During the last decade, alloying Pt with early transition metals (M) such as Fe, Co, Ni, and Cu with well-controlled size, shape, and composition represents the leading research activities in the proton exchange membrane fuel cells (PEMFCs) community and dramatic enhancements in oxygen reduction reaction (ORR) activity and reduced usage of Pt have been achieved. However, Pt-M alloy nanoparticles (NPs) usually adopt a disordered face-centered cubic (fcc) solid-solution structure in which M and Pt are randomly distributed. When these Pt-M NPs are exposed to the corrosive conditions in acid-based PEMFCs, M is subject to fast etching that leaves a defective Pt surface with much low-coordinated Pt atoms, which leads to the structural and performance degradation. Engineering the crystal structure of Pt-M (M = transition metal) nanoalloys to chemically ordered ones (L10 or L12 structures) have drawn increasing attention in ORR electrocatalysis due to their robustness and high resistance against M etching in acid. However, preparing small-sized (&lt; 5 nm) Pt-M NPs with both high monodispersity and chemical ordering is not an easy task. Generally, disorder-order phase transformation is realized by high temperature annealing (&gt; 500 °C), which easily causes NP sintering and leads to the difficulty in preparing small-sized PtM NP catalysts with high quality. Although NPs coated with a layer of robust oxide can be stabilized against sintering, such protective layers as well as the conventional spherical Pt-M alloy precursors would greatly limit atom mobility during phase transformation. As a result, most of the reported intermetallic Pt-M systems are only partially ordered, which essentially compromise the benefits of intermetallic structure on ORR catalysis. Our previous work has demonstrated that the rational design of Pt-Fe precursor could facilitate the atom diffusion kinetics and enable the formation of fully ordered 8 nm PtFe NPs with extremely high ORR activity and stability. In this presentaion, our recent achievements in prepareing ultrasmall (&lt; 5 nm) intermetallic Pt-M NP catalysts for ORR catalysis through different strategies will be introduced and relevant structure-performance correlations will be discussed.

Ultrasmall structurally ordered PtM nanocrystals as efficient and robust oxygen reduction catalysts

2019 Spring National Meeting

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