Origin of oxygen reduction reaction activity on "Pt ₃Co" nanoparticles: Atomically resolved chemical compositions and structures

Rotating disk electrode measurements of acid-treated "Pt ₃Co" nanoparticles showed specific oxygen reduction reaction (ORR) activity (~0.7 mA/em _Pt ² at 0.9 V vs RHE in 0.1 M HC10 ₄ 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 ~ 1.4 mA/em _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 "Pt ₃Co" 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 "Pt ₃Co" 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 "Pt ₃Co" nanoparticles relative to Pt can be attributed to the presence of surface Pt segregation.