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 Ptenriched 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 ∼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 “Pt3Co” NPs. SECTION: Surfaces, Interfaces, Catalysis P exchange membrane fuel cells (PEMFCs) are promising energy conversion devices for automotive applications, but the slow oxygen reduction reaction (ORR) kinetics catalyzed by Pt-based catalysts limits their efficiency and cost. Studies have shown that Pt-alloy surfaces can weaken binding of oxygenated species, and enhance intrinsic ORR activities relative to bulk Pt. For example, acid leaching of bulk Pt alloys can result in Pt enrichment in the near-surface regions causing intrinsic ORR activity two to three times higher than Pt. Recently, aberration-corrected scanning transmission electron microscopy (STEM) imaging of acid-leached Ptalloy nanoparticles (NPs) has provided evidence of the presence of skeleton Pt-enriched surface regions similar to those proposed for bulk Pt surfaces (Scheme 1a). However, the specific activities of Pt-alloy NPs decrease over time during exposure to fuel cell operating conditions, especially under voltage cycling. It is hypothesized that the activity loss of Pt-alloy NPs results from thickening of Pt shells (Scheme 1a) during PEMFC voltage cycling, which might be associated with reduction and deposition of Pt ions dissolved from small particles on the surface of large particles in an Oswald ripeninglike process. Therefore, it is of great importance to understand how the surface composition of Pt-alloy NPs changes during voltage cycling. In this study, we employ aberration-corrected STEM energydispersive spectroscopy (EDS) mapping with a spatial resolution of ∼0.5 nm to study the surface compositional changes of “Pt3Co” NPs in a PEMFC membrane electrode assembly (MEA) cathode before and after voltage cycling. Although recent aberration-corrected STEM spectroscopy studies have revealed surface compositions of as-synthesized Pt-alloy NPs, understanding of alloy NP stability is very limited. Our recent conventional STEM studies have used EDS analysis of Pt and Co from discrete spots across individual “Pt3Co” NPs to show the growth of Pt-enriched surface regions and the development of core/shell NPs upon PEMFC potential cycling. However, the low spatial resolution of ∼2.5 nm only allows for the analysis of “Pt3Co” NPs of ∼10 nm or greater and fails to resolve the Pt shell that is expected to have an average thickness of ∼1.5 nm. Here we report EDS Pt and Co maps of individual NPs, which clearly resolve skeleton Pt-enriched regions of “Pt3Co” NPs (obtained from acid leaching) in the pristine MEA cathode, and the formation of nearly pure Pt shells greater than 1 nm thick on the NP surface in the voltage-cycled MEA cathode. Aberration-corrected STEM high-angle annular dark-field (HAADF) imaging of pristine and cycled MEA cathode cross sections, having high sensitivity to mass contrast, shows Pt loss Received: December 6, 2011 Accepted: December 29, 2011 Published: December 29, 2011 Letter