The detailed structure and composition (surface and bulk) as well as catalytic activity for oxygen reduction of electrochemically dealloyed PtCu3 thin films have been investigated. Synchrotron-based anomalous X-ray diffraction (AXRD) reveals that a Pt enriched surface region (∼1.0 nm thick) and a Cu depleted interior (atomic ratio different from that of PtCu3) are formed in the dealloyed film, and we directly observe a compressive lattice strain in the Pt surface region. The dealloyed PtCu3 thin films show a ∼2.4 fold increase in the specific oxygen reduction activity over pure Pt thin films as measured by a rotating disk electrode (RDE). Our results show that the enhanced catalytic activity of the dealloyed Pt−Cu film is primarily due to the compressive strain in the surface layer (ligand effect is very weak). We compare our results on thin films to related results on nanoparticles. These studies provide a better understanding of the structure − composition and structure − activity relationships in Pt-skeleton structures prepared by dealloying base-metal-rich alloys.

We establish relationships between the atomic structure, composition, electrocatalytic activity, and electrochemical corrosion stability of carbon-supported Pt-Co alloy nanoparticles in electrode catalyst layers. These Pt-Co catalysts have received much attention for use as cathode layers in polymer electrolyte membrane fuel cells (PEMFCs) because of their favorable oxygen-reduction-reaction (ORR) activity and suspected corrosion stability. We reported an enhancement of activity of low-temperature Pt 50 Co 50 of 3 times that of pure carbon supported Pt catalysts. The use of synchrotron X-ray diffraction has enabled structural characterization of the alloy nanoparticles both before and, importantly, after electrocatalysis under fuel cell like conditions. From this, a detailed picture of the relative activity and stability of Pt-Co alloy phases as a function of synthesis conditions has emerged. We have investigated the structure, composition, chemical ordering, and concentration of Pt-Co alloy phases in (i) a dry, freshly synthesized nanoparticle catalyst, (ii) the catalytic electrode layer in a proton-conducting polymer electrolyte before electrocatalytic activity, and (iii) the same electrode layer after electrocatalytic activity. We find that Pt 50 Co 50 catalysts annealed at 600°C consist of multiple phases: a chemically ordered face-centered tetragonal (fct) and two chemically disordered face-centered cubic (fcc) phases with differing stoichiometries. The Co-rich fcc phase suffers from corrosive Co loss during the preparation of conducting polymer electrode layers and, more significantly, during the ORR electrocatalysis. Most importantly, these fcc phases exhibit high catalytic activities for ORR (about 3× compared to a pure Pt electrocatalyst). Pt 50 Co 50 catalysts annealed at 950°C consist mainly of the fct Pt 50 Co 50 phase. This phase shows favorable stability to corrosion in the conducting polymer electrode and during electrocatalysis, as the relative intensities of fcc(111)/fct(101) peak ratio remained consistently around 0.5 before and after preparation of conducting polymer electrode layers and before and after electrochemical measurements; however, it exhibits a lower catalytic ORR activity compared to the low-temperature fcc alloy phases (about 2.5× compared to a pure Pt electrocatalyst). Our results demonstrate the complexity in these multiphase materials with respect to catalyst activity and degradation. By understanding of the relationships between crystallographic phase, chemical ordering, composition, and the resulting electrochemical activity and corrosion stability of fuel cell catalysts within polymer-electrolyte/catalyst composites, we can move toward the rational design of active and durable catalyst materials for PEMFC electrodes.

Anomalous small angle X-ray scattering (ASAXS) is shown to be an ideal technique to investigate the particle size and particle composition dynamics of carbon-supported alloy nanoparticle electrocatalysts at the atomic scale. In this technique, SAXS data are obtained at different X-ray energies close to a metal absorption edge, where the metal scattering strength changes, providing element specificity. ASAXS is used to, first, establish relationships between annealing temperature and the resulting particle size distribution for Pt25Cu75 alloy nanoparticle electrocatalyst precursors. The Pt specific ASAXS profiles were fitted with log-normal distributions. High annealing temperatures during alloy synthesis caused a significant shift in the alloy particle size distribution towards larger particle diameters. Second, ASAXS was used to characterize electrochemical Cu dissolution and dealloying processes of a carbon-supported Pt25Cu75 electrocatalyst precursor in acidic electrolytes. By performing ASAXS at both the Pt and Cu absorption edges, the unique power of this technique is demonstrated for probing composition dynamics at the atomic scale. These ASAXS measurements provided detailed information on the changes in the size distribution function of the Pt atoms and Cu atoms. A shift in the Cu scattering profile towards larger scattering vectors indicated the removal of Cu atoms from the alloy particle surface suggesting the formation of a Pt enriched Pt shell surrounding a Pt-Cu core. Together with XRD and TEM, ASAXS is proposed to play an increasingly important role in the mechanistic study of degradation phenomena of alloy nanoparticle electrocatalysts at the atomic scale.