| Nowadays,the problems of environmental pollution and energy shortage become more and more serious.Concerning to the environmental pollution and energy shortage problems caused by the overuse and depletion of traditional fossil fuels,governments all over the worlds have put great efforts into the development of clean and renewable energy to replace fossil energy and reduce the environmental problem.Proton exchange membrane fuel cells（PEMFCs）have been considered as a potential green energy device for electric vehicle and portable power due to its high energy conversion efficiency,eco-friendly nature and reliability.However,high cost and low durability of fuel cells catalyst has been the major obstacles for commercialization of PEMFCs.Therefore,exploring new active catalysts,especially inexpensive,reliable and highly active catalysts toward oxygen reduction reaction at the cathode with sluggish kinetics,for the industrialization of PEMFCs is of great importance.Although some progress has been made in the development of platinum-free catalyst,their activity and stability are still insufficient in the practical applications of fuel cells.Tailoring structure and composition of catalyst in nanoscale for development of low platinum catalyst with excellent performance and stability has drew great attention these days.In consideration of only the surface or near-surface layer atoms provide active sites for catalytic reactions,it is very critical to rationally design a catalyst with Pt-enriched surface to enhance the utilization efficiency of Pt for reduction of the usage.Meanwhile,the electrocatalytic activity of the core-shell nanostructured catalyst can be significantly improved by controlling the ligand effect and the stress interaction originated from the lattice mismatch between the core substrate and Pt based shell.In this thesis,we aimed at improving the usage of platinum through designing a series of highly active and stable core-shell structures via seed-mediated synthesis,electrochemical deposition and Galvanic replacement reactions.In the meanwhile,we gain a fundamental understanding of fuel cell catalyst structures and their corresponding catalytic reaction mechanisms.We believe that this thesis project can provide valuable experimental and theoretical knowledge for electrocatalysis while promoting the commercialization progress of the fuel cells.The main research contents and results of this thesis are as follows:（1）Pt based core-shell nanostructures have been extensively studied as one of the most promising electrocatalysts to significantly reduce the loading of the expensive Pt catalyst for fuel cells but it is very challenging to fabricate such a structure to possess proper metal components while making small and uniformly distributed nanoparticles for high catalytic performance.In the second chapter,a facile one-pot approach with oleylamine as a solvent,surfactant and reductant is innovated to successfully prepare small and uniformly dispersed Au@PtCu core–alloy shell nanoparticles.The structure feature of the nanoparticles was confirmed by HAADF-STEM-EDS and XRD.The results reveal that the core-shell structured Au@PtCu/C catalyst exhibits significantly enhanced electrochemical active surface area,and thus possesses much more available catalytic active sites for methanol electrooxidation reaction.Meanwhile,the present of Au core and Cu in shell could modify the electronic structure of Pt for further enhancement of the poisoning tolerance ability of the active sites,thus producing improved specific activity and stability toward methanol oxidation reaction.（2）Various researches have demonstrated that the catalytic activity of Pt-based nanocatalyst is closely related to the surface structure of nanocrystal.The high-index surfaces usually present higher catalytic activity since it holds a high density of atomic kinks,steps and ledges,which usually serve as high active sites for breaking chemical bonds.Nevertheless,the size of PtCu nanoparticles with high-index facets reported in the literature usually larger than 30 nm or even hundreds nanometers.It remains great challenge to fabricate such a structure to increase the usage of noble Pt while making uniformly distributed small nanoparticles for high catalytic performance.In the third chapter,Pd nanocubes with average edge length of 8.4 nm were served as seeds for mediating growth of uniform Pd@PtCu concave octahedrons with a mean edge length of13.7 nm.The results reveal that the present of Pd nanocube seeds could efficiently reduce the size of the as-prepared concave octahedrons,thus increasing the utilization efficiency of Pt.The concave structure of the particles is confirmed by TEM.Atomic-resolution HAADF-STEM image displays that the as-prepared concave octahedron holds high-index surface of{530}and{740}.Towards oxygen reduction reaction,the specific activity at 0.9 V of the Pd@PtCu concave octahedron catalyst is 3.36 mA cmPt-2,which is 2.56 and 11.42 times higher than that of Pd@PtCu octahedron(1.31 mA cmPt-2)and commercial Pt/C(0.294 mA cmPt-2)catalyst,respectively.The enhanced activity toward oxygen reduction reaction of the Pd@PtCu concave octahedron catalyst can be ascribed to Cu incorporated into Pt to form PtCu alloy with a concomitant lattice contraction,resulting a down shift of the d-band center,thus weaken the chemisorption of hydroxyl species and accelerate desorption.Furthermore,the high-index facet of the Pd@PtCu concave octahedron such as{530}and{740}surface has a high density of atomic steps and kinks as high active sites for breaking O-O bonds,which also contributes to the great enhancement of the catalytic activity.（3）Development of an inexpensive but high performance ORR catalyst with low Pt loading is very important for the commercialization of fuel cells.Herein galvanic exchange-formed ultra-low Pt loaded porous Ag-Pd binary alloy nanotubes（Ag-Pd@Pt）was prepared as a catalyst toward oxygen reduction reaction（ORR）.With highly dispersed active but ultra-low Pt atoms（Pt:about 5.8 wt.%）on the porous binary alloy surface,the Ag-Pd@Pt catalyst increases mass activity toward ORR by 1.82 times than commercial Pt/C while possessing excellent operation stability.DFT calculation reveals that Ag-Pd@Pt catalyst possesses higher oxygen and weaker OHad absorption ability owing to the Ag-Pd alloy core decorated Pt electronic structure in comparison to pure Pt surface.Besides,the self-supported and porous structure could significantly enhance the stability and mass transport,which is favorable for improvement of catalytic performance.（4）A self-supported porously structured catalyst usually has improved activity and enhanced stability.In the fifth chapter,a two-step template method was used to prepare interconnected PdCu@Pt nanowire network catalyst,in which PdCu nanowire network was synthesized by the use of Plurinic F127 as a micelle template,followed by the use as a template to be galvanically replaced with[PtCl4]2-in solution.The core-shell structure could significantly enhance the utilization efficiency of Pt,while the lattice of Pt shell is decorated by PdCu core to modify its electronic structure,thus largely improving the ORR performance.Besides,the networked nanowires could not only enhance the mass transport during ORR,but also reduce the particle agglomeration for improved stability.This work provided a facile approach to synthesize highly active and stable low-Pt catalyst.In brief,we have prepared four different Pt-based core-shell nanostructured catalysts and investigated their electrocatalytic behaviors.The results reveal that the core-shell structure can greatly improve the electrochemical active surface area.Both geometric and electronic structures of catalytic sites can be regulated by the components and interactions between core and shell to improve the electrocatalytic activity and stability.This thesis work offers a facile approach to greatly reduce the high cost and improve stability for the Pt-based catalysts,thus holding great promises to promote the commercialization of PEMFCs. |
