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 “Pt 3 Co” 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 “Pt 3 Co” 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 “Pt 3 Co” NPs after PEMFC voltage cycling using energy-dispersive spectroscopy mapping in an aberration-corrected scanning transmission electron microscope with subnanometer resolution. Acid-treated “Pt 3 Co” NPs were found to have Pt-enriched shells of ∼0.5 nm, whereas the Pt-enriched-shell became thicker (∼1–6 nm) after PEMFC voltage cycling, where greater shell thicknesses were associated with larger “Pt 3 Co” NPs.