Origin of Oxygen Reduction Reaction Activity on "Pt3Co" Nanoparticles: Atomically Resolved Chemical Compositions and Structures

Rotating disk electrode measurements of acid-treated "Pt3Co" nanoparticles showed specific oxygen reduction reaction (ORR) activity (similar to 0.7 mA/cm(pt)(2) at 0.9 V vs RHE in 0.1 M HClO4 at room temperature), twice that of Pt nanoparticles. Upon annealing at 1000 K in vacuum, the ORR activity at 0.9 V was increased to similar to 1.4 mA/cM(pt)(2) (four times that of Pt nanoparticles). High-resolution transmission electron microscopy and aberration-corrected high-angle annular dark-field in the scanning transmission electron microscope was used to reveal surface atomic structure and chemical composition variations of "Pt3Co" nanoparticles on the atomic scale. Such information was then correlated to averaged Pt-Pt distance obtained from synchrotron X-ray powder diffraction data, surface coverage of oxygenated species from cyclic voltammograms, and synchrotron X-ray absorption spectroscopy. It is proposed that ORR activity enhancement of acid-leached "Pt3Co" relative to Pt nanoparticles is attributed to the formation of a percolated structure with Pt-rich and Pt-poor regions within individual particles, while the increase in the specific ORR activity of annealed "Pt3Co" nanoparticles relative to Pt can be attributed to the presence of surface Pt segregation.