Metallic Single Atom Layer And Metal Clusters Catalysts For Fuel Cells Application

Fuel cells are regarded as ideal candidates for widely used power energy in the future due to their high energy conversion efficiency and environmetally friendly.Electrocatalysts are the core components and key materials for fuel cells,which determine the life,cost,efficiency and reliability of fuel cells.At present,Pt-containing materials are most widely used in fuel cell as cathode and anode catalysts.The low reserves and high price of platinum are bound to seriously hamper the large-scale application of fuel cell power technology.Therefore,design and fabrication of high performance and low cost catalysts is crucial for the widespread application of fuel cells,aiming to weakening or even fully eliminating the high dependency on precious metal Pt.Herein,five works were carried out in this thesis as followed:(1)Fabrication of single-atom-layer Pd and its catalytic performance.Precious metal films containing only one or a few layers of atoms have attracted great interest from the scientific and industrial communities because their quantum size and surface effects have enhanced their performance and minimized the usage of precious metal in various applications.We report that the freestanding and self-stabilized single-atom-layer palladium was generated in the angstrom-sized interlayer space of layered minerals by controlling the intercalation of Pd,where the metal only grows and spreads out in two dimensions.The results show that Pd single-atom-layer with a two-dimensional size of 5?50nm retains high metallic crystal structure,and the thickness of one-atom-thick Pd is 0.3?0.4nm.Moreover,the two-dimensional size of metal crystal is an important factor for the existence of stable single-atom-layer structure,and the single-atom-layer structure can be stabilized effectively when the size is greater than 30 nm.The results of electrochemical tests show that Pd single-atom-layer catalyst has very high atom utilization and catalytic activity,the mass activity reached 0.257A/mg for oxygen reduction,which is 64.7%higher than the commercial Pt/C.It also exhibited 4.8 times higher than that of Pd nanoparticles for formic oxidation.(2)Fabrication and catalytic performance of single-atom-layer Pd Co.We report that the freestanding and self-stabilized single-atom-layer Pd Co alloy with an atomic ratio of Pd to Co of 1:1 was generated in the angstrom-sized interlayer space of layered minerals by simultaneously controlling the intercalation of Pd and Co,where the metal only grows and spreads out in two dimensions.The results show that Pd Co single-atom-layer with a two-dimensional size of 1?2nm has high crystallization degree and strong lattice contraction,and the thickness of one-atom-thick Pd Co is 0.4?0.5nm.The strong lattice contractions provide a strong stabilization effect to maintain Pd Co single-atom-layer nanosheets as small as 1.76 nm.The results of electrochemical tests show that the utilization of single-atom-layer was highly reached to 95.92%,indicating that almost all Pd atoms are exposed to the electrode-electrolyte interface and utilized.The single-atom-layer Pd Co alloy exhibits an evidently enhanced mass activity toward oxygen reduction turned out to be nearly six times higher than commercial Pt nanoparticles.Its mass activity is also eight times higher than that of Pd nanoparticles for formic acid oxidation.(3)Studies of coordination structure and electron structure of single-atom-layer metal and its alloy.In this study,we analyzed the coordination structure and surface electron structure of single-atom-layer metal crystal by combining X-ray absorption spectrum and X-ray photoelectron energy spectrum.The results indicate the as-constructed single-atom-layer has two coordination features,i.e.six-coordination and eight-coordination,consistent with the coexistence of(111)and(200)single-atom-layer planes.It was further found that their valence band and empty band exhibit an unusual split under such absolute two-dimensional coordination/bonding,differs from that of the conventional metal crystal,i.e.the band splitting forms a lower band in the x-y plane and a higher band in the z-direction,which greatly improves the stability and reactivity of single-atom-layer metal crystals.(4)Modulation of iridium-based alloy catalyst by a trace metals for hydrogen oxidation.The OH_ad species are generally believed to affect the HOR/HER process,while the mechanism is still debatable.Therefore,clearing the role of OH adsorption during HOR/HER process is essential for developing high performance HOR/HER catalysts under alkaline electrolyte.We designed an approach to study the impact of trace level of metal impurities on the surface of Ir alloy nanocrystals during HOR-pathway under alkaline medium by using Ir and Ir Fe alloy nanoparticles as model catalysts.The Fe content of Ir Fe/C is gradually altered by chemical and electrochemical de-alloying methods,and the surface of Ir/C surface is modified by trace metals with different oxyphilicity(Fe³⁺,Ru³⁺,Au³⁺),which regulated the adsorption properties of OH on the surface of Ir based catalysts.The electrochemical tests indicate that the de-alloyed Ir Fe/C and Ir/C modified with trace Ru exihibit higher HOR/HER activity.Furthermore,it is also found that too strong or too weak OH adsorption on the Ir catalysts has a negative contribution to HOR/HER-pathway under alkaline medium.Therefore,a volcano-shaped relationship between the adsorption strength of OH and the HOR/HER catalytic activity was revealed.(5)Regulation of Ru clusters catalyst by oxidation degree for hydrogen oxidation.Ruthenium is generally used as a property-promoter rather than as the main catalyst for the hydrogen oxidation,because of the preferential formation of a surface oxidation state at the anode potential of 0.1?0.3 V vs.RHE,which leads to few effective active sites for hydrogen oxidation.Herein,we report the Ru clusters are interphase-oxidized by Ti O₂ to yield half-filled 4d-orbitals,and greatly promote HOR activity and inoxidizability under alkaline conditions.Furthermore,the discussion is made on the impact of oxidation ways and degree on activity and antioxidant,and the reasons behind improved performance are further revealed.The X-ray absorption spectrum and X-ray photoelectron energy spectrum results confirmed that the Ru clusters in IO-Ru-Ti O₂/C are preoxidized near to a level of Ru^iv,and maintain a metallic surface with abundant metallic Ru-Ru bonds.These features decrease the adsorption of oxygen species during the HOR,and promote the bonding of H intermediate species.The results of electrochemical tests shows that sub-2 nm Ru clusters can efficiently catalyze the HOR at a potential of up to 0.9 V(vs RHE)without being oxidized.As a result,a mass activity of 907A/g _Ru@0.05 V was achieved for the IO-Ru-Ti O₂/C catalyst,which is17.5 and 1.5 times greater than those of the Ru/C and Pt Ru/C catalysts,ranking among the most active HOR catalysts in alkaline media reported to date.