Preparation And Investigation Of Core-shell Structured Pt Monolayer Catalysts And Carbon Based Non-Pt Catalysts

Proten Exchange Membrane Fuel Cell (PEMFC) is considered to be the most potential fuel cell that can be applied to large-scale transportation, military and other purposes. However, the commercialize of PEMFC is hindered by several difficulties, one of them is the extensive use of expensive platinum-based catalysts.Recently, the emergence of core-shell structured low-platinum catalysts and carbon-based non-precious catalysts has boosted the development of PEMFC, they can greatly reduce the cost of the whole cell by lowering the amount of Pt or eliminating the usage of Pt. In fact, the investigation of these two types of catalysts has become a significant topic in the area of PEMFC.In the thesis, we illustrated several low-platinum catalysts prepared by under potential depositon technique and organic-gol method, and explored their catalytic performance toward oxygen reduction reaction and formic acid oxidation. In order to exploit the structure and capability of as-prepared catalysts, numerous characterization tools were applied, like X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Thermal gravimetric analysis (TGA), Microscopy (TEM, STEM), Rotating disk electrode (RDE) and Membrane electrode assemble (MEA). By the help of experimental results, we investigated the feasible kinetic of catalytic reaction on catalysts’surface, revealed the relation between structures and properties of electro-catalysts and interpreted the possible origins for improved catalytic activities of the refined electro-catalvsts.Four major works were presented here:Firstly, under potential deposition (UPD) technique was used to form Ru@Pt core-shell structured catalysts with controllable thickness. We optimized the number of Pt layers in catalysts, and explored the ORR kinetics on the surface of Pt shells. A ORR catalytic activity sequence of Pt2ML-Ru/C> Pt3ML-Ru/C> PtML-Ru/C was concluded from experimental results. Among Ru@Pt catalysts, Pt2ML-Ru/C showed best ORR catalytic performance with specific activity of0.62mA cm-2and the mass activity of0.96mA (mg Pt)-1, both exceeded the2015Targets for platinum-based catalyst from DOE of US. Compared with commercial Pt/C, the Pt mass activity in Pt2ML-Ru/C was2.8times higher.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images vividly demonstrated Pt2ML-Ru/C had a structure with Ru as the core and Pt as the shell. Experimental and theoretic results concluded that the adsorption and activation energy of mediates on Pt, like O*and OH*, identified the ORR catalytic efficient of the catalyst. Therefore, the Pt shell in Pt2ML-Ru/C showed even better ORR catalytic activity than Pt/C since it successfully balanced the O-O bond breaking and O-H bond forming.Secondly, unsupported3-dimensional (3D) Fe-N-C framework catalyst Fe1.1-N-C-850-6was obtained by pyrolysis of poly-aniline. The catalyst displayed both excellent ORR activity and sustainable stability as the cathode catalyst in0.1M HClO4solution. It possessed a half-wave potential of785mV (vs. RHE) in polarization curve and stayed unchanged after5,000cycles of accelerate stability test. Experimental results indicated that the catalytic activity and morphology of these Fe-N-C catalysts were influenced by thermal temperature, Fe precursor contents and the addition of (NH4)2S2O8. XPS and Raman spectra proved the exist of pyridinic-N, pyrrolic-N and graphite-N in Fe1.1-N-C-850-6catalyst. The electron density of C atom would be changed by covalenting with N, and this might contributed to form active sites in the catalyst. Comparing with the Fe-N-C catalyst without3-D framework, Fe1.1-N-C-850-6owned a BET surface area as high as890m2g-1which could provide more places for the reaction. Because of that, Fe1.1-N-C-850-6displayed better ORR activity. Revealed by RDE test, the ORR kinetic on Fe1.1-N-C-850-6was believed to follow4-electron pathway.Thirdly, Four different Miller indexed platinum single crystals, Pt (111), Pt (110), Pt (100) and Pt (211), were obtained by a simple and safe annealing method. In0.1M H3PO4solution, enormous ORR over-potential was detected due to competitive adsorption of anionic H2PO4-and O2on the surface of all Pt single crystals. However, polarization curves of ORR and the corresponding K-L plots indicated that first-ordered kinetics was followed on these Pt surfaces. Since structure-sensitive absorbent H2PO4-had distinct adsorption strength on different surface structure, the ORR activity decreased as Pt (110)> Pt (211)≈Pt (100)> Pt (111). The ORR activity of Pt (111) was seriously influenced by the adsorption of H2PO4-due to its high coordination number and active surface. Nevertheless, the high-index Pt single crystal, Pt (211), reduced the negative impact from the adsorption of H2PO4-and showed better ORR in H3PO4solution. This might owing to the special stepped structure on the surface.Fourthly, A series of PdPt-CeO2/C anode electro-catalysts for HCOOH oxidation were assembled by reducing PdPt alloy on CeO2modified carbon supports using organic gel method. Electrochemical tests indicated the best catalyst was Pd15Pt1-15CeO2/C, the one with a CeO2loading of15%, and PdPt alloy amount of20%while the Pd to Pt ratio was15:1. In0.5M H2SO4+0.5M HCOOH solution, Pd15Pt1-15CeO2/C showed a on-set potential reached-0.1V (vs. Ag/AgCl) and current density as high as1.75mA (mg metal)-1at0.1V (vs. Ag/AgCl). Comparing with Pd/C, the HOOH oxidation reaction on the catalyst negatively shifted0.1V and had a60%improvement of current density. We proposed that "direct pathway" was favored for the oxidation reaction of HCOOH on Pd15Pt1-15CeO2/C not only because of the structure and electron effect induced by PdPt alloy but also came from the collaboration of CeO2-Each components represented their own effort to the catalysts:(a) By embedded into the lattice of Pd to form the PdPt alloy, Pt launched a strong electronic effect and altered the adsorption characters of Pd;(b) CeO2displayed bi-functionally by providing-OH groups to eliminate active mediates produced at low potential.