Structural Characterization and Enhanced Oxygen Reduction Activity of Chemically-Ordered Intermetallic PtFeNi Catalysts
The fct-PtFeNi catalysts with different atomic compositions of Fe and Ni (Pt50Fe35Ni15, Pt50Fe25Ni25, Pt50Fe15Ni35) were prepared using a simple solid-state impregnation method; the mixture of metal (Pt, Fe, Ni)-salts and carbon blacks were annealed under H2/N2 at a temperature of 800°C. The prepared fct-PtFeNi catalysts exhibit about 3 times higher ORR mass activity, compared with the commercial Pt catalyst. Especially, the mass activity of Pt50Fe35Ni15 is about 0.62 A/mgPt; it is higher than that of fct-Pt50Fe25Ni25 and fct-Pt50Fe15Ni35 (about 0.5 A/mgPt). To discuss more about the enhancement of the ORR activities in the fct-PtFeNi alloys, the surface structures of the catalysts were examined by in-situ XAFS measurements. The EXAFS analysis of Pt-L3 edge reveals that the distance of Pt-Pt bonds in the fct-PtFeNi catalysts is about 2.72 nm, which is shorter than that of commercial Pt catalyst. Interestingly, the coordination number of Pt oxides (Pt-O or Pt-OH) in the fct-Pt50Fe35Ni15 is more increased at high potential of 0.8~1.2V, compared with other fct-PtFeNi catalysts with the lower content of Fe. The tendency of the formation of Pt oxides is in accord with that of the ORR activities in the fct-PtFeNi with the different compositions of Fe and Ni. Therefore, it is inferred that fct-PtFeNi with higher content of Fe would form more Pt oxides, leading to a faster reaction rate of oxygen reduction. This study brought out new insights; the surface structures of chemically-ordered fct-Pt alloys, such as the Pt-Pt bond distance and the formation of Pt oxides, strongly affect the ORR activities, as those of the previously-reported disordered Pt-alloys do as well. It is believed that our findings can aid in developing fct-Pt alloys with more enhanced catalytic activity and durability for PEFCs.