Fabrication Of Transition Metal Supported On Nitrogen-doped Carbon Nanocomposites And Their Application For The Catalytic Oxygen Reduction Reaction

The problems arose from the ever-increasing global energy demand and worsening environmental impact owing to the utilization of fossil fuels,which force researchers to develop substitutes for traditional energy resources.Fuel cell technology,as a promising energy conversion device,provides clean and sustainable power.However,the kinetically sluggish oxygen reduction reaction（ORR）at the cathode greatly affects the energy conversion efficiency.Currently,precious metal platinum（Pt）based catalysts are generally considered as the most effective catalysts for ORR.Unfortunately,a series of issues including high cost,scarcity of natural resources,insufficient durability,and crossover effect still restrict the large-scale commercialization of Pt-based electrocatalysts.Therefore,extensive research has been devoted to the development of non-Pt materials,which need to be of low cost but have comparable or even better activity than Pt in fuel cells.Thus far,various catalysts such as non-precious metals or metal oxides（e.g.,Co,Fe,Fe₃O₄,CoO,Co₃O₄,and ZrO₂,etc.）,heteroatom-doped metal-free carbons（e.g.,one-dimension（1D）carbon nanotubes,two-dimension（2D）graphene/carbon nanoribbons,and three-dimension（3D）porous carbons）,and metal-coordinated nitrogen-doped carbon（M-N_x/C）materials have been widely investigated in ORR process.Among them,iron-and cobalt-based materials have been demonstrated as one of the best alternatives for Pt/C because of their excellent catalytic activity and stability.In this thesis,electrocatalytically active materials with high efficiency（transition metal or transition metal oxide）were integrated with1D carbon nanotubes or 2D graphene with excellent physical and chemical properties to solve the main challenges of pure metal or metal oxide,such as poor conductivity,corroded in electrolyte,metal particle agglomeration and monotonous pore structure which were not conducive to ion/proton transmission and diffusion.Novel synthesis strategies by means of nano-confined self-assembly or in situ self-assembly through electrostatic attraction have been established to construct metal oxide hollow nanostructures coupled with 2D carbon nanosheets and 1D nitrogen-doped carbon nanotubes decorated with carbon-coated cobalt nanoparticles,so that the interefaces between electrochemical active materials and electrolyte could be optimized,resulting in fast ion or proton transport pathways,structure stability and excellent electrocatalytic properties.A series of novel results are described as follows:1.Using 2D nanostructured montmorillonite（MMT）as a template,the ferric oxide hollow nanostructures anchored on 2D porous carbon nanosheets composite（Fe₂O₃/PCS）were fabricated via electrostatic attraction and intercalation of polypyrrolidone-ferric nitrate in MMT,followed by carbonization reduction and template removal treatment.The macroporous struture and sheet thickness of carbon nanosheets in Fe₂O₃/PCS hybrid could be precisely controlled.The micro-environment created by the hollow structure of Fe₂O₃ could regulate the interaction between the active site and the substrate,thereby improving the utilization of the catalysis active materials.The conductivity of Fe₂O₃/PCS has been greatly improved by 2D porous carbon nanosheets.Furthermore,the excellent structural stability was benefited from the synergistic effect of metal oxide hollow nanoparticals and the carbon composite interfaces.Therefore,the ORR catalytic performance of Fe₂O₃/PCS was significantly higher than that of hollow iron oxide coupled with carbon in bulk structure（Fe₂O₃/HCB）with an onset potential of-0.113 V（vs Ag/AgCl）and current density of-4.0mA·cm^-2 at-0.6 V,as well as the good catalytic stability in 0.1 M KOH.2.Employing 1D halloysite nanotubes（HNTs）as a template,nitrogen-doped carbon nanotubes decorated with carbon-coated cobalt nanoparticles（Co@C-NCNTs）were constructed from zeolitic imidazolate framework ZIF-67.Electrostatic force-induced preadsorption of coordination center ions on the surface of HNTs led to in situ nucleation and confined growth of a thin layer of ZIF-67.Subsequent carbonization and template removal procedure gave rise to NCNTs with open end and large inner cavity,thin wall of moderate graphitization,and decoration with carbon-coated Co nanoparticles.More intersting,the synthetic strategy could be easily extended to the preparation of various NCNTs with tunable graphitization and metal decoration from different ZIFs.Benefited from 1D nanotubes with more exposed active surface area and convenient channels for the transport of electrons and reactants,the resulting Co@C-NCNTs exhibited enhanced catalytic performance for ORR with an onset potential of-0.1 V vs Ag/AgCl,high current density(-4.42 mA·cm^-2 at-0.6 V),and approximate 4e^-transfer process in 0.1 M KOH.In addition,Co@C-NCNTs showed higher durability and remarkable methanol tolerance capability both in alkaline and acidic solutions superior to the commercial Pt/C.The present strategy for structure-control electrocatalysts has created a new pathway for the fabrication of promising cathode catalyst for fuel cell applications.