Synthesis,Performance Optimization And Stability Investigation Of One-Dimensional Multi-component Electrocatalysts

Fuel cells have extended broad commercial application prospects in transportation,stationary and portable power generation equipment because of their advantages of high efficiency,no environmental pollution,quick start-up and low noise,thus will help to alleviate global energy supply and clean environment issues.Extensive attention has paid from academic and industry.Among all the existing fuel cells,proton exchange membrane fuel cells(PEMFCs)have widely been used in fields of hybrid electrical vehicle,portable electronic devices,and cogeneration systems along with their characteristics such as simple preparation,low operating temperature,and high power density.Nowadays,high cost and the shortened lifespan caused by poisoning of intermediates are two major problems for proton exchange membrane fuel cells.These problems are closely related to the catalysts.The high cost and stability of catalysts seriously restrict the further commercial promotion of fuel cells.At present,it has become a hot spot in the field of catalysis to optimize and control the structure and components of catalysts on nano/atomic scale,and to establish their relationship with catalytic activity and stability.In this dissertation,a series of noble metal electrocatalysts with controllable one?dimensional(1D)nanostructures were synthesized by ultrathin and ultralong tellurium nanowires as templates.The relationship between composition,structure and catalytic properties were studied.Tellurium nanowires is a very suitable template candidate material with many advantages such as high reactivity,well dispersibility and scalable preparation.1D electrocatalysts with ultralong and ultrathin structure were easily prepared through combination of tellurium nanowire templates and simple route,thereby effectively improving stability of the catalysts.In addition,the catalyst activity can be further increased through synergistic effects by rationally regulating the components of catalysts.Based on the understanding of catalytic reaction mechanism,this paper focuses on preparation,composition control,performance and mechanism of 1D multi-component noble metal catalysts with tellurium nanowires as templates.The main results are as follows:1.A method for large-scale preparation of ultrathin core-shell nanowires has been demonstrated.The galvanic reaction between palladium nanowires and the platinum precursor was induced by bromide ions to generate Pd@Pt nanowires with controllable composition and sheH thickness.The bromide ions play an important role in initiating and promoting the galvanic reactions,and are beneficial for formation of PtBr62-with lower redox potential.In addition,bromide ions could effectively reduce the reaction rate via affecting the balance of displacement reaction,thereby obtaining a smooth platinum shell with controllable coverage.The thickness of shell layer can be precisely controlled by changing amount of the bromide ions.Thickness of platinum shell has a direct relationship with oxygen reduction performance.The performance would be the best.in case of the platinum shell have fully covered palladium nanowires and the shell thickness is about three atomic layers.Not enough coverage or too thick shell is not beneficial for oxygen reduction reaction.1D Pd@Pt core-shell nanowires were prepared using tellurium nanowires as templates.The mildness and simplicity of this sacrificial template method provided a good basis for large-scale synthesis of catalysts.2.1D ternary PtNiTe electrocatalysts were prepared by partially sacrificial template method.Unreacted tellurium atoms were retained in the final catalysts,reducing amount of noble metal and stabilization for 1D structural framework.Although platinum and nickel components were added at the same time,the reaction kinetics of displacement between tellurium and different metals was different,due to differences in the standard reduction potentials,thereby affecting the composition ratio of final products.Based on this,composition and structure of nanowires could be controlled by changing the addition sequence and amount of precursors,effectively controlling and utilizing the gap time between different precursors.The structure of final electrocatalysts might be 1D homogeneous structure,core-shell structure and rough structure.Controllability and diversity of surface composition and structure of the catalyst provide a platform for optimization of catalyst performance and mechanism research.3.Based on the high activity of tellurium nanowire templates,a simple displacement reaction was taken to replace part of tellurium nanowires to prepare quaternary PtPdRuTe nanotubes with controllable composition and morphology.The hollow structure of nanotubes comes from the Kirkendall effect during the reduction process.Platinum atoms with relatively higher valence state and lower molar volume induce palladium atoms to diffuse outward and thus forming nanotube structures.Therefore,the amount of palladium has a direct influence on thickness of wall.Platinum with lower redox potential tends to replace tellurium atoms on the surface,but palladium is beneficial to react with tellurium atoms beneath surface.thereby improving the interaction of heteroatoms.Palladium was introduced into ternary PtRuTe nanotubes,providing a protective PtPd layer on the catalyst surface.Stability of the components is maintained,and the electronic structure of surface platinum was modified because of the protective layer,thus improving the catalysts properties.