Structural, compositional and electrochemical characterization of Pt-Co oxygen-reduction catalysts.

Pt-Co thin-film electrocatalysts have been characterized using low-energy ion-scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and electrochemistry (EC). For comparative purposes, LEISS and EC were also carried out on a bulk Pt(3)Co(111) single crystal. The extensive experimental work resulted in the establishment of the surface phase diagram of the alloy film marked by a substantial divergence between the composition at the interface and that in the interior. When a dual-layer deposit of Pt and Co was annealed at high temperatures, alloy formation transpired in which the outermost layer became single-crystalline and enriched in Pt. The preferential surface segregation of Pt, however, was not sufficient to generate a platinum-only overlayer or "skin". Invariably, Co was found to co-exist with Pt, independent of the substrate [Mo(110) or Ru(0001)] employed; Pt(3)Co was the most favored composition. The same result, the absence of a Pt skin, was likewise indicated at the post-thermally-annealed surface of the bulk Pt(3)Co(111) monocrystal. For alloy-film surfaces more enriched in Pt than Pt(3)Co, the topmost layer was constituted primarily, but not exclusively, of Pt(111) domains. The proclivities of the alloys towards enhanced catalysis of the oxygen-reduction reaction were assessed in terms of their voltage efficiencies, as manifested by the open-circuit potential (OCP) in O(2)-saturated sulfuric acid electrolyte. The Pt(3)Co surface, whether from the thin film or the bulk single crystal, exhibited the highest OCP, a significant improvement over pure Pt but still appreciably lower than the thermodynamic limit. The degradation of the Pt(3)Co thin-film surface was predominantly due to Co corrosion. A minimal amount was spontaneously dissolved upon simple immersion in solution; slightly higher dissolution occurred at potentials above the OCP. The fraction that was not immediately corroded proved to be stable even after prolonged periods at potentials more positive than the OCP.