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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.

Dealloyed Pt-Cu alloy nanoparticles are active oxygen reduction electrocatalysts; they are formed from Cu-rich alloy precursors during a selective Cu atom dissolution (dealloying) process. The surface of Cu-rich particle precursors is prone to oxidation under ambient air conditions, which may critically affect the aging behavior of the precursors. Here, we present a systematic stability and aging study of a carbon-supported Pt25Cu75 alloy nanoparticle catalyst precursor. We study the impact of the aging of the catalyst material on its electrocatalytic performance for the oxygen reduction reaction (ORR) after dealloying. We obtain a practical insight into the electrochemical behavior of the materials in the formats of powders, inks, and films. Our studies suggest that the Pt-Cu precursors show a stable catalytic performance when aged as dry powders in air. Ink samples, however, reach their maximum ORR activity of up to 1.3 A/mgpt with aging for 24-48 h after which they deteriorated in performance. Finally, catalyst thin films were the most sensitive to aging in air and generally deteriorated rapidly after just one day. Our results provide practical insights and guidelines regarding the stability and handling of the nanoparticle catalyst powder. [ABSTRACT FROM AUTHOR]

Dealloyed Pt25Cu75 bimetallic nanoparticle electrocatalysts exhibit up to six times higher oxygen reduction reaction activities than pure nanoparticle Pt catalysts at 0.9 V/ reversible hydrogen electrode (RHE). The active form of the catalyst is formed in situ from Pt-Cu precursor material using voltammetric dealloying. The effects of composition of recursors as well as effects of the annealing temperature and duration on the catalyst activity are studied. We vary the composition between Pt25Cu75 and Pt75Cu25 and change the annealing conditions from 600 to 950°C and for 7 and 14 h. X-ray diffraction and electrochemical analyses are used to obtain insight on the structural details of the catalyst samples. Information regarding the extent of alloying, atomic ordering, the Pt and Cu compositions, and distributions on the nanoparticles and particle (crystallite) sizes is correlated with the trends observed from mass and specific activities of the catalysts. It was found that an annealing duration of 14 h offers little or no benefit to catalytic activities compared to 7 h. Dealloyed Pt25Cu75 annealed for 7 h, at 800°C yielded an optimal active material with respect to the extent of alloying and particle size growth and exhibited the highest Pt mass-based and favorable specific catalytic oxygen reduction reaction (ORR) activity. The occurrence and role of a noncubic Pt50Cu50 Hongshiite phase is discussed. [ABSTRACT FROM AUTHOR]

Abstract: We report on synthesis–structure–activity–stability relationships of Pt3Co nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). We have synthesized Pt3Co alloy electrocatalysts using liquid impregnation techniques followed by reductive annealing at high and low temperatures. We have performed detailed structural X-ray diffraction (XRD)-based structural characterization (symmetry, lattice parameters and composition) of individual Pt–Co alloy phases before and, importantly, after electrochemical rotating disk electrode (RDE) measurements. This enables us to directly evaluate the corrosion stability of various Pt–Co alloy phases under typical fuel cell cathode conditions. Pt3Co prepared at low annealing temperatures (600°C) resulted in multiple phases including (i) a disordered face-centered cubic (fcc) Pt95Co5 phase and (ii) an ordered face-centered tetragonal (L10) Pt50Co50 phase; high temperature annealing (950C) resulted in a single ordered primitive cubic (L12) Pt3Co phase. The ordered alloy phases in both catalysts were not stable under electrochemical treatment: The ordered face-centered tetragonal (fct) phase showed corrosion and dissolution, while the ordered primitive cubic (L12) Pt3Co phase transformed into a disordered structure. The ordered primitive cubic structure exhibited higher resistance to sintering. Low annealing temperatures resulted in higher Pt surface-area specific activities for ORR. Kinetic Tafel analysis confirmed a general shift in the formation potential of oxygenated surface species, such as Pt–OH, for both alloy catalysts. Reduced OH coverage alone proved insufficient to account for the observed activity trends of the two alloy catalysts. [Copyright &y& Elsevier]