Structural Characterization and Enhanced Oxygen Reduction Activity of Chemically-Ordered Intermetallic PtFeNi Catalysts

The polymer electrolyte fuel cells (PEFCs) are excellent candidates as next-generation electric generators for vehicles, residential uses, laptop, etc. However, there are many issues to be solved to enlarge commercially uses. One of issues is related to Pt catalysts for oxygen reduction reaction (ORR) on a cathode electrode. Pt catalysts show low ORR activity and low durability, leading to lower performance in PEFCs. Our earlier studies found the chemically ordered face centered tetragonal (fct) intermetallic Pt-alloy catalysts (L1₀ type super-lattice structure) can satisfy both of higher ORR activity and long-term durability, whereas many of the previously-reported Pt-alloys cannot. It is considered that the high performance of fct-Pt alloy catalysts would result from the chemically-ordered structures, but it is unclear how they work to enhance ORR activity and durability. Thus, in this presentation, we will discuss the relationship between the ORR activities and surface structures in fct-Pt alloys by the detailed characterization of the catalyst’s surface structures.

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/mg_Pt; it is higher than that of fct-Pt₅₀Fe₂₅Ni₂₅ and fct-Pt₅₀Fe₁₅Ni₃₅(about 0.5 A/mg_Pt). 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.