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

Abstract

Rotating disk electrode measurements of acid-treated "Pt 3 Co" nanoparticles showed specific oxygen reduction reaction (ORR) activity (∼0.7 mA/cm Pt 2 at 0.9 V vs RHE in 0.1 M HClO 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/cm Pt 2 (four times that of Pt nanoparticles). High-resolution transmission electron microscopy and aberrationcorrected high-angle annular dark-field in the scanning transmission electron microscope was used to reveal surface atomic structure and chemical composition variations of "Pt 3 Co" 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 3 Co" 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 3 Co" nanoparticles relative to Pt can be attributed to the presence of surface Pt segregation.

Understanding the electrocatalysis of oxygen reduction on platinum and its alloys

“…Nonetheless, several research groups have reported the synthesis of ''Pt-skin'' structures upon carbon-supported Pt-alloy nanoparticles. 90,151,168,169 They achieved this either through hightemperature annealing in an inert or reducing atmosphere or by electrochemical cycling in a CO-saturated electrolyte. Each of these studies demonstrated a higher ORR activity than for a standard leached ''Pt-skeleton'' catalyst.…”

Synergetic Effect of Surface and Subsurface Ni Species at Pt−Ni Bimetallic Catalysts for CO Oxidation

“…Both the compressive strain effect and the ligand effect contribute to the modification of the surface catalytic properties. 12,32,51,52 Moreover, the Pt-skin surfaces are indeed the stable surface structure in acid solutions for the ORR reactions 9,14,[17][18][19]53 and in the reductive atmospheres for the hydrogenation reactions. [54][55][56] In contrast, there is no consistent picture for the active surface structure of the Pt-based bimetallic catalysts in oxidation reactions, such as the CO oxidation.…”

Components for PEM Fuel Cells: An Overview

“…Several Pt alloys, including late transition metals such as Ni, Co, Cr and Fe, together with partially dealloyed core-shell catalysts derived from Pt-Cu nanoparticles, are considerably more active than Pt and have been studied intensively [145][146][147][148][149][150][151][152][153][154][155]. The reasons for the higher catalytic activity of Pt based binary catalysts have been reported to be due to (i) an increase in the resistance to particle sintering, (ii) surface roughening due to removal of some base metal, increasing the Pt surface area (iii) preferred crystallographic orientation (iv) geometric factors (decreased Pt-Pt bond distance) [156], (v) dissolution of the more oxidisable alloying component [157], (vi) change in surface structure [158] or electronic factors (increased Pt dband electron vacancy of the Pt skin layer originating from the bulk alloys) (vii) oxygen adsorption differences due to modified anion and water adsorption [159,160] .…”