Intermetallic nanoparticles (NPs) are a class of metallic alloy NPs within which metal atoms are bonded via strong d-orbital interaction and ordered anisotropically in a specific crystallographic direction. Compared to the common metallic alloy NPs with solid solution structure, intermetallic NPs are generally more stable against chemical oxidation and etching. The strict stoichiometry requirement, well-defined atom binding environment and layered atomic arrangement also make intermetallic NPs an ideal model for understanding their physical and catalytic properties. This talk focuses on monodisperse intermetallic L10-structured and hard magnetic MPt NPs, especially Pt-, Pd-, and Au-based L10-NPs, as robust catalysts for electrochemical reduction and oxidation reactions. One example is core/shell L10-FePt/Pt with ~2 atomic layers of Pt shell for oxygen reduction reaction (ORR). The smooth thin Pt shell is compressed due to its epitaxial binding with the L10-FePt core. The structure is chemically stable against fast/deep Fe etching and is catalytically more active than the Pt counterpart, showing superior mass activity and much better durability under the fuel cell testing conditions even after 30,000 fuel cell testing cycles at 80°C. When Fe is replaced by Co in the L10-FePt/Pt structure, the Pt shell in the L10-CoPt/Pt structure is compressed even more and its ORR catalysis is further enhanced. At 60°C in 0.1 M HClO4, the hard-magnet L10-CoPt better stabilizes Co (5% loss after 24 h) than the soft-magnet A1-CoPt (34% loss in 7 h). L10-CoPt/Pt achieves mass activities (MA) of 0.56 A/mgPt initially and 0.45 A/mgPt after 30,000 voltage cycles in the membrane electrode assembly at 80°C, exceeding the DOE 2020 targets on Pt activity and durability (0.44 A/mgPt in MA and &lt;40% loss in MA after 30,000 cycles). Another example is core/shell L10-MPt/AuPt (M = Fe or Co) with 2 atomic layers of AuPt alloy shell for electrooxidation reactions. Au in the MAuPt structure facilitates MPt structure transformation from disordered solid solution structure to ordered intermetallic L10-structure, forming core/shell L10-MPt/AuPt. The L10-structure stabilizes M and significantly enhances the catalysis of the PtAu surface towards electro-oxidation of formic acid (11.97 A/mgPt), methanol (1.49 A/mgPt) or even ethanol (1.55 A/mgPt) in 0.1m HClO4. The L10-CoPt/AuPt catalyst is stable, with negligible degradation in mass activities and Co/Pt/Au composition changes after 10000 cycles.

Intermetallic hard magnetic MPt nanoparticles as robust electrocatalysts for electrochemical reactions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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