Sub-Nanometer-Resolution Elemental Mapping of Pt3Co Nanoparticle Catalyst Degradation in Proton-Exchange Membrane Fuel Cells

The efficiency of proton exchange membrane fuel cells (PEMFCs) is limited largely by sluggish oxygen reduction reaction (ORR) kinetics, even when promoted by Pt-based alloy nanoparticles (NPs). Acid-leached Pt alloys such as Pt3Co have shown considerably higher specific (2-5 times) and mass (2 to 3 times) ORR activity than Pt NPs. However, the specific activity enhancement of Pt3Co NPs decreases during PEMFC operation, which has been attributed to the formation of a Pt-enriched shell near the NP surfaces. In this study, we report direct evidence of surface Pt and Co compositional changes in acid-treated Pt3Co NPs after PEMFC voltage cycling using energy-dispersive spectroscopy mapping in an aberration-corrected scanning transmission electron microscope with subnanometer resolution. Acid-treated Pt3Co NPs were found to have Pt-enriched shells of similar to 0.5 nm, whereas the Pt-enriched-shell became thicker (similar to 1-6 nm) after PEMFC voltage cycling, where greater shell thicknesses were associated with larger Pt3Co NPs.