Preparation And Electrocatalytic Performance Of Core-shell Fe₂O₃/Pt Nanoparticles

Direct methanol fuel cells (DMFCs) in portable electronic devices and transportation applications have attracted increased attention. Electrocatalytic materials with high activity are critically needed to enhance the performance of DMFCs. Currently, Pt is the most active and indispensable material for methanol oxidation. However, Pt is a precious metal and therefore very expensive. Therefore, further enhancements in Pt utilization efficiency and catalytic activity are fundamentally important for the widespread use of DMFCs. Arranging Pt as thin shells on other substrates with a core-shell or decorated structure greatly enhances the utilization of Pt because of the improved dispersion of the metal. This method also improves material properties because of the strain and ligand effects exerted by the core substrate on the supported Pt overlayer.Iron oxide nanoparticles in the amorphous state are more interesting than their crystallized counterparts when used as a catalyst because the former can form dangling bonds and yield higher surface-bulk ratios. More importantly, amorphous iron oxides do not exhibit the long-range order characteristic of a crystal. The short-range orders of these nanoparticles at the atomic length scale are due to the nature of chemical bonding in the particles. The lattice defects of nanoparticles have distinct implications for mediating atomic arrangement as well as tuning the electronic structure and coordination of the outer shell. Based on this concept, arranging Pt as thin shells on an amorphous iron oxide core may improve its catalytic performance.In this paper, core-shell Fe2O3/Pt nanoparticles with amorphous iron oxide cores are successfully synthesized by a two-step chemical reduction strategy. The Pt loading can be adjusted using this preparation technique to obtain a series of chemical compositions with varying amounts of Pt precursors. The morphology, structure, and composition of the as-prepared nanoparticles are characterized by transmission electron microscopy, X-ray diffraction, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. Electrocatalytic characteristics are systematically investigated by electrochemical techniques, such as cyclic voltammetry, chronoamperometry, impedance spectroscopy, potentiodynamic technology and in situ Fourier transform infrared spectroscopy.The physical characterization results showed that the as-prepared Fe2O3/Pt nanoparticles have near-spherical shapes and show slight agglomeration; The Fe2O3nanoparticles have an amorphous structure. Typical peaks characteristic of the face-centered cubic lattice structure of the Pt shell in Fe2O3/Pt (Fe:Pt=1:1) nanoparticles are observed. Furthermore, the diffraction peak intensity of Pt gradually decreases with decreasing Pt contents in the Fe2O3/Pt nanoparticles. For Fe/Pt atomic ratios larger than1:0.5, the diffraction peak of Pt is not observed. It also were founded that the electronic effect between Pt and Fe2O3increases with increasing Fe contents in the Fe2O3/Pt nanoparticles.The electrochemical characterization for methanol oxidation results showed that the as-made Fe2O3/Pt nanoparticles exhibit superior catalytic activity with lower peak potential and enhanced CO2selectivity toward methanol electrooxidation compared with the E-TEK40%Pt/C catalyst. The highest activity is achieved by core-shell Fe2O3/Pt nanoparticles with an Fe/Pt atomic ratio of1:0.5(A g-1of Pt) or1:0.33(mA cm-2). These nanomaterials also show much higher structural stability and tolerance to the intermediates of methanol oxidation. The enhanced catalytic performance of the core-shell Fe2O3/Pt catalysts is attributed to two main reasons. First, the presence of amorphous Fe2O3cores modifies the dispersion and electronic structure of Pt and influences the chemi-adsorption of methanol and poisoning species, such as CO. Second, amorphous iron oxide nanoparticles facilitate the development and formation of a microcrystalline Pt thin film, which strongly influences surface atom densities and chemical reactivity.The electrochemical characterization for oxygen reduction results showed that the catalytic activity of core-shell Fe2O3/Pt nanoparticles first increases and then decreases with decreasing Pt content. The highest activity for the oxygen reduction is achieved by core-shell Fe2O3/Pt nanoparticles with an Fe/Pt atomic ratio of1:0.5or1:0.33. When the concentration of methanol in the electrolyte increased, the catalytic activity of the core-shell Fe2O3/Pt nanoparticles for oxygen electrochemical reduction obviously reduced. However, the peak current of methanol electro-chemical oxidation significantly increased.