu foil is confirmed by Auger electron spectroscopy(AES)and X-ray photoelectron spectroscopy(XPS)characterization technology.Theoretical evidence supports that the interaction between the Cu and C atoms in form of C cluster atop the Cu plane greatly enhances the catalytic activity of adjacent Cu atoms and lowers energy barriers for stepwise decomposition of CH4.The 13CH4 isotope data demonstrate that the carbon source for graphene growth origins from both the carbon impurities in the Cu foil and the decomposed CH4,unraveling the dual roles of carbon impurities.Compared with Cu foil,the C-Cu complex catalyst shows higher catalytic activity for CH4decomposition.Moreover,the carbon of C-Cu complex catalyst can be integrated sequentially into the graphene lattice to in-situ create defects during the CVD process.Therefore,the graphene film with extremely high defect density of 5.9×1011 cm-2 and relatively high crystallinity can be successfully synthesized at a low temperature of700??The defective graphene film exhibits a specific capacitance of 10.6?F/cm2(about fivefold over pristine graphene)and excellent oxygen reduction reaction(ORR),hydrogen evolution reaction(HER)performances.This work proposes a referential strategy to realize a series of low temperature processes associated with activation of C-H bond by constructing an absolutely novel catalyst model of metal-carbon complex.Secondly,chemical doping and morphology engineering are paramount to enhance graphene-based materials catalytic performance toward HER.In order to optimize the growth kinetics of graphene and carbon nanotubes in the in-situ preparation of graphene/carbon nanotube composite,we propose a strategy with differentiation growth temperature to synthesize heteroatoms-doped three-dimensional(3D)carbon nanotubes/graphene hierarchical architecture(N,S-CNTs/N,S-G)on layered double oxide substrates,in which the N,S-doped CNTs are in-situ grown on both sides of N,S-doped graphene,by two differentiated CVD processes.The high concentrations of N and S dopants(up to 6.5 at.%)provide sufficient catalytic active sites for HER,while the CNTs seamless graft on graphene ensures the excellent electric conductivity of N,S-CNTs/N,S-G hybrids.The electrochemical test results show that the 3D N,S-CNTs/N,S-G composites display superior electrocatalytic activity for HER with an onset potential of 62 m V vs.RHE(achieve current density of 1 m A cm-2)and a small overpotential of 126 m V at 10 m A cm-2,which outperforms N,S-CNTs,N,S-G,N-G and most of reported chemical doped carbon-based composites.The strategy proposed in this study provides a new way for the in-suit fabrication of other heterogeneous composite catalyst.Finally,the conductivity and HER activity of graphene can be enhanced by doping transition metals and nitrogen,and a synergistic effect will be established between Mo S2and transition metals in graphene by combining Mo S2 with transition metal and nitrogen doped graphene to form composite catalyst.The existence of this synergistic effect can significantly improve the HER performance of the composite catalyst.Currently,it has become the development direction of composite catalyst to develop a simple and effective synthesis strategy,which can directly compound the components and realize the comprehensive synergism among the components.Based on this,we develop a simple method for preparation composite catalyst of Mo S2 and transition metal doped graphene.Mo S2 nanosheets and Co,N,S doped graphene are prepared and compounded together at the same time by a recrystallization and one-step heating treatment processes.The composite catalyst Mo S2@Co-N@NSG shows a nanosheet structure,and 2H-Mo S2nanosheets with a size of 2.0 nm?3.0 nm are loaded on the graphene layer.The structures of pyridine-N,pyrrole-N,graphite-N and Co-N,as well as the abundant Mo S2edge sites provide a large number of catalytic active sites for the composite catalyst.The electrochemical test results show that the Mo S2@Co-N@NSG composites display superior electrocatalytic activity for HER with a small overpotential of 116 m V at 10m A cm-2.And the mass specific activity of Mo S2@Co-N@NSG is eightfold over Mo S2@NSG,Co-N@NSG and NSG catalysts.This work provides a new approach for the simplified and large-scale preparation of other metal sulfide and transition metal doped graphene composite catalysts with high hydrogen evolution activity.In this thesis,the energy storage and electrocatalytic performance of graphene-based materials are significantly improved by introducing defects in graphene,doping with heteroatoms and constructing three-dimensional structure with carbon nanotubes and combining Mo S2 with transition metal and nitrogen doped graphene,revealing the important roles of defects/doping,morphological structure and synergistic effect on the performance of graphene-based electrode materials and providing a new way for development of graphene-based electrode materials. |
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