Dealloyed PtCo 3 and PtCu 3 catalysts supported on high surface area carbon (HSC), which were synthesized under different conditions, were tested as cathode electrodes in proton exchange membrane fuel cells. The dealloyed PtCu 3 / HSC gave higher initial oxygen reduction reaction (ORR) kinetic activity but much worse durability in a voltage cycling test. Detailed characterization was undertaken to develop insights toward the development of catalysts with both high activity and good durability. In situ X-ray absorption spectroscopy (XAS) analysis showed that dealloyed PtCu 3 / HSC exhibited stronger bulk Pt−Pt compressive strains and higher bulk d-band vacancies (attributed in part to a greater ligand effect induced by Pt−Cu bonding) than dealloyed PtCo 3 /HSC, factors which can be expected to correlate with the higher initial activity of dealloyed PtCu 3 /HSC. Annular dark field (ADF) imaging and electron energy loss spectroscopy (EELS) mapping demonstrated that a strong majority of metal nanoparticles in both dealloyed PtCu 3 /HSC and PtCo 3 /HSC have variants of core−shell structures. However, the most prevalent structure in the dealloyed PtCo 3 /HSC gave multiple dark spots in ADF images, approximately half of which were due to Co-rich alloy cores and half of which arose from voids or surface divots. In contrast, the ADF and EELS data for dealloyed PtCu 3 /HSC suggested the predominance of Pt shells surrounding single Cu-rich cores. Further work is needed to determine whether the contrast in durability between these catalysts arises from this observed structural difference, from the differences between the corrosion chemistry of Cu and Co, or from other factors not addressed in this initial comparison between two specific catalysts.

A carbon-supported, dealloyed platinum-copper (Pt-Cu) oxygen reduction catalyst was prepared using a multi-step synthetic procedure. Material produced at each step was characterized using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), electron energy loss spectroscopy (EELS) mapping, x-ray absorption spectroscopy (XAS), x-ray diffraction (XRD), and cyclic voltammetry (CV), and its oxygen reduction reaction (ORR) activity was measured by a thin-film rotating disk electrode (TF-RDE) technique. The initial synthetic step, a co-reduction of metal salts, produced a range of poorly crystalline Pt, Cu, and Pt-Cu alloy nanoparticles that nevertheless exhibited good ORR activity. Annealing this material alloyed the metals and increased particle size and crystallinity. TEM shows the annealed catalyst to include particles of various sizes, large (>25 nm), medium (12–25 nm), and small (<12 nm). Most of the small and medium-sized particles exhibited a partial or complete coreshell (Cu-rich core and Pt shell) structure with the smaller particles typically having more complete shells. The appearance of Pt shells after annealing indicates that they are formed by a thermal diffusion mechanism. Although the specific activity of the catalyst material was more than doubled by annealing, the concomitant decrease in Pt surface area resulted in a drop in its mass activity. Subsequent dealloying of the catalyst by acid treatment to partially remove the copper increased the Pt surface area by changing the morphology of the large and some medium particles to a “Swiss cheese” type structure having many voids. The smaller particles retained their core-shell structure. The specific activity of the catalyst material was little reduced by dealloying, but its mass activity was more than doubled due to the increase in surface area. The possible origins of these results are discussed in this report.

Following earlier work of Huggins and Nix [Ionics6, 57 (2000)], several recent theoretical studies have used the shrinking core model to predict intraparticle Li concentration profiles and associated stress fields. A goal of such efforts is to understand and predict particle fracture, which is sometimes observed in degraded electrodes. In this paper we present experimental data on LiCoO2 and graphite active particles, consistent with previously published data, showing the presence of numerous internal pores or cracks in both positive and negative active electrode particles. New calculations presented here show that the presence of free surfaces, from even small internal cracks or pores, both quantitatively and qualitatively alters the internal stress distributions such that particles are prone to internal cracking rather than to the surface cracking that had been predicted previously. Thus, the fracture strength of particles depends largely on the internal microstructure of particles, about which little is known, rather than on the intrinsic mechanical properties of the particle materials. The validity of the shrinking core model for explaining either stress maps or transport is questioned for particles with internal structure, which includes most, if not all, secondary electrode particles.

A novel synthesis technique has been developed that yields monodisperse Pt particles in electrostatically stabilized suspensions without the use of structure directing organic surfactants. The approach uses stannous chloride as both reducing and stabilizing agent to form multifaceted Pt single crystal nanoparticles and clusters of less than 20 atoms. These particles may be assembled into layered electrode structures having well-controlled Pt loadings without precipitation onto organic supports or sintering to remove organic residues, both of which are known to yield particle aggregation and the formation of nonregular structures. Consequently, the particles may be used for fundamental investigations on the effect of platinum dispersion on catalytic activity never previously possible. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of these particles provides the first direct evidence that peak oxygen reduction reaction (ORR) activity with increased catalyst dispersion is associated with the crystal to cluster transition and a change in reaction mechanism as reflected by the change in the Tafel slope from 120 mV/decade for the crystals to 220 mV/decade for the clusters at high current density. ORR mass activities obtained at 0.9 V versus reversible hydrogen electrode (RHE) from rotating disk electrode (RDE) experiments in perchloric acid were found to systematically vary from a minimum of about 18 A/g for the atomic clusters, to about 48 A/g for the single crystals, to a peak activity of 74 A/g for transitional structures (twice the value measured on commercial catalyst). Furthermore, the peak electrochemically active area (ECA) obtained from proton underpotential deposition is found to occur well within the atomic cluster regime.

Layer‐by‐layer (LBL) assembly of carbon nanoparticles for low electrical contact resistance thin film applications is demonstrated. The nanoparticles consist of irregularly shaped graphite platelets, with acrylamide/ββ‐methacryl‐oxyethyl‐trimethyl‐ammonium copolymer as the cationic binder. Nanoparticle zeta (ζζ) potential and thereby electrostatic interactions are varied by altering the pH of graphite suspension as well as that of the binder suspension. Film thickness as a function of zeta potential, immersion time, and the number of layers deposited is obtained using Monte Carlo simulation of the energy dispersive spectroscopy measurements. Multilayer film surface morphology is visualized via field‐emission scanning electron microscopy and atomic‐force microscopy. Thin film electrical properties are characterized using electrical contact resistance measurements. Graphite nanoparticles are found to self‐assemble onto gold substrates through two distinct yet overlapping mechanisms. The first mechanism is characterized by logarithmic carbon uptake with respect to the number of deposition cycles and slow clustering of nanoparticles on the gold surface. The second mechanism results from more rapid LBL nanoparticle assembly and is characterized by linear weight uptake with respect to the number of deposition cycles and a constant bilayer thickness of 15 to 21 nm. Thin‐film electrical contact resistance is found to be proportional to the thickness after equilibration of the bilayer structure. Measured values range from 1.6 mΩ cm−2 at 173 nm to 3.5 mΩ cm−2 at 276 nm. Coating volume resistivity is reduced when electrostatic interactions are enhanced during LBL assembly.