Research On Oxygen Reduction Mechanism Of Pt-(Au,Ni,Pd)Alloy Nanostructures

Proton exchange membrane fuel cells(PEMFCs)are considered to be a potential alternative power source because of its high conversion efficiency,low pollution,light weight,and high power density for commercial applications.Pt catalysts are considered to be active cathode electrocatalysts for ORR.In order to reduce the cost of Pt and enhance catalytic activity,Pt-based bimetallic catalysts and catalysts formed by alloying transition metals into Pt have been an important focus in the design of catalystsfor the ORR.In this dissertation,Pt-based alloyed nanocatalysts with controlled compostions have been synthesized by surfactant-free and a single-phase method.Their compositions and structure were characterized by transmission electron microscopy(TEM),X-ray Photoelectron Spectroscopy(XPS)high energy x-ray diffraction(HE-XRD)etc modern techniques.The catalytic activity of alloyed nanocatalysts based on atomic-scale structure for oxygen reduction reaction was studied by electrochemical method and DFT calculation for the correlation between compositions and structure for catalytic activity.The main achievements of this dissertation were summarized as follows:1.Pt-Au alloy nanowires featuring composition-tunable and parallelly-bundled Boerdijk-Coxeter helix type of structure with(111)-dominant facets have been demonstrated for the first time as composition-and facet-tunable catalysts for ORR.The(111)facet dominance and the Pt-Pt lattice shrinking at a Pt:Au ratio of?76:24,as revealed by HE-TEM and HE-XRD/PDFs analyses of the PtnAu100-n NWs with different bimetallic compositions,have been found to play a major role in the enhancement of the electrocatalytic activity.Pt76Au24 exhibited a maximum activity for ORR.For this reason,at an optimum atomic ratio of Pt:Au close to 76:24 in PtAu alloy,there must be a unique surface atomic which produces a maximal charge transfer and a balance the Pt and Au surface sites to achieve the optimal electrocatalytic activity and stability,as demonstrated by the significantly-improved electrocatalytic activity and stability of PtAu NWs than Pt NWs.The synergy of which is further substantiated by DFT modeling results.In comparison with the catalysts derived from the nanoparticle counterpart with a similar composition and lattice shrinking,the nanowire catalysts were shown to display a higher catalytic activity and stability for ORR.The important role of the(111)facet-dominant surfaces in the catalytic energy of the nanowires is discussed in terms of a combination of bifunctional and the atomic-scale alloying properties,which shines a fresh light into the design and engineering of alloy nanowire catalysts with high catalytic and electrocatalytic activity and durability.2.Pt-Ni alloyed nano wires with composition-tunable and(111)(200)-dominant facets have been demonstrated for the first time as composition-and facet-tunable catalysts for ORR.The relative ratio of(200)facets vs.all facets((200)and(111))being identified is found to be dependent on the composition.As Pt%increases from 0 to?45%,(200)facets have been observed.As Pt%increases from?50%to higher%,the ratio showed sharp increase in the range of 10 to 25%.The emergence of 200-facets reduces the coordination numbers for the stress for the near-surface Pt atoms.In other words,the Pt-Pt bond length is effectively increased.There is a lattice expansion when Pt%is greater than?40%and less than?90%but a small lattice shrinking when Pt%is less than-40%and greater than-90%.This result is also confirmed by in-house XRD data.By incorporating Ni in Pt,there is a compressive strain as a result of the smaller atomic size for Ni,and the charge transfer between Pt and Ni would lower the d band center for Pt weakening the O binding,which enhanced the catalytic activity for ORR and Pt59Ni41 showed the maximum mass activity.In comparison with PtNi nanoparticles(NPs),the PtNi NWs display a remarkably higher electrocatalytic activity and stability.3.We have synthesized(111)facets dominant Pt-Pd alloyed nanowires with controlled compositions by surfactant-free thermal method.The morphology and structure were characterized by HR-TEM and XRD.Pt-Pd nanowires featured(111)facets were shown by HR-TEM.The incorporation of Pd into Pt can change the lattice constant.There is a lattice shrinking at Pt%lowering 20%and lattice expansion at Pt%exceeding 20%.Alloying Pt with Pd results from the lowering of the lattice constant and the Pt-Pt bond distances.Pd adsorbed OOH stronger than Pt,and Pd binded OH and O less strong than Pt.The OOH adsorption is directly related to the first electron reduction step and the less strong adsorption of OH/O favors O and OH reduction to water.Pt78Pd22 can better reduce the adsorbed O and OH than other compositions.Besides the alloying effect,the bicontinuous void space of Pt-Pd provides the integral molecular transport paths and the interconnected nanoscale framework facilitates the transport of electrons and adsorbed reaction intermediates.4.Sub-10 nm Pt-Pd nanoparticles loaded onto a conducting carbon support featuring(111)facets were prepared by a simple one-step method.No external reagent was used and a secondary amine species having a high affinity toward the closest-packed Pt(111)facets was generated in situ in the reaction.Dimethylamine so generated can be washed away from the catalyst surface,leading to bare Pt-Pd surfaces,so Pt-Pd catalysts were directly used for testing electrocatalysis without any surface treatment.The presence of carbon support in the reaction medium is crucial as it anchors the Pt nanoclusters generated at an early stage of thereaction and lets them evolve into shapes.Pt62Pd38 has a highest mass activity,which is confirmed from DFT calculation showing the adsorbed energy exhibited intermediate value and facilitated the breakage of O-O.5.One dimensional(ID)nanostructures represent apromising morphology paradigm that may overcome some inherent drawbacks of OD NPs.The 1D smooth single-crystalline planes could minimise the number of undesirable low-coordination defect sites which are less catalytically active and vulnerable to oxidation and decomposition.Besides,1D nanostructure can facilitate the electron transport by the path directing effects in catalyst electrodes and thus enhancing the reaction kinetics on the catalyst surface.Furthermore,bicontinuous void space of 1D nanostructure provides the integral molecular transport paths and the interconnected nanoscale frame work facilitates the transport of electrons and adsorbed reaction intermediates.