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

Area of Science:

• Materials Science
• Surface Science
• Electrochemistry

Background:

• Platinum-cobalt alloys are promising electrocatalysts for the oxygen-reduction reaction (ORR).
• Understanding surface composition and stability is crucial for optimizing catalyst performance.
• Thin-film catalysts offer tunable properties compared to bulk materials.

Purpose of the Study:

• To characterize Pt-Co thin-film electrocatalysts and a bulk Pt(3)Co(111) single crystal.
• To establish the surface phase diagram of Pt-Co alloy films.
• To assess the ORR catalytic activity and stability of these materials.

Main Methods:

• Low-energy ion-scattering spectroscopy (LEISS)
• X-ray photoelectron spectroscopy (XPS)
• Low-energy electron diffraction (LEED)
• Temperature-programmed desorption (TPD)
• Electrochemistry (EC)

Main Results:

• Pt-Co alloy films exhibit a significant divergence between surface and bulk composition after annealing.
• The outermost layer of annealed films is enriched in Platinum (Pt) but not a pure Pt skin; Pt(3)Co is the favored composition.
• Pt(3)Co surfaces, from both thin films and bulk crystals, show the highest ORR activity (OCP), outperforming pure Pt.
• Cobalt (Co) corrosion is the primary degradation mechanism for Pt-Co thin films, occurring even at open-circuit potential.

Conclusions:

• Pt-Co alloy formation leads to Pt enrichment on the surface, enhancing ORR catalysis.
• The absence of a pure Pt skin and the presence of Co are consistent across different substrates.
• Cobalt corrosion is a critical factor limiting the stability of Pt-Co electrocatalysts.
• Further research is needed to mitigate Co corrosion and achieve the thermodynamic potential for ORR.