Search

We present synchrotron x-ray investigations of thin film La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 − δ (LSCF) model cathodes for solid oxide fuel cells, grown on electrolyte substrates by pulse laser deposition, in situ during half-cell operations. We observed dynamic segregations of cations, such as Sr and Co, on the surfaces of the film cathodes. The effects of temperature, applied potentials, and capping layers on the segregations were investigated using a surface-sensitive technique of total external reflection x-ray fluorescence. We also studied patterned thin film LSCF cathodes using high-resolution micro-beam diffraction measurements. We find chemical expansion decreases for narrow stripes. This suggests the expansion is dominated by the bulk pathway reactions. The chemical expansion vs. the distance from the electrode contact was measured at three temperatures and an oxygen vacancy activation energy was estimated to be ~ 1.4 eV.

Infiltrated La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) sintered porous powder cathodes for solid oxide fuel cells have been investigated by synchrotron ultra-small angle x-ray scattering (USAXS). We demonstrated that atomic layer deposition (ALD) is the method for a uniform coating and liquid-phase infiltration for growing nanoscale particles on the porous LSCF surfaces. The MnO infiltrate, grown by ALD, forms a conformal layer with a uniform thickness throughout the pores evidenced by USAXS thickness fringes. The La0.6Sr0.4CoO3 (LSC) and La2Zr2O7 (LZO) infiltrates, grown by liquid-phase infiltration, were found to form nanoscale particles on the surfaces of LSCF particles resulting in increased surface areas. Impedance measurements suggest that the catalytic property of LSC infiltrate, not the increased surface area of LZO, is important for increasing oxygen reduction activities.

Oxygen reduction and transport in electrochemical applications are strongly affected by the reactant gas composition, including impurities such as water. Understanding the relationship between oxygen reactivity and humidity is key for attaining stability and high efficiency of solid oxide fuel cells, which are especially attractive for transportation applications as low emission power generation devices. Both short- and long-term effects of moisture have previously been associated with use of ambient air. To better understand these effects, we study oxygen exchange kinetics at the cathode surface and the cathode/electrolyte interface using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) epitaxial thin films as a model-surface system. In situ synchrotron X-ray techniques evaluate the oxygen reduction reaction (ORR) kinetics in response to environmental variables. The results suggest a clear enhancement of the ORR rate upon short term exposure of the perovskite-structured films to water. On the basis of these measurements, along with in situ X-ray characterization of Sr segregation and computational first-principles studies, we suggest a model for this increased ORR activity that may lead to further improvements by stabilizing active cathode surface configurations.

In situ synchrotron X-ray diffraction was used to investigate oxygen surface exchange behavior in La0.6Sr0.4Co0.2Fe0.8O3−δ/Gd2O3-doped CeO2/Y2O3-stabilized ZrO2 (LSCF/GDC/YSZ) epitaxial thin-film h...

Design of cost-effective electrocatalysts with enhanced stability and activity is of paramount importance for the next generation of energy conversion systems, including fuel cells and electrolysers. However, electrocatalytic materials generally improve one of these properties at the expense of the other. Here, using density functional theory calculations and electrochemical surface science measurements, we explore atomic-level features of ultrathin (hydroxy)oxide films on transition metal substrates and demonstrate that these films exhibit both excellent stability and activity for electrocatalytic applications. The films adopt structures with stabilities that significantly exceed bulk Pourbaix limits, including stoichiometries not found in bulk and properties that are tunable by controlling voltage, film composition, and substrate identity. Using nickel (hydroxy)oxide/Pt(111) as an example, we further show how the films enhance activity for hydrogen evolution through a bifunctional effect. The results suggest design principles for this class of electrocatalysts with simultaneously enhanced stability and activity for energy conversion. Development of electrocatalysts with high stability and activity is a critical challenge. Here, the authors combine simulations with in situ experiments to identify principles underlying simultaneously enhanced stability and activity of ultrathin (hydroxy)oxide films on transition metal substrates.

The methods used to improve catalytic activity are well-established, however elucidating the factors that simultaneously control activity and stability is still lacking, especially for oxygen evolution reaction (OER) catalysts. Here, by studying fundamental links between the activity and stability of well-characterized monometallic and bimetallic oxides, we found that there is generally an inverse relationship between activity and stability. To overcome this limitation, we developed a new synthesis strategy that is based on tuning the near-surface composition of Ru and Ir elements by surface segregation, thereby resulting in the formation of a nanosegregated domain that balances the stability and activity of surface atoms. We demonstrate that a Ru0.5Ir0.5 alloy synthesized by using this method exhibits four-times higher stability than the best Ru-Ir oxygen evolution reaction materials, while still preserving the same activity.

Understanding the functional links between the stability and reactivity of oxide materials during the oxygen evolution reaction (OER) is one key to enabling a vibrant hydrogen economy capable of competing with fossil fuel-based technologies. In this work, by focusing on the surface chemistry of monometallic Ru oxide in acidic and alkaline environments, we found that the kinetics of the OER are almost entirely controlled by the stability of the Ru surface atoms. The same activity-stability relationship was found for more complex, polycrystalline and single-crystalline SrRuO(3) thin films in alkaline solutions. We propose that the electrochemical transformation of either water (acidic solutions) or hydroxyl ions (alkaline solutions) to di-oxygen molecules takes place at defect sites that are inherently present on every electrode surface. During the OER, surface defects are also created by the corrosion of the Ru ions. The dissolution is triggered by the potential-dependent change in the valence state (n) of Ru: from stable but inactive Ru(4+) to unstable but active Ru(n4+). We conclude that if the oxide is stable then it is completely inactive for the OER. A practical consequence is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate of the oxide is neither too fast nor too slow.

Synchrotron-based X-ray techniques have been used to study in situ the structural and chemical changes of film cathodes during half-cell operations. The X-ray techniques used include X-ray reflectivity (XR), total-reflection X-ray fluorescence (TXRF), high-resolution diffraction (HRD), ultra-small angle X-ray scattering (USAXS). The epitaxial thin film model cathodes for XR, TXRF, and HRD measurements are made by pulse laser deposition and porous film cathodes for USAX measurements are made by screen printing technique. The experimental results reviewed here include A-site and B-site segregations, lattice expansion, oxidation-state changes during cell operations and liquid-phase infiltration and coarsening of cathode to electrolyte backbone.

Resonant anomalous X-ray reflectivity (RAXR) is a powerful technique for measuring element-specific distribution profiles across surfaces and buried interfaces. Here, the RAXR technique is applied to characterize a complex oxide heterostructure, La0.6Sr0.4Co0.2Fe0.8O3−δ, on NdGaO3, and the effects of data sampling and model-dependent fitting procedures on the extracted elemental distribution profile are evaluated. The strontium profile through a 3.5 nm-thick film at 973 K and at an oxygen partial pressure of 150 Torr (1 Torr = 133.32 Pa) was determined from the measured RAXR spectra. The results demonstrate thatin situRAXR measurements can provide key insights into temperature- and environment-dependent elemental segregation processes, relevant, for example, in assessing the cathode performance of solid oxide fuel cells.

To better understand the response of oxygen vacancy concentration to applied potential, the lattice parameter of pulsed laser deposited La0.6Sr0.4Co1-xFexO3-δ thin films was monitored using in situ X-ray diffraction. We demonstrate that the chemical expansion under applied potential depends on the cathode morphology, which determines the contribution of different reaction pathways. We investigated applied potential dependent lattice expansion on La0.6Sr0.4Co1-xFexO3-δ with 3 different Co:Fe ratios in an attempt to connect bulk chemical expansion data to thin films. We find that the chemical expansion trends in thin films are different than expected from bulk data.

Active in alkaline environment: The activity of nickel, silver, and copper catalysts for the electrochemical transformation of water to molecular hydrogen in alkaline solutions was enhanced by modification of the metal surfaces by Ni(OH)(2) (see picture; I = current density and η = overpotential). The hydrogen evolution reaction rate on a Ni electrode modified by Ni(OH)(2) nanoclusters is about four times higher than on a bare Ni surface.

Design and synthesis of materials for efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl ions to oxygen in alkaline environments is of paramount importance in reducing energy losses in water–alkali electrolysers. Here, using 3d-M hydr(oxy)oxides, with distinct stoichiometries and morphologies in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) regions, we establish the overall catalytic activities for these reaction as a function of a more fundamental property, a descriptor, OH–M2+δ bond strength (0 ≤ δ ≤ 1.5). This relationship exhibits trends in reactivity (Mn < Fe < Co < Ni), which is governed by the strength of the OH–M2+δ energetic (Ni < Co < Fe < Mn). These trends are found to be independent of the source of the OH, either the supporting electrolyte (for the OER) or the water dissociation product (for the HER). The successful identification of these electrocatalytic trends provides the foundation for rational design of ‘active sites’ for practical alkaline HER and OER electrocatalysts. Efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl ions to oxygen in alkaline environments is important for reducing energy losses in water–alkali electrolysers. Insight into the activities of hydr(oxy)oxides on platinum catalyst surfaces for hydrogen and oxygen evolution reactions should prove significant for designing practical alkaline electrocatalysts.

This Letter reveals new findings on the influence of noncovalent interactions on the electrochemical interface. Using surface X-ray scattering, we demonstrate that the barium cations are located at 3.4 A away from the surface, suggesting that they are partially hydrated, though not adsorbed at the surface. The effect of the cation on the oxygen reduction reaction (ORR) ranges from significant (Pt) to little (Au), depending on the nature of the metal and cation. Finally, we show that these results, as obtained on well-defined single-crystal surfaces, correlate well with observations on high surface area nanoparticle catalysts.

We investigated the relative contribution of the native surface and of the grain boundaries intersecting the surface to the chemical surface exchange of La0.7Sr0.3MnO3 (LSM) thin films. The chemical surface exchange coefficients, kchem, were determined using electrical conductivity relaxation (ECR). Two kchem values were obtained, indicating two surface exchange processes occurred in parallel. These two kchem values had different activation energies (Ea), which were interpreted as belonging to (1) the native surface response of individual grains/variants and (2) the variant / grain boundaries of the textured films. The first process has an activation energy of Ea,1 ≈ 1.57 eV, while Ea,2 ≈ 0.87 eV. The kchem,2 values are almost 3 orders of magnitude higher than the kchem,1 values at low temperatures. At higher temperatures, the data can be fit with one kchem that has an intermediate value of Ea, indicative of a transition region.

We present a rigorous approach for the shape design of supported metal nanoparticle catalysts, morphologically identical to each other and epitaxially grown on strontium titanate substrates using electron beam lithography. We predict the particle shapes using Wulff construction based on density functional theory calculations of surface energies. Then, according to the theoretical predictions, we are able to tweak morphologies of the already produced nanocrystals by changing annealing conditions. The ability to design, produce and characterize the catalyst nanoparticles allows us to relate microscopic morphologies with macroscopic oxygen-reduction activities in perchloric acid [Komanicky et al., J. Am. Chem. Soc. 131 (2009) 5732]. The unexpectedly high oxygen-reduction activities proportional to inactive (1 0 0) facets led us to suggest a model where the reaction intermediates can cross over to neighboring facets in nanoscale proximity.

Monodisperse Pt3Co nanoparticles with size controlled from 3 to 9 nm have been synthesized through an organic solvothermal approach and applied as electrocatalysts for the oxygen reduction reaction. Electrochemical study shows that the Pt3Co nanoparticles are highly active for the oxygen reduction reaction and the activity is size-dependent. The optimal size for maximal mass activity was established to be around 4.5 nm by balancing the electrochemically active surface area and specific activity.

We investigated the underlying physical and chemical causes of the improvement of electrochemical activity of the cathode in solid oxide fuel cells (SOFC) under long term cathodic or anodic polarization, termed 'current conditioning', through in situ X-ray absorption spectroscopy (XAS). The electrodes were powder La0.8Sr0.2MnO3 (LSM) made with spray pyrolysis and thin films of LSM and La0.8Ca0.2MnO3 (LCM) grown by pulsed laser deposition (PLD), all on single crystal Yttria stablized Zirconia (YSZ) substrates. Changes in the XAS of the Mn K, La L3 as well as the Sr K edge for LSM during current conditioning were studied. Our results shows no significant oxidation state changes for the Mn, but instead suggests that the redistribution of cation concentration profiles inside the pervoskite electrode may be the important factor in current conditioning.

The development of electrocatalytic materials of enhanced activity and efficiency through careful manipulation, at the atomic scale, of the catalyst surface structure has long been a goal of electrochemists. To accomplish this ambitious objective, it would be necessary both to obtain a thorough understanding of the relationship between the atomic-level surface structure and the catalytic properties and to develop techniques to synthesize and stabilize desired active sites. In this contribution, we present a combined experimental and theoretical study in which we demonstrate how this approach can be used to develop novel, platinum-based electrocatalysts for the CO electrooxidation reaction in CO(g)-saturated solution; the catalysts show activities superior to any pure-metal catalysts previously known. We use a broad spectrum of electrochemical surface science techniques to synthesize and rigorously characterize the catalysts, which are composed of adisland-covered platinum surfaces, and we show that highly undercoordinated atoms on the adislands themselves are responsible for the remarkable activity of these materials.

We present contact angle measurements of water on single-crystal rutile TiO 2 (110) surfaces, exposed to ambient air, or protected in dry air. Our measurements indicate that the surfaces exposed to ambient air are hydrophobic, with a contact angle of θ=61(5)°. However, the well-protected dry surface also exhibits some hydrophobic tendency, with θ=32(5)°. It is known that UV irradiation transforms both surfaces superhydrophilic, with 0=0° [R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431-432]. We also present preliminary X-ray crystal truncation rod measurements on the hydrophobic TiO 2 (110) surface, and of the effect of UV illumination on the surface.

We report CO-induced lifting of the hexagonal surface reconstruction on Au (001). Using in-situ surface x-ray scattering, we determined a pressure-temperature phase diagram for the reconstruction and measured the dynamical evolution of the surface structure in real time. Our observations provide evidence that, under certain conditions, even macroscopic Au surfaces, much larger than catalytic Au nanoparticles [M. Haruta, Catal. Today 36, 153 (1997)], can exhibit some of the reactive properties and surface transitions observed in systems known to be catalytically active such as Pt (001)., 4 Figures. Accepted as a Letter to Journal of Physical Chemistry C

We used in situ Se K-edge X-ray spectroscopy to characterize Ru nanoparticles chemically modified with submonolayers of selenium (Se/Ru) [Cao et al. J. Electrochem. Soc. 2006, 153, A869]. X-ray powder diffraction verified that the Se/Ru catalyst had metallic Ru cores. The in situ X-ray absorption near edge structure taken at the open circuit potential showed that there were both elemental and oxidized selenium on the as-prepared Se/Ru samples. All selenium oxide was reduced to the elemental form of selenium by applying negative potentials. By applying positive potentials, selenium was subsequently reoxidized. The analysis of the extended X-ray absorption fine structure shows the appearance of selenium hydration (Se-OH{sub 2}) in a deaerated solution, which was not observed during the oxygen reduction reaction. We present evidence that Se-free Ru atoms play an important role in the ORR activity of the Se/Ru catalyst studied in this paper.

We review structures of high-density CO monolayers on Pt(1 1 1) surfaces in CO-saturated electrolytes or in gaseous CO at near atmospheric pressure, using surface X-ray scattering (SXS) and scanning tunneling microscopy (STM). In electrolytes, we confirmed the well-known (2 × 2)-3CO and (√19 × √19)-13CO structures and were able to study the transition between them. For gas-phase studies, we were able to stabilize extremely well-ordered CO monolayers by emersion transfer from an electrochemical cell. We found that the hexagonal close-packed (2 × 2)-3CO structure is the equilibrium phase at room temperature in ∼1 atm CO gas pressure. This commensurate (C) phase transforms continuously to an incommensurate (IC) phase at elevated temperature (a second-order phase transition). We also confirm that the (√19 × √19)-13CO structure is stable at lower CO partial pressure. This C phase transforms discontinuously to an IC phase (a first-order phase transition). A tentative phase diagram and a brief review of structure details of the (2 × 2)-3CO and (√19 × √19)-13CO phases will be presented.

In the present study, we used a surface-science approach to establish a functional link between activity and stability of monometallic oxides during the OER in acidic media. We found that the most active oxides (Au ≪ PtIrRu ≪ Os) are, in fact, the least stable (Au ≫ PtIrRu ≫ Os) materials. We suggest that the relationships between stability and activity are controlled by both the nobility of oxides as well as by the density of surface defects. This functionality is governed by the nature of metal cations and the potential transformation of a stable metal cation with a valence state of n = +4 to unstable metal cation with n+4. A practical consequence of such a close relationship between activity and stability is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate is neither too fast nor too slow.

Three of the fundamental catalytic limitations that have plagued the electrochemical production of hydrogen for decades still remain: low efficiency, short lifetime of catalysts and a lack of low-cost materials. Here, we address these three challenges by establishing and exploring an intimate functional link between the reactivity and stability of crystalline (CoS2 and MoS2) and amorphous (CoSx and MoSx) hydrogen evolution catalysts. We propose that Co(2+) and Mo(4+) centres promote the initial discharge of water (alkaline solutions) or hydronium ions (acid solutions). We establish that although CoSx materials are more active than MoSx they are also less stable, suggesting that the active sites are defects formed after dissolution of Co and Mo cations. By combining the higher activity of CoSx building blocks with the higher stability of MoSx units into a compact and robust CoMoSx chalcogel structure, we are able to design a low-cost alternative to noble metal catalysts for efficient electrocatalytic production of hydrogen in both alkaline and acidic environments.

Resonance anomalous surface x-ray scattering (RASXS) technique was applied to electrochemical interface studies. It was used to determine the chemical states of electrochemically formed anodic oxide monolayers on platinum surface. It is shown that RASXS exhibits strong polarization dependence when the surface is significantly modified. The polarization dependence is demonstrated for three examples; anodic oxide formation, sulfate adsorption, and CO adsorption on platinum surfaces. s- and p- polarization RASXS data were simulated with the latest version of ab initio multiple scattering calculations (FEFF8.2). Elementary theoretical considerations are also presented for the origin of the polarization dependence in RASXS., Comment: 10 pages. To be published in Rad. Phys. Chem as a part of proceedings to X05, Melbourne, Australia, July, 2005

In-situ Se K edge X-ray spectroscopy measurements were carried out for selenium modified ruthenium nanoparticles during oxygen reduction. X-ray powder diffraction verified that the samples had pure Ru cores. In-situ XANES showed that there were significant amounts of oxidized Se in the Ru/Se samples that could be reduced by negative potentials. Selenium in the sample was found to oxidize at electrode potentials of 900 mV and higher (vs. RHE). Analysis of extended x-ray absorption fine structure shows an enhancement of the Se-Ru bonds during oxygen reduction reaction.

Stability and dissolution of platinum single crystal surfaces were investigated with atomic force microscopy and inductively coupled plasma mass spectroscopy. Both macroscopically flat low- index surfaces of (111), (100), and (110), and (111)-(100) nanofaceted surfaces [Komanicky et al., J. Phys. Chem. B, 2005, 109, 23543.] were investigated. A clear difference was observed between the large low-index surfaces and the nanofaceted surfaces. In the low-index surfaces, the platinum oxide formation passivates the surfaces, resulting in a lower overall dissolution rates at higher potentials. On the other hand, the nanofaceted surface dissolve faster at a higher potential indicating the edges and corners are the main sources of dissolution. The differences in the dissolution behaviors between (111), (100), and (110) surfaces will also be discussed.

Methods for the fabrication of large areas of nanoscale features with controlled period and intraperiod organization are of interest because of the potential for high-throughput mass production of nanoscale devices. Due to their potential in this regard, much recent attention has been devoted to self-organization processes, in which processing causes the spontaneous emergence of a nanoscale pattern. The short-range order can be quite high but some envisaged applications require long-range order, which is destroyed by uncontrolled topological defects arising spontaneously from the self-organization process. A potentially successful hierarchical fabrication strategy is the fabrication of controlled features at a small, but lithographically tractable, length scale by methods such as conventional mask or optical-standing-wave lithography, in order to guide a self-organization process at the finest length scale. Topographic patterning has been used for templating the local disorder in two-dimensional (2D) self-assembled monolayers and for templating defect organization or elimination in three-dimensional (3D) colloidal crystallization. Topography has also been used to manipulate semiconductor quantum-dot placement, composition, and strain, through its effect on stress, surface energy, and mobility. 3D short-range ordering of grown-in quantum-dot short-period superlattices can result from multilayer growth; nanoscale topographic templating of the first layer could dramatically accelerate the development of order and lead to true long-range order. Lithographically and focused ion beam (FIB)-patterned topographies have recently been used to template quantum-dot growth in linear chains, periodic 2D lattices, and in more complex configurations that are promising for novel nanoelectronic architectures, such as quantum cellular automata. The finest features have been templated by serial writing with a FIB, a prohibitively expensive process for mass production that might be circumvented by using a hierarchical fabrication strategy. Here we report the influence of patterned boundaries on the primary material of complementary metal oxide semiconductor (CMOS) technology, i.e., a Si(001) substrate, in guiding self-organized topographic ripples spontaneously appearing during uniform irradiation with low energy Ar ions. We show that the long-range order of the features can be greatly enhanced by this lateral-templating approach. The emerging pattern can be manipulated by changing the boundary spacing and misorientation with respect to the projected ion-beam direction. We develop a scalar figure of merit, a dimensionless topological defect density, to characterize the degree of order of the pattern. At small boundary separation, greatest order is observed when the separation is near an integer multiple of the spontaneously arising feature size. The defect density is exceedingly low up to a critical misorientation angle, beyond which topological defects develop in proportion to the incremental misorientation. These results suggest the potential utility of lateral templating ion-irradiation-induced topographic patterns for highthroughput fabrication at sublithographic length scales. Sputter patterning is the spontaneous formation of oneand two-dimensional arrays of topographical ripples and dots on the surfaces of solids eroded by a directed ion beam. This phenomenon has been observed for a wide range of ion species, energies, and incident angles on a variety of materials, including glass, semiconductors, metals, and insulators. Spontaneously emerging feature sizes, which are selected by the kinetics and, consequently, are continuously tunable via the operating conditions, have been reported as small as 15 nm. In general, controlling the behavior of such driven systems is important in many areas of science and engineering. The success of lateral templating raises the possibility of controlling the morphology at a fine length scale in mass production. Because ion irradiation is currently used for doping control in the mass production of semiconductor devices, there may be little impediment to its rapid uptake for morphology control. We investigate the control of the self-organized sputter pattern by using lithography to control only the boundaries of the field over which morphological features evolve. The system chosen for study was silicon with argon-ion irradiation under conditions known to create a topographically rippled surface with small ordered domains at a spontaneously arising “natural” wavelength of k= 140 nm. Details are reported in the Experimental section. The morphological development of templated and non-templated regions was compared after exposure under identical conditions. Figure 1a shows an atomic force microscopy (AFM) topograph of a templated substrate after ion bombardment. The initial configuration consists of a series of 200 nm mesas and 200 nm valleys with a periodicity of 400 nm, as shown in the top trace in Figure 1b. Figure 1a and the bottom trace of Figure 1b C O M M U N IC A IO N S

We present a novel model system for nanoparticle electrocatalysts. A surface consisting of alternating (100) and (111) facets, several nanometers across and nearly 1 microm long, were self-assembled by annealing Pt single crystal surfaces initially cut at the midpoint between [111] and [100] directions, i.e., Pt(1+ square root of 3 1 1). The formation of these self-assembled arrays of nanofacets was monitored by in-situ surface X-ray scattering. These surfaces were further characterized with scanning probe microscopy and cyclic voltammetry. We found that the Pt(1+ square root of 3 1 1) surface is flat with less than 1 nm rms roughness when it was annealed in argon/hydrogen atmosphere. Then the surface forms nanofacets when it is annealed in pure air. This nanofaceting transition was completely reversible and reproducible. We investigated effects of CO adsorption on the voltammetric characteristics of both hydrogen-annealed and air-annealed surfaces. We found that CO-adsorption/desorption cycles in CO containing electrolyte solution result in considerable modification of blank cyclic voltammograms for the both surfaces. We attributed these differences to the electrochemical annealing of surface defects due to the increased mobility during the cycles.

Incorporation of nanoparticles into devices such as solid oxide fuel cells (SOFCs) may provide benefits such as higher surface areas or finer control over microstructure. However, their use with traditional fabrication techniques such as screen-printing is problematic. Here, we show that mixing larger commercial particles with nanoparticles allows traditional ink formulation and screen-printing to be used while still providing benefits of nanoparticles such as increased porosity and lower sintering temperatures. SOFC anodes were produced by impregnating ceria–gadolinia (CGO) scaffolds with nickel nitrate solution. The scaffolds were produced from inks containing a mixture of hydrothermally-synthesised nanoparticle CGO, commercial CGO and polymeric pore formers. The scaffolds were heat-treated at either 1000 or 1300 °C, and were mechanically stable. In situ ultra-small X-ray scattering (USAXS) shows that the nanoparticles begin sintering around 900–1000 °C. Analysis by USAXS and scanning electron microscopy (SEM) revealed that the low temperature heat-treated scaffolds possessed higher porosity. Impregnated scaffolds were used to produce symmetrical cells, with the lower temperature heat-treated scaffolds showing improved gas diffusion, but poorer charge transfer. Using these scaffolds, lower temperature heat-treated cells of Ni–CGO/200 μm YSZ/CGO-LSCF performed better at 700 °C (and below) in hydrogen, and performed better at all temperatures using syngas, with power densities of up to 0.15 W cm−2 at 800 °C. This approach has the potential to allow the use of a wider range of materials and finer control over microstructure.

Advances in the theoretical understanding of interfacial electrochemistry have, over the past decade, permitted the extension of periodic Density Functional Theory studies, which have traditionally been applied to probe chemistry at gas/solid interfaces, to electrochemical systems where potential-dependent structural and chemical phase transformations occur at liquid/solid interfaces. Indeed, such techniques have been employed to study a surprisingly wide class of electrochemical processes, ranging from electrocatalysis to corrosion. In this talk, we will discuss how these strategies can be applied to predict the detailed, atomic-scale properties of monolayer (hydroxy)oxide base metal films on precious metal and precious metal alloy substrates as a function of applied potential. We will describe the analysis of Moire patterns, variable oxidation states, and three-phase boundaries of these films, and we will demonstrate how these results may be used to predict the shape and oxidation states of supported two-dimensional nanoislands as a function of electrochemical conditions. We will next discuss how the three-phase boundaries may promote hydrogen evolution rates in alkaline environments, and we will close with some perspectives on the possibilities for electrocatalyst design offered by these systems. If time permits, we will also discuss a new approach, based on a combination of DFT and kinetic Monte Carlo simulations, that permits direct simulation of polarization curves as a function of sweep rate and that rigorously predicts intrinsic reaction pathways for both electroreduction of nitrates and NO in acidic environments.

In developing cost-effective complex oxide materials for the oxygen evolution reaction, it is critical to establish the missing links between structure and function at the atomic level. The fundamental and practical implications of the relationship on any oxide surface are prerequisite to the design of new stable and active materials. Here we report an intimate relationship between the stability and reactivity of oxide catalysts in exploring the reaction on strontium ruthenate single-crystal thin films in alkaline environments. We determine that for strontium ruthenate films with the same conductance, the degree of stability, decreasing in the order (001)(110)(111), is inversely proportional to the activity. Both stability and reactivity are governed by the potential-induced transformation of stable Ru(4+) to unstable Ru(n4+). This ordered(Ru(4+))-to-disordered(Ru(n4+)) transition and the development of active sites for the reaction are determined by a synergy between electronic and morphological effects.

We will briefly review in situ synchrotron x-ray investigation of model thin film cathode systems for solid oxide fuel cells. The film cathodes examined in this study are (La,Sr)MnO3_δ (LSM), (La,Sr)CoO3_δ (LSC), and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) thin films epitaxially grown on YSZ single crystal substrates by the pulse laser deposition technique. We find in all cases that Sr is enriched or segregated to the surface of the film cathodes. We concluded that the Sr enrichments or segregations are mainly the results of annealing because they do not depend on whether the cathodes are electrochemically biased or not during annealing. However, at least in the case of LSCF, we find that B-site Co segregates rather uniformly to the surface and the segregation responds sensitively and reversibly to the electrochemical bias.

Controlled arrangement of nanometer-scale Ni(OH)2 clusters on platinum electrode surfaces increases the catalytic activity for the hydrogen evolution reaction in KOH solution by a factor of eight relative to state-of-the-art metal and metal oxide catalysts.

Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in alkaline environments is one key to reducing the high overpotentials and associated energy losses in water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni(OH)(2) clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)(2) clusters promoted the dissociation of water and the production of hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li(+)-induced destabilization of the HO-H bond, resulting in a factor of 10 total increase in activity.

We have performed coherent x-ray scattering experiments on the hexagonally reconstructed Pt (001) surface to study the temperature-dependent surface dynamics. By correlating speckle patterns collected at the (001) anti-Bragg position we are able to measure surface dynamics when the averaged incoherent x-ray scattering appears static. In the temperature range above the rotational epitaxy transition and below the roughening transition (1750 K - 1830 K), we have observed well-defined oscillatory autocorrelations of speckles that persist for tens of minutes, in addition to the expected thermal decorrelation. The observed oscillations indicate surface dynamics due to "step-flow" motion. This is shown with a simple model in which the phase of the scattered x-rays from the steps within the illumination area is retained in the coherent x-ray scattering. This demonstrates a possibility that x-ray speckles can be used to monitor the real-space real-time evolution of surfaces in addition to the traditional decorrelation measurements., Comment: 12 pages, 3 figures

Monodisperse Pt(3)Co nanoparticles have been synthesized with size control via an organic solvothermal approach. The obtained nanoparticles were incorporated into a carbon matrix and applied as electrocatalysts for the oxygen reduction reaction to investigate the effects of particle size and pretreatment on their catalytic performance. It has been found that the optimal conditions for maximum mass activity were with particles of approximately 4.5 nm and a mild annealing temperature of about 500 degrees C. While the particle size effect can be correlated to the average surface coordination number, Monte Carlo simulations have been introduced to depict the nanoparticle structure and segregation profile, which revealed that the annealing temperature has a direct influence on the particle surface relaxation, segregation and adsorption/catalytic properties. The obtained fundamental understanding of activity enhancement in Pt-bimetallic alloy catalysts could be utilized to guide the development of advanced nanomaterials for catalytic applications.

We present coherent speckled x-ray diffraction patterns obtained from a monolayer of surface atoms. We measured both the specular anti-Bragg reflection and the off-specular hexagonal reconstruction peak for the Au (0 0 1) surface reconstruction. We observed fluctuations of the speckle patterns even when the integrated intensity appears static. By auto-correlating the speckle patterns, we were able to identify two qualitatively different surface dynamic behaviors of the hex reconstruction depending on the sample temperature., 15 pages, 4 figures. Accepted to Phys. Rev. Lett. 9/2009

We produced millions of morphologically identical platinum catalyst nanoparticles in the form of ordered arrays epitaxially grown on (111), (100), and (110) strontium titanate substrates using electron beam lithography. The ability to design, produce, and characterize the catalyst nanoparticles allowed us to relate microscopic morphologies with macroscopic catalytic reactivities. We evaluated the activity of three different arrays containing different ratios of (111) and (100) facets for an oxygen-reduction reaction, the most important reaction for fuel cells. Increased catalytic activity of the arrays points to a possible cooperative interplay between facets with different affinities to oxygen. We suggest that the surface area of (100) facets is one of the key factors governing catalyst performance in the electrochemical reduction of oxygen molecules.

Publisher Summary This chapter discusses the resonance elastic and inelastic x-ray scattering processes for the in-situ investigation of electrochemical interfaces. X-ray-in and x-ray-out methods with high-energy and high-brilliance x-rays are some important techniques that can be used for the study of electrochemical interfaces. The most widely used technique is x-ray absorption spectroscopy (XAS). An advantage of the x-ray techniques is that they are highly versatile. They have been powerful in case of both single-crystal electrode interfaces and nano-structured electrochemical interfaces. In addition, they serve as promising tools for the surfaces of nanoparticles. This chapter elaborates on the potential new x-ray applications beyond ordinary XAS. Most techniques are possible only because of third-generation synchrotron x-ray sources. The scattering probability of x-rays at an atom can be enhanced when the photon energy is close to an absorption edge of the atom. This is similar to classical resonance phenomena. The discreteness of electronic energy levels in atoms or molecules is inherently quantum mechanical in nature. A quantum mechanical description of the resonance phenomena has also been reviewed in the chapter.

We fabricated and characterized one- and two- dimensional nanoscale arrays of platinum for study of model catalysts. One-dimensional arrays of nanoscale facets were fabricated by annealing a high-index plan of platinum single crystals. The high-index plane forms rows of alternating two low-index facets, (111) and (100), widths of which are ~10 nanometers. Two-dimensional arrays were fabricated lithographically from the epitaxial films of platinum grown on SrTiO3 substrates. Electron beam lithography was used to create precisely registered square arrays of millions of identical platinum nanocrystals with ~30 nm in diameter.

Polarization dependence of resonant anomalous surface x-ray scattering (RASXS) was studied for interfaces buried in electrolytes or in high-pressure gas. We demonstrate that RASXS exhibits strong polarization dependence when the surface is only slightly modified by adsorption of light elements such as carbon monoxide on platinum surfaces. s- and p-polarization RASXS data were simulated with the latest version of ab initio multiple scattering calculations (FEFF8.2). Elementary considerations are additionally presented for the origin of the polarization dependence in RASXS., 15 pages. Submitted to Phys. Rev. Lett

Mesa structures fabricated on Si(111) surfaces have been found experimentally to develop step arrays with large spacing of the order of a micron or more after annealing at temperatures where sublimation becomes important. Ridges around the edges initially develop during annealing and form barriers to step motion before eventually breaking down. This produces an array of steps of the same sign with a few wide terraces. Computer simulations using one dimensional Burton, Cabrera and Frank (BCF) theory including attachment-detachment rates and step-step repulsion for this configuration show that the terraces evolve under different dynamics depending on the terrace widths. For large terrace widths, sublimation dominates the step dynamics and the Ehrlich-Schwoebel effect is negligible. Sinusoidal terrace width distributions result in this case. The experimentally measured step distribution has such a sinusoidal shape suggesting that the step dynamics is sublimation dominated on the mesas after ridge breakdown.

Steps with spacings of microns form on top of mesas fabricated on Si(111) that is annealed at temperatures where sublimation becomes important. Upon annealing, mesas first develop ridges along their edges, effectively creating craters which then become step-free by a step flow process described in the literature. Due to the miscut of the average surface from (111), ridge breakdown occurs on one edge of each mesa as sublimation proceeds. The breakdown point then acts as a source of steps which spread out over the mesa surface. The distribution of steps in the resulting step train depends on the sublimation rate, direct step-step interaction and the diffusive exchange of atoms among the steps. Computer simulations using BCF (Burton, Cabrera and Frank) theory provide insight into the controlling processes. The results suggest that self-organization of the wide terrace distributions in the low step density area occurs under sublimation. We compare the experimental and predicted step distributions., Comment: 11 pages, 3 figures with 7 subfigures revised for submission to Surface Science

Pentacene stands out as a model molecule among organic semiconductors due to its ability to form well-ordered films that show a high field effect mobility. We discuss the processes involved in pentacene film growth, emphasizing differences with respect to inorganic films. The influence of growth parameters such as the substrate nature and temperature, the deposition rate, and the kinetic energy of the molecular beam on the structure and morphology of pentacene films are discussed. Finally, we overview recent attempts to model pentacene film nucleation and growth, and draw attention to the role of dislocations.