Program - Symposium U: Materials for Catalysis in Energy

The development of energy storage technologies having a great potential to be scaled-up is highly demanded by the energy sector. Many researchers have proposed the all-vanadium redox flow battery (VRB) as a promising grid energy storage system for renewable energy due to its low cost, modular configuration, flexible operation, fast power response, high energy efficiency, long service lifetime and environmental compatibility. However, this technology shows some drawbacks, such as a relatively poor energy-to-volume ratio, with a power-to-weight ratio of ca. 25-35 Wh kg-1, which is quite low if compared to other rechargable battery types. Within this framework and taking into account that the electrochemical kinetic limitations of VRB is found in the positive side ([VO2]+/[VO]2+ redox couple), we have focused on the development of novel electrodes with high catalytic activity towards the [VO2]+/[VO]2+ redox reaction. These novel electrodes have been prepared on carbon-supported Pt-based electrodes in combination with iron or cupper metals (e.g. Fe-Pt /C and Cu-Pt/ C electrodes) in order to assess the role of both. These electrodes have demonstrated higher electrocatalytic activities than the simplex Pt catalyst/C towards the positive reaction of VRB. Different stoichiometric series of (Fe-Pt) and (Cu-Pt) nanoparticles were prepared in order to comprehensive elucidation of the optimal stoichiometry to achieve the best electrocatalytic performance of the bimetallic nanoparticles on carbon-supported. The physical and electrochemical properties of the as-prepared electrocatalysts are investigated by High- resolution transmission electron microscopy and energy diffraction x-ray, cyclic voltammetry, spectroscopy impedance electrochemical, chronoamperometry and chronopotentiometry. The carbon-supported electrodes containing (Cu-Pt or Fe-Pt) nanoparticles have shown a very positive synergistic contribution of both metals, which is attributed to the Pt-OH, Fe-OH and Cu-OH bonds that favor the adsorption of a large amount of vanadium ions. As a result, the electron and oxygen tranfer processes involved in the [VO2]+/[VO]2+ redox reaction become accelerated at the decorated electrodes open wide possibility for the electrode improvement.

One of the major challenges toward commercialization of proton exchange membrane fuel cells (PEMFCs) is the development of active and stable cathode catalysts [1]. The current industry standard for PEMFC cathodes consists of Pt nanoparticles supported onto high surface area carbon (Pt/C) [2]. Pt/C has high specific and mass activities but limited stability under operating conditions [1]. One morphology that is known to exhibit excellent stability and high per-site specific activity is flat Pt, as found for bulk Pt samples [3]. However, Pt utilization in bulk Pt catalyst is low, leading to low mass activities and greater costs [2]. In an effort to combine the stability and per-site activity of bulk Pt and mass activity of Pt/C nanoparticles, we directed Pt morphology at the nano-scale and synthesized an extended 3D mesoporous Pt network with double gyroid (DG) structure. The high surface area DG Pt shows better specific activity for the oxygen reduction reaction (ORR) than commercial Pt/C, as measured by the intrinsic kinetic current density per mass and per surface area. An accelerated stability test performed by potential cycling up to 10,000 cycles demonstrates that DG Pt thin film catalyst maintains over 80% of the initial electrochemical surface area (ECSA), while Pt/C nanoparticulate catalyst loses over half of its ECSA. Consequently, the meso-structured Pt film successfully combines excellent durability and high turnover frequency (TOF) of bulk Pt with the high surface area and mass activity of Pt/C nanoparticles. To further improve activity of meso-structured catalyst, we synthesized DG alloy catalysts and evaluated their ORR activity and stability. References: [1] W. Vielstich, H. A. Gasteiger, A. Lamm, & H. Yokokawa. Handbook of Fuel Cells: Fundamentals, Technology, and Applications. (John Wiley & Sons Ltd, 2009). [2] Gasteiger, H. A., Kocha, S. S., Sompalli, B. & Wagner, F. T. Appl. Catal., B 56, 9-35, (2005). [3] Komanicky, V. et al. J. Electrochem. Soc. 153, B446-B451, (2006).

We have compared the rates of CO formation on Cu and Cu oxide surfaces during the electrochemical reduction of CO2. Rates are measured in aqueous KHCO3 electrolyte saturated with continuously flowing CO2 using a two-compartment cell with Nafion membrane separator and three-electrode configuration. Copper foil electrodes are used “as received” after cleaning using either HCl etching or H3PO4 electropolishing. Copper oxide electrodes are prepared by electrochemical deposition onto Cu foils from CuSO4 and lactic acid bath. The CO2 reduction behavior of the metallic Cu electrodes is similar to previous literature reports. Hydrogen formation is the main reaction at potentials less cathodic than –1.4 V(SCE). At -1.4 V(SCE) the formation of CO becomes significant, and CH4 is eventually observed at cathodic potentials beyond –1.6 V(SCE). The behavior of the Cu oxide electrodes shows a complex transient response. At constant potential (–1.4 V(SCE)), there is a large, transient cathodic current, accompanied by a color change of the electrode. The integrated current during this transient correlates with the thickness of the electrochemically deposited film, and indicates that most of the film is being reduced at this potential. After the initial oxide reduction transient, the current density stabilizes and the formation rates of H2 and CO show a more slowly varying transient behavior. The H2 formation rate increases with time on stream, and begins to approach the rate observed on freshly cleaned Cu foil. The magnitude of the difference below the rate on Cu metal correlates with the thickness of the initial oxide layer. In contrast, the CO formation rate is at least one order of magnitude higher on the (reduced) Cu oxide samples than on Cu foil at the same potential. The CO formation rate increases with the thickness of the initial oxide layer, but decays significantly with time on stream. These results are consistent with a model in which the initial Cu oxide layer is rapidly reduced, creating a mixture of highly dispersed Cu clusters (which continue to evolve with time) and residual Cu oxide clusters (which continue to decay). Some fraction of the underlying Cu foil substrate may also become exposed during the film reduction process. Hydrogen formation occurs primarily at Cu metal surfaces, and is inhibited by the presence of larger amounts of Cu oxide (cf. the thickness dependence noted above) but becomes more facile as the Cu oxide layer becomes more fully reduced (cf. time-on-stream results). In contrast, the CO formation rate is enhanced on the Cu oxide clusters, or possibly on highly dispersed Cu clusters that are formed by the Cu oxide reduction process.

The noble metal electrocatalysts commonly used in hydrogen fuel cells have severe limitations in their cost and availability, and also require undesirably high overpotentials to operate at useful current densities. We have chosen to search for potential replacements among complex oxides of early transition metals (Ti, Nb, Ta) which are likely to strongly resist dissolution in acidic conditions. Although these oxides are typically wide band gap semiconductors, starting compositions and synthetic conditions have been chosen to produce reduced transition metal oxides which are black in color indicating high carrier concentrations and the potential for good electronic conductivity. The catalytic performance of these compounds (in both acidic and basic media) has been measured, and the dependence of activity on structure, physical properties, and electron counts will be discussed for this general family of materials.

Carbon nanotubes (CNT) have shown promising characteristics as alternative catalyst support material for proton exchange membrane (PEM) fuel cell. Previous studies on CNT based electrode have suggested that CNT support with high surface area and chemical stability could enhance the electrocatalyst’s activity and stability by their active interaction. However, CNTs are reportedly difficult to coat with metal particles on their inert surface without surface oxidation treatment. In addition, an additional carbon black based gas diffusion layer (GDL) is still necessary for CNT supported catalyst thus limiting its overall effectiveness. In this work, a complex Pt/CNT based electrode has been fabricated by in-situ growth of a dense CNT layer on plain carbon paper using thermal chemical vapour deposition (CVD) technique followed by direct sputter deposition of Pt nanoparticles onto the CNT layer. Furthermore, by modifying the CNT catalyst, aligned CNTs perpendicular to the substrate has been obtained. The in-situ grown aligned CNT layer on carbon paper showed tunable diameter and density under different growth processes. Transmission electron microscopy (TEM) micrographs showed a very good dispersion of Pt nanodots on the surface of the CNTs. Raman spectroscopy demonstrated an ID/IG of 0.42, revealing a high crystalline of the aligned CNTs. The maximum power density based on Pt loading is 14.7 W/mg Pt for the Pt/CNT-based electrode, compared to 12.4 W/mg Pt for the Pt/VXC72R-based electrode under a very low 0.04 mg/cm^2 Pt loading. In-situ characterization techniques such as cyclic voltammetry, electrochemical impedance spectroscopy and accelerated degradation tests (ADT) have been carried out to fully evaluate the Pt/aligned CNT based electrode. Results confirmed that the Pt/CNT catalyst prepared by the combined fabrication method can yield higher Pt utilization as well as superior electrochemical stability, demonstrating that an optimum aligned CNT layer is able to provide extremely high surface area and porosity which can serve both as the GDL and catalyst layer simultaneously. Moreover, the reduced charge transfer and mass transport resistance of the electrode further suggest that this integrated CNT GDL and catalyst layer has an intrinsic structural merit, where the high mass transport was attributed to the unique aligned structure of CNTs resulting in reduced water flooding, higher conductivity and ease of airflow pathway. ADT showed that the aligned CNTs excellent corrosion resistance properties, most probably due to the high crystallinity of the ACNTs.

Nanocomposite materials consisting of platinum deposited on carbon nanotubes are emerging electrocatalysts for the oxygen reduction reaction in PEM fuel cells. However, these materials albeit showing promising electrocatalytic activities suffer from unacceptable rates of corrosion during service. This study demonstrates an effective strategy for creating highly corrosion-resistant electrocatalysts utilizing metal oxide coated carbon nanotubes as a support for Pt. The electrode geometry consisted of a three-dimensional array of multi-walled carbon nanotubes grown directly on Inconel and conformally covered by a bilayer of Pt/niobium oxide. The activities of these hybrid carbon-metal oxide materials are on par with commercially available carbon-supported Pt catalysts. We show that a sub-nanometer interlayer of NbO2 provides effective protection from electrode corrosion. After 10,000 cyclic voltammetry cycles in the 0.5 V to 1.4 V range, the loss of electrochemical surface area, reduction of the half-wave potential, and the loss of specific activity of the NbO2 supported Pt were 10.8%, 8 mV and 10.3%, respectively. Under the same conditions, the catalytic layers with Pt directly deposited onto carbon nanotubes had a loss of electrochemical area, reduction of half-wave potential and loss of specific activity of 47.3 %, 65 mV and 65.8%, respectively. The improved corrosion resistance is supported by microstructural observations of both electrodes in their post-cycled state. First principles calculations at the density functional theory level were performed to gain further insight into changes in wetting properties, stability and electronic structure introduced by the insertion of the thin NbO2 film.

We are developing novel Pd-Pt bimetallic catalysts that have much less platinum content but drastically enhanced activity and durability for ORR. By focusing on a number of well-defined, highly tunable, and precisely controllable systems, we have addressed the following issues: i) how does the facet or arrangement of atoms on the surface affect the ORR activity; ii) how do the thickness and morphology of the Pt overlayer affect the coupling between the Pd and the Ptm; and iii) how does the morphology of a bimetallic electrocatalyst contribute to the enhancement in stability by altering the sintering or ripening kinetics. In this talk, I will focus on our recent progress in the design and synthsis of these novel bimettalic catalysts.

Advances in nanotechnology have demonstrated the important roles of particle size, shape, and component distribution in determining nanocatalysts' performance for applications in renewable energies. For example, low-Pt-content, active, and durable metal nanocatalysts are required for commercialization of fuel cells and for lowering the cost of hydrogen generation through water electrolysis. Enhanced catalytic performances were found on several core-shell type nanoparticles with narrow particle size distribution[1-3]. There is a great need for methods that can produce them in high quality at low cost for large scale commercialization. This presentation will discuss the synthesis methods based on ethanol as the solvent and reducing agent to fabricate core-shell nanocatalysts without any capping agents. One example is Ru(core)-Pt(shell) nanoparticles supported on carbon black and nanotubes with or without functional groups. Besides structural characterization of the nanocatalysts, in-depth discussion will be given on how to evaluate catalysts’ performance for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) on catalyzed gas diffusion electrodes and in the membrane electrode assembly tests in water electrolyzers and fuel cells, respectively. The results revealed that the HER-HOR on Pt in acids can be considered as barrier-less, i.e., the reaction rate is determined solely by the frequency factor, A, in the Arrhenius equation, k = A exp(-Ea/RT) = A, when the activation barrier, Ea = 0. Thus, HER and HOR are completely reversible and their current densities are determined by the number of collisions between reactants and Pt surface, as well as the electronic and protonic resistances in the circuit. Therefore, gaining high effective Pt surface area with as little Pt as possible and reducing mass transport limitation of the reactants are the main routes to enhance catalysts’ performance. To achieve these goals, the synthesis conditions were fine tuned to control the density of nucleation sites on various carbon supports, to enhance crystallographic ordering during particle growth, and to form suitable Pt shell structure. Functional groups were introduced to assist synthesis process or modifying the hydrophilicity of the catalysts. Other examples, such as, Pd(core)-Pt(shell) nanoparticles for oxygen reduction reaction and Ir/IrOx nanoparticles for oxygen evolution reaction may be briefly discussed. (1) Wang, J. X.; Brankovic, S. R.; Zhu, Y.; Hanson, J. C.; Adzic, R. R. Journal of the Electrochemical Society 2003, 150, A1108. (2) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Topics in Catalysis 2007, 46, 249. (3) Wang, J. X.; Inada, H.; Wu, L. J.; Zhu, Y. M.; Choi, Y. M.; Liu, P.; Zhou, W. P.; Adzic, R. R. Journal of the American Chemical Society 2009, 131, 17298.

One of the critical steps in the SOFC and Li-air battery operation leading to large overpotentials and charge-discharge hysteresis is the kinetics of the oxygen oxidation reaction (ORR). It is important thus to explore the mechanisms that control this reaction which remain elusive, largely due to the lack of experimental techniques capable of probing ORR on the nanoscale. Oxygen vacancies play a significant role in determining the functionality of electro-resistive devices, non-volatile memories based on resistive switching and solid oxide fuel cells. Traditionally, the study of the role of oxygen vacancies in these processes is limited by high activation temperature and macroscopic measurement techniques. Here, we demonstrate spatially resolved local probing of the thermodynamics and kinetics involving the generation and diffusion of oxygen vacancies by utilizing chemical expansivity of these oxides upon application of concentrated electric fields. Using Band excitation Electrochemical strain microscopy (ESM), a strongly confined electric field at tip is used to drive the oxygen vacancies in these oxide materials and the resulting localized electrochemical strain is detected. A high frequency periodic bias is applied on the oxide material and the PFM tip acts as a probe of the local displacement arising due to migration of oxygen vacancies. Mapping the loop opening as a function of the final bias allows establishment of the onset and kinetics of the diffusion process. Signal relaxation measurements enable us to locally characterize the diffusion dynamics of the vacancies. Correlated mapping of the local oxygen vacancy movement and diffusivity has been achieved with a resolution of ~ 30 nm. The mapping of vacancies is shown on purely ionic oxides (Yttrium stabilized zirconia and Samarium doped Ceria). Systematic mapping of ORR/OER activity on bare and Pt-functionalized yttria-stabilized zirconia (YSZ) surfaces is demonstrated. This approach allows directly visualization of ORR\OER activation process at the triple-phase boundary. The electrical field-dependence of ionic mobility is explored to determine the critical bias required for the onset of electrochemical transformation, potentially allowing to deconvolute reaction and diffusion processes in the fuel cell system on a local scale. The results of ESM performed at elevated temperatures and controlled oxygen environments on solid electrolytes will also be discussed. Acknowledgement This material is based upon work supported by research division of Materials Science and Engineering, Office of basic energy sciences, DOE.

Heteroatom modification of carbon based materials is becoming increasingly important within the context of greener technologies, as it provides a useful tool to moderate physical and chemical properties for specific applications. Concerning the oxygen reduction reaction in fuel cells, scientists seek to find sustainable alternatives to the current state of the art platinum based catalysts. We present a one pot, hydrothermal synthesis of nitrogen and sulfur dual doped carbon aerogels, based on our previously published glucose and albumin system.^[1] This template-free approach gives rise to controlled morphologies with surface areas from 190 m² g^-1 to 320 m² g^-1. Two additives, thienyl–cysteine (TC) and 2-thiophene carbaldehyde (TCA), were used for sulfur incorporation, giving rise to distinct morphologies and varying doping levels of sulfur. Nitrogen doping levels of ~ 5 wt% and sulfur doping levels of 1 wt% (using TCA) to 4 wt% (using TC) could be obtained. A thermal pyrolysis step was used to further tune the aerogel conductivity and heteroatom binding states. By comparing solely nitrogen-doped with nitrogen/sulfur doped carbon aerogels, we show that the presence of sulfur improves the overall electrocatalytic activity of the carbon material in both basic and acidic media. Compared to commercially available Vulcan carbon, our materials exhibit superior catalytic performance in 0.1 M HClO₄. Carbon materials are known for their poor ORR catalytic activity in acidic media, which are the standard working conditions of commercial proton exchange membrane fuel cells.^[2] These findings are hence expected to be relevant to future research concerning the improvement of non-precious ORR catalyst. 1. White, R.J., et al., A sustainable synthesis of nitrogen-doped carbon aerogels. Green Chemistry, 2011. 2. Zhang, J., PEM fuel cell electrocatalysts and catalyst layers: fundamentals and applications. 1 ed. Vol. 53. 2008: Springer. 1137.

The alloying of Pt with iron-group metals has been studied for the promotion of catalytic reactions and alleviation of catalyst poisoning. The use of in situ optical methods to identify molecular species on electrocatalyst surfaces is critical to optimizing and developing catalysts. In the present work, benzenethiol (BT) adsorption/desorption at Pt metal, Pt₃Co and Pt₄Ni alloy films under electrochemical conditions are investigated with in situ Raman spectroscopy and cyclic voltammetry (CV). When adsorbed to a surface, BT exhibits an angle-specific Raman spectrum that features an easily-tracked, strong peak at 1572 cm^-1. This peak is evaluated over potentials of 200-1300 mV (vs SHE). Complete desorption of BT occurs at 1300 mV of positive-going sweeps for all three film compositions. During negative-going sweeps, the BT peak reappears on Pt₃Co films at 850 mV, which is 50 mV lower than on Pt and Pt₄Ni. Furthermore, at 200 mV, the recovered BT signal on Pt₃Co is only ~25% as intense as when first deposited. The Pt and Pt₄Ni films both recover 55-65% of their signal; however, the normalized BT signal is ~20% stronger on Pt than Pt₄Ni during a second positive-going sweep. These results provide spectroscopic evidence that Pt₃Co and Pt₄Ni alloys are more easily cleaned after BT poisoning than Pt metal. None of the three film compositions tested display a sulfate spectral feature between 1000 and 1200 cm^-1, indicating an absence of residual S species after BT desorbs. Features in the low-frequency region of the Raman spectra further confirm that the metal–S bond is broken during electrooxidative desorption of BT. A more complete picture of potential-dependent BT-metal interactions is obtained when Raman spectroelectrochemical data are compared to CV stripping data. BT can be oxidatively removed from electrocatalysts surfaces by CV cycling between 200 mV and 1300 mV. Pt₄Ni requires the least number of CV cleaning cycles to recover maximum electrocatalytic activity, followed by Pt₃Co. The maximum extent of electrocatalytic activity recovered is highest for Pt₃Co. The ease of cleaning, based on the trends from the electrochemical measurements, corroborates the conclusion of the Raman spectroelectrochemical study that Pt₃Co is the most S-resistant of the electrocatalyst compositions investigated.

It is known that atomically flat, Au 111 surfaces are inactive for gas phase CO oxidation. Several decades ago, it was found that when gold is nanoscaled to less than 5 nm in diameter on certain metal oxide supports, it becomes one of the most active CO oxidation catalysts. Gold has also been shown to be electrocatalytically active for CO oxidation. The physical origins are widely debated. Possible explanations are: 1. Size dependent coordination number effects; 2. Support directed coordination number effects; 3. Gold lattice strain; 4. Support electronic promotion; 5. Gold surface oxidation; 6. Support activation of oxygen. It is our objective to establish a nanoscaling factor hierarchy from this list by measuring the physical, electronic, and electrocatalytic factors for the highly active Au on TiO2 system. Our approach entails the design of a well defined, flat TiO2 support and the deposition of Au nanoparticles using a contamination free, size controlled e-beam technique. This design approach is well suited for determination of size, shape and lattice strain that we measure by Scanning Electron Microscopy and Transmission Electron Microscopy. Additionally, we measure the electronic structure using the 5d subshell sensitive hard x-ray valence band photoelectron spectroscopy to help disentangle the previous nanoscaling effects. There is a trend in the valence band electronic structure that we attribute to a surface coordination number effect. Finally, we measure the CO electrocatalytic oxidation activity versus Au nanoparticle size on TiO2 in both base (0.1 M KOH) and acid (0.1 M H2SO4). The results show that in both base and in acid, there is an optimum in activity with size at 5 nm in diameter. It is hypothesized that the optimum in activity is related to an optimal average coordination number at 5 nm that is able to bind OH- not too strongly or weakly.

The use of semiconductors to split water into H₂ fuel and O₂ gas is a promising technology for renewable producing chemical fuels that can be stored and transported. The best stable semiconductor discovered thus far to have activity for visible-light-driven solar water splitting is (GaN)_1-x(ZnO)_x, but even for this material the maximum quantum efficiency with visible light is still only about 6%. The average and local crystal structure of this compound has been investigated using a broad range of tools (x-ray and neutron powder diffraction and pair distribution function studies, ¹⁴N and ⁷¹Ga solid state nuclear magnetic resonance, scanning and transmission electron microscopy). All of these techniques suggest the presence of an intergrowth defect that is prevalent for Zn-rich but not Zn-poor compositions, and which is present in sufficient quantities to appear in bulk measurements (including powder diffraction patterns). The role that this defect may play in influencing the optical and charge transport properties of this phase will be discussed.

Carbon-supported nanomaterials can be done with different techniques or methods. The composition and morphology of the nanomaterials determines its future application. The direct methanol fuel cell (DMFC) cathode has the problem of requiring high cost catalysts and its degradation due to methanol crossover through the membrane. Also, the deposition methods for the catalysts are complex and time consuming. To solve these drawbacks is necessary to find a catalyst with high methanol tolerance and simple methodology for its deposition. Palladium (Pd) has resulted in active catalysts for the oxygen reduction reaction with high methanol tolerance. The research focus is the development of palladium nanostructures on carbon support for oxygen reduction reaction (ORR) using sputtering and electrospinning techniques . The synthesis of palladium thin films and nanoshells was carried out with dc-magnetron sputtering and electrospinning techniques on highly ordered pyrolytic graphite (HOPG) surface. Electrospun polymer fibers mats of poly(ethylene) oxide (PEO) were used as templates for the Pd shell nanostructures formation. The Pd thin films and nanoshells thicknesses were between 25 nm to 95 nm of Pd. The Pd nanoshells were characterized by electrochemical and surface analysis techniques. Scanning electron microscopy and energy-dispersive X-ray fluorescence spectroscopy (SEM/EDS) were used to study the morphology and composition of the Pd nanoshells. Electrocatalytic activity towards ORR and methanol tolerance in oxygen saturated 0.5 M H2SO4 solution was done. Palladium nanoshells have activity for the oxygen reduction reaction and present higher methanol tolerance than platinum catalysts. This research presents a novel approach for the synthesis of electrocatalysts for fuel cells.

Low-temperature fuel cells based on methanol/formic acid have attracted growing attention as a promising power source due to the high energy density and the convenient storage, transport of the small organic molecules. However, nanoparticle (NP) catalysts studied to date for methanol/formic acid oxidation reaction (MOR/FAOR) are prone to be severely poisoned by CO or other intermediate species during the catalytic process. Here we report a reliable chemical synthesis of catalysts based on high-quality FePt bimetallic NPs and MFePt (M= Co, Ni, Pd etc.) trimetallic NPs. With exquisitely tailoring on the composition and structure, these multimetallc NP catalysts exhibit exciting enhancement in activity and CO-tolerance for MOR/FAOR compared to traditional Pt NP catalysts. The present design and synthesis provide a robust approach to practical catalysts for MOR/FAOR.

Photoelectrochemical water splitting has become an attractive approach for hydrogen (H₂) production in view of energy and environmental issues. Since the first report of a TiO₂ thin-film photoelectrode, a variety of semiconductor electrodes and devices have been investigated. To date, performances with conversion efficiency as high as 10% have been demonstrated for electrodes based on stacked III-V semiconductors prepared by the MOCVD technique. However, these electrodes have limited corrosion resistance in aqueous electrolytes and are expensive for practical applications. On the other hand, the well-studied transition metal oxides are corrosion-resistant and inexpensive, but conversion efficiencies for these electrodes are not sufficiently high yet due to the lack of optical and photoelectrochemical properties required for realizing high photocurrents and H₂ evolution rates. Chalcopyrite p-type semiconductors such as CuInSe₂, CuGaSe₂, CuInS₂ and their mixed crystals are used as absorber layers in thin film solar cells. Due to their high absorption coefficient, 1-2-μm thick layers are enough to absorb the most part of the incident solar radiation. A wide range of band gap values (1.0-2.4 eV) can be obtained by changing the In/Ga and/or Se/S ratios. These specific properties are also attractive for the use of a photocathode for H₂ production. Although there have been a few reports in which photoelectrochemical properties of the series of chalcopyrite families for H₂ production were discussed, there have been little work on efficient H₂ production. In the present study, therefore, we attempted to fabricate a CuInS₂-based photocathode for efficient H₂ production. Polycrystalline CuInS₂ films were fabricated by sulfurization of electrodeposited Cu and In metallic precursor films. Structural analyses revealed that the CuInS₂ film formed compact agglomerates of crystallites with grain sizes of ca. 0.5-1.5 μm. Photoelectrochemical characterization revealed that the film was p-type with a flat band potential of 0.3-0.4 V (vs Ag/AgCl at pH), which is suitable for water reduction but cannot be for water oxidation. Upon loading Pt deposits, the film worked as a H₂ liberation electrode under cathodic polarization. Moreover, by introduction of n-type thin layers such as CdS and ZnS on the CuInS₂ surface before the Pt loading, appreciable improvements of H₂ liberation efficiency were achieved: for the CdS modified sample, spectral response data showed incident photon to current efficiency as high as 20% at wavelengths ranging from ca. 500 to 750 nm.

ZnO/Ag belonging to a semiconductor–metal composite possesses versatile material characteristics such as the introduction of the charge transfer and the suppression of electron–hole recombination. Great efforts thereby have been devoted for preparation of ZnO/Ag heterostructures with various morphologies such as clusters (0D) and dendrites (1D). In this study, polygon Ag nanosheet capable of high surface–to–volume ratio and large flat plane was used, which is able to support a stable atmosphere of vertical growth for high–density ZnO nanowires (NWs) and hence provide more ZnO/Ag junctions. Besides, we demonstrated the ZnO/Ag heterostructure in the presence of ZnO NWs and Ag nanosheets by means of a two–step growth process. Ag nanosheets were fabricated by utilizing polyol reduction and CN- etching, and then the seed–mediates method was employed in order to deposit ZnO NWs robustly on Ag nanosheets for the anisotropic growth. The experimental results show that the single crystalline Ag nanosheets with the thickness of 20–30 nm enlarge its area along 〈110〉 and ⅓〈422〉. ZnO NWs arrays are vertically assembled on both sides of Ag nanosheets (111) facets. Meanwhile, these interfaces of ZnO/Ag heterostructures can boost the electron–hole pairs separation, so the enhancement of the photocatalytic activity can be achieved by observing the representative pollutant concentrations.

Alternative energy sources are being extensively explored such as wind and solar energy. Due to their intermittency, it will be difficult for grids to handle a large percentage of their energy coming from these sources. This energy could be stored in chemical bonds by converting CO₂ and water to fuels and oxygen, creating a carbon neutral cycle. Heterogeneous catalysts studied for the CO₂ reduction reaction in aqueous environments consist mainly of metals, which produce varied product distributions but all require large overpotentials.¹ There has also been some work done exploring chemical modification of metal surfaces.^2-4 This study is focused on lowering overpotentials required for and tuning the product distribution of electrochemical CO₂ reduction on metals. A custom electrolysis cell was designed with a large working electrode area to electrolyte volume ratio to increase sensitivity for detection of liquid products. Electrolysis experiments were run potentiostatically for 1 hour. Gas Chromatography (GC) and Nuclear Magnetic Resonance (NMR) were used to detect gas and liquid products, respectively. Several potentials were chosen for each catalyst tested to explore current densities ranging from about 0.5 to 15 mA/cm². Pt metal was explored, and CO₂ reduction products were detected in small quantities as early as -0.6V vs. RHE. Consistent with literature, H₂ was by far the dominant product with formate (<0.5%) as a minor product.¹ However, CO gas, methane, and methanol were also detected in small amounts (<0.5%). The platinum surface was then modified with a thin film of polyaniline (PANi) in an effort to modify Pt reactivity. The PANi was electrodeposited in sulfuric acid using cycling voltammetry. The PANi-Pt catalysts show a small but noticeable increase in efficiency for formate and CO production and a decrease in methanol and methane production compared to the pure Pt metal catalyst. The mechanism for this change in activity is being explored. ACKNOWLEDGMENT The authors would like to thank Chevron, the Stanford Graduate Fellowship, and the National Science Foundation for student funding and the Global Climate and Energy Project (GCEP) for project funding. REFERENCES 1. Hori et al. Electrochimica Acta, 39, 1833-1839, 1994. 2. K. Ogura et al. J. Electrochem. Soc., 145, 3801-3809, 1998. 3. B. Aurian-Blajeni et al. J. Electroanal. Chem., 149, 291-293, 1983. 4. R. Aydin et al. J. Electroanal. Chem., 535, 107-112, 2002.

The discovery of photoelectrochemical splitting of water on titanium dioxide (TiO₂) by Fujishima and Honda in 1972 has initiated a considerable boom of semiconductor-based photocatalyst research. TiO₂ is probably the most promising photocatalyst being environmentally friendly, with low cost, good photocatalytic activity and excellent photostability. However, the large band gap of TiO₂ (~3.2 eV for anatase TiO₂ and ~3.0 eV for rutile TiO₂) restricts its applications in visible light region. In recent researches, nitrogen doping can decrease the band gap of n-type TiO₂ due to the mixing of N 2p states with O 2p states. Decoration of TiO₂ with metals and metal oxides such as Pt, Au, Pd, PdO, Ni, and Ag, has been found to enhance the photocatalytic properties. In these cases, low cost Pd-based catalysts are more suitable for industrial application as compared with Pt-based catalysts (approximately 20%-25% of that of Pt metal). In this study, we are combining the efforts of Pd-based nanoparticle decoration with nitrogen doped TiO₂ (N-TiO₂) synthesis in order to develop novel and efficient photocatalytic materials that are easy to produce in industrial quantities. First, TiO₂ are annealed at 600 °C for 12 hours in ammonia to achieve N-doped TiO₂. Palladium precursor was dissolved in acetone and mixed with N-TiO₂ by ultrasonic agitation. After evaporating the solvent, the samples were calcined in air at 300 °C for 2 hours, and then reduced in 15%H₂-85%Ar flow at 500 °C for different time. The microstructure, morphology, size and chemical composition of various N-TiO₂-Pd series nanoparticles synthesized and used in the work were characterized by using synchrotron radiation X-ray diffraction, transmission and scanning electron microscopy as well as X-ray photoelectron spectroscopy. When we applied N-TiO₂-Pd photocatalyst in the water ethanol mixture (molar ratio ~ 3:1), the hydrogen evolution rates were as high as 73,400 μmol/g.h under the UV-B radiation (Sankyo Denki, G8T5E UV-B lamps, 8W × 6 piece). Our study confirms that optimal Pd/PdO nanoparticles decorated on N-TiO₂ surface increase the photocatalytic activity. Furthermore, the ideal reduction time for N-TiO₂-Pd series photocatalysts is ~15 min. The results of the study display that N-TiO₂-Pd photocatalysts are suitable for the generation of renewable energy .

Cu/ZnO based catalysts have been commonly used to achieve for hydrogen production from methanol steam reforming under relatively low operating temperature. According to previous works so far, it has been known that Cu morphology in the conventional Cu/ZnO bulk catalysts synthesized by co-precipitation methods is one of the main issues related to the active sites toward high performance reforming. The advanced methods to control Cu morphology have been investigated as introducing the additive metals or varying process conditions of precipitation or calcination. In this work, we synthesized novel Cu nanoparticle/ZnO nanowire catalysts and investigated their performance with the variation of Cu morphology according to the changes of Cu deposition and reduction time. Our results showed clearly that the morphology of Cu nanoparticle on single crystal ZnO nanowire play a great role in the methanol steam reforming processes. In addition, the morphology changes of Cu during chemical reaction were investigated systematically. Finally, we will discuss the optimal morphology of Cu nanoparticle on ZnO nanowire toward best catalytic performance.

The Ru85Se15 nanoparticles supported on commercial Vulcan XC-72R or multi-wall carbon nanotubes (MWCNTs) were synthesized by microwave assisted polyol method with different solution pH. The Ru85Se15 nanoparticles on citric acid (CA)-treated supports prepared at pH=7 exhibited the most uniform particle distribution, higher degree of graphitization on supports, and four-electron ORR mechanism. Using lower loading of 0.138 mg Ru cm-2, the maximum power densities (Pmax) for the Ru85Se15/CA-MWCNTs and Ru85Se15/CA-XC72R were 380 mW cm-2 at 1430 mA cm-2 and 336 mW cm-2 at 1230 mA cm-2, respectively, with oxygen, while 166 mW cm-2 at 710 mA cm-2 and 126 mW cm-2 at 510 mA cm-2, respectively, with air. The Pmax of 103 mW cm-2 with air for the Ru85Se15/CA-MWCNTs could be retained (38% loss), while 46 mW cm-2 for Ru85Se15/CA-XC72R (64% loss) upon 6000 cycles. The four-electron ORR mechanism and highly graphitized MWCNTs might be responsible for the high performance and durability of Ru85Se15/CA-MWCNTs.

The Solid Oxide Fuel Cell (SOFC) which employs a ceramic electrolyte represents the most efficient and clean way to generate electricity by electrochemical reaction between fuel and oxygen at high temperature in the range of 800-1000 oC. In addition, SOFCs have many advantages such as multi-fuel capability and simplicity of system design. However, the degradation of component performance and cell lifetime problems occur due to high operation temperature. To solve these problems, the lowering operation temperature have been achieved by using a thin film electrolyte on an anode supported cell and high conductivity oxide ion conductors such as the doped LaGaO3 and scandia doped zirconia. While the electrolyte resistance decreases, cathode polarization limits the performance of SOFCs at low temperature (below 800 oC). LSCF(La0.6Sr0.4Co0.2Fe0.8O3) is the most candidate cathode. However, it exhibits insufficient electrocatalytic activity at intermediate temperature operation. The addition of noble metals to LSCF is known to improve the physicochemical properties. The purpose of this study is to improve power density of anode-supported flat-tube SOFCs at intermediate temperature such as 600-750 oC by using silver infiltration method on LSCF cathode. Because silver metal has good catalytic activity and high electrical conductivity, it improves the catalytic activity for oxygen reduction. The solution containing Ag was infiltrated into the LSCF cathode. Also Ag-glass composite materials was studied as the interconnect for intermediate temperature. The anode-supported flat-tube SOFC with Ag composite interconnect was manufactured and showed a good performance in a temperature range of 600-750 oC.

MOF-based catalysis has recently emerged as a burgeoning subfield of heterogeneous catalysis. The catalytic functions of MOFs are endowed by active metal sites and/or reactive organic groups that constitute the frameworks of MOFs. In addition, the incorporation of catalytic metal nanoparticles (NPs) into the cavities of MOFs can also impart catalytic function. Herein we will present the preparation of redox active MOFs supported Pd nanocatalysts via the redox-couple-driven method, and their catalytic applications toward CO oxidation. We prepared Pd NPs on the redox-active Ni-carboxylate-based MOF (Pd@ra-MOF) for CO oxidation. The Pd@ra-MOF was found to be highly active catalyst for CO oxidation. Importantly, we found that the catalytically more active composite, PdO-NiO/C, was generated in situ by thermal transformation of Pd@ra-MOF during CO oxidation reaction, which is stable with repeated reaction runs. We believe that this work provides a new avenue to the MOF-based metal catalysts and can be extended to other MOFs that are constructed with redox active species.

Hydrogen production via photoelectrochemical water splitting is a promising energy storage avenue to circumvent the intermittency of solar insolation. No known semiconductor satisfies all material requirements for high efficiency operation and viable options for the oxygen evolving photoanode are particularly sparse. With band edges straddling the water redox potentials, absorption in the visible-light region, and facile synthetic routes, tantalum nitride and oxynitride are attractive photoanode candidates. However, their performance is potentially hindered by several charge transport limitations, including poor (i) bulk charge transport, (ii) charge transport across grain boundaries, and (iii) charge transfer across the interface at the back contact. The primary goal of this work is to understand which of these mechanisms limits overall performance. To answer this question, emphasis is placed on designing an electrode architecture to isolate the losses associated with each aspect of charge transport. Thin films (10 nm – 500 nm) of tantalum nitride (Eg = 2.1 eV) were synthesized by oxidation of tantalum metal in air above 500°C and subsequent nitridation in ammonia above 800°C. Synthesis of a pure tantalum oxynitride (Eg = 2.4 eV) phase is challenging but can be accomplished by nitridation in humid ammonia under the right conditions. Characterization of the photoelectrode morphology by scanning electron microscopy and crystallinity by x-ray diffraction will be presented to support our conclusions. The photoactivity of the films as a function of thickness was studied and relevant metrics such as the absorbed-photon-to-current efficiency will be reported. The relationship between film thickness and electrode performance is complicated by several competing effects that govern charge transport. These effects include minimizing the distance photogenerated charge carriers must travel to be collected, changing the degree of crystallinity, and varying the surface texturing which results in a different electrochemically active surface area. Our results show that the variation in crystal grain size is slight with a minimal impact on performance, while changes in film morphology lead to drastically different photocurrents. Minority charge carrier (hole) transport is believed to be limiting in these tantalum based material systems. Continuing efforts will focus on designing an advanced architecture to maximize the optical absorption while maintaining the optimal charge transport benefits of the very thin film geometry.

The development of a cost effective process for the electrochemical reduction of CO₂ to fuels and chemicals could enable a shift to a sustainable energy economy. Coupled to a renewable energy source such as wind or solar, such a process could generate carbon neutral fuels or fine chemicals that are conventionally produced from petroleum. The key to developing such a process is the discovery of a catalyst capable of performing the conversion at a low overpotential selectively to the desired product. To gain insight into the factors important to designing better catalysts, we began by studying metallic copper. It is well known that copper is capable of catalyzing the electroreduction of CO₂ into hydrocarbons with high current efficiency. To further our understanding of CO₂ reduction on copper, we employed an experimental method capable of accurate current and voltage measurement coupled with sensitive product detection. Using a custom electrochemical cell with a large electrode area and small electrolyte volume led to higher concentrations of minor liquid phase products. We used onstream gas chromatography to detect gas phase products and ex-situ liquid phase product analysis with NMR to measure the product distribution at a range of potentials. Our results are in good agreement with past studies which have reported methane, ethylene, CO, formate, ethanol, 1-propanol, allyl alcohol, acetaldehyde, propionaldehyde, acetate, and methanol as products. In addition to the reported products we also detected several minor products that have not been reported before to our knowledge: ethylene glycol, glycolaldehyde, hydroxyacetone, acetone, and glyoxal. This expanded knowledge of the products of CO₂ electroreduction on copper and their voltage dependence leads us to consider a possible mechanistic pathway based on the enol tautomers of the multicarbon products being the active species on the electrode surface. Hopefully, a better understanding of CO₂ electroreduction on copper can lead to the development of new and better catalysts for this important reaction.

The electrochemical hydrogen evolution reaction (HER), 2H+ + 2e- → H2, is critical to the production of hydrogen fuel via water splitting. The best known HER catalysts are precious metals such as platinum. In order to make electrochemical hydrogen fuel synthesis economically feasible, highly active and stable HER electrocatalysts composed of inexpensive and abundant materials must be developed. Several studies have shown that molybdenum sulfide (MoS2) has potential as a low cost, active HER catalyst, but previous techniques for synthesizing MoS2 catalysts have required ultra-high vacuum or high temperature processing steps, which make these procedures expensive and incompatible with some substrates (1, 2). In order to develop a practical molybdenum sulfide catalyst, further efforts to create scalable synthetic techniques are required. In this study, we developed a simple wet chemical synthesis for a nanostructured molybdenum sulfide catalyst. Unlike previous synthesis techniques, this method requires no high temperature treatment or separate sulfidization step, yields a high density of catalytically active sites, and enables catalyst deposition onto many substrates. To synthesize the catalyst, precursor salts containing molybdenum and sulfur ions were combined in an aqueous acid to create nanoparticles, which were collected via centrifugation. The particles were then deposited onto a glassy carbon disk substrate via drop casting to create a catalyst film. Physical and chemical characterization revealed that the catalyst is an amorphous molybdenum sulfide material. Electrochemical polarization curves collected in a rotating disk electrode configuration showed that the molybdenum sulfide catalyst has good HER activity compared to other non-precious metal catalysts. Electrochemical capacitance measurements were used to determine the catalyst surface area and calculate a lower bound turn over frequency. Finally, electrochemical stability tests indicated that the catalyst degrades after extensive reductive cycling, but some activity was recovered after refreshing the electrolyte. This room temperature, wet chemical synthetic technique successfully created a nanostructured molybdenum sulfide catalyst film with high activity for the HER. Developing strategies to improve the catalyst stability has the potential to further increase the performance of this material. 1. T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 317, 100 (2007). 2. Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara and T. F. Jaramillo, Nano Letters, 11, 4168 (2011).

MoS2 is a semiconducting material consisting of sulfur-molybenum-sulfur tripledecker layers loosely bound by van der Waals interactions. Through its bandgap of 1.9 eV at the monolayer limit and band alignment, it is a material with great potential for photocatalytic water splitting. Single layer MoS2 can be exfoliated mechanically similar to graphene. While this method is simple, it is hard to control and not amendable to mass production of thin films. Solution-based processes have been proposed and may provide a scalable source of a mixture of single and multilayer material. Here we show an alternative avenue for the fabrication of MoS2 monolayers: growth of MoS2 on a sulfur-preloaded copper surface. In contrast to all other methods, this route has the potential of providing exclusively monolayer material, as the sulfur source is only available until the substrate is covered. Practically, this approach is related to the growth of graphene monolayers on copper or ruthenium films, where segregation of carbon to the surface is employed in aggregating a carbonaceous layer that transforms into graphene under the correct conditions. Small MoS2 triangles of a few nanometers in size have been grown previously on gold in a dilute H2S athmosphere. Here we show significantly larger patches, tens of nanometers in size. In contrast to gold, copper forms a multitude of sulfur surface coverages and also readily absorbs sulfur into the bulk. Thus, we can preload the substrate with a specific amount sulfur using an easy to handle liquid precursor, benzenethiol. In previous work we have shown that heating to below 400K removes the phenyl group of benzenethiol reliably from copper leaving sulfur coverages behind. STM imaging shows that MoS2 flakes can grow across substrate step edge which is a critical requirement for large size CVD growth on a metal substrate. The flakes exhibit a Morie pattern due to lattice mismatch between the MoS2 and the Cu substrate; this can be used to locate the position of underlying Cu substrate at a atomic resolution.

The structure and chemical composition of Pd nano particles (15 and 35 nm) exposed to pure CO and mixtures of CO and O2 at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar. In this talk, the X-ray Diffraction results will be presented. Our investigation shows that under CO exposure in a flow reactor, the lattice parameter of the nanoparticles change from the nominal parameter of palladium to a larger lattice parameter. This lattice parameter change is also observed when the gas flow is composed of CO-rich CO/O2 mixtures (with respect to stoechiometric ratio for CO2 production). The lattice parameter decreases back to nominal Pd value when the CO/O2 ratio is under stoechiometric. This lattice parameter change, which is reversible, is consistent with carbon dissolving into the Pd particles forming PdCx [1,2]. This phenomenon demonstrates that dissociation of CO on Palladium is possible, and that the excess carbon readily dissolves into the lattice. This result contrasts with the results obtained on single crystals, where CO dissociation was not observed,[3] and is an argument in favor of the existence of a material gap. 1 M. Maciejewski and A. Baiker J. Phys. Chem. 98, 285-290 (1994) 2 N. Seriani, J. Harl, F. Mittendorfer, G. Kresse J. Chem. Phys. 131, 054701 (2009) 3 V. V. Kaichev, I. P. Prosvirin, V. I. Bukhtiyarov, H. Unterhalt, G. Rupprechter, H-J. Freund J. Phys. Chem. B 107, 3522-3527 (2003)

Solar energy provides an abundant potential source of renewable clean energy provided there exists an efficient method of energy storage. Photocatalytic water splitting can be used to store solar energy in the form of chemical bonds, particularly those of hydrogen (H₂) which can then be used as a fuel. However, the water splitting reaction is severely limited by the high overpotential costs of the water oxidation half reaction to produce oxygen (O₂). Furthermore, there exists a need for a non-precious metal catalyst to drive the oxygen evolution reaction (OER) at reasonable overpotentials to make the overall water splitting process more economical. Focusing on non-precious transition metal oxides as potential catalysts, we have identified CoTiO₃ as a novel catalyst for OER. Attempting to combine the stability of TiO₂ with the high OER activity of cobalt oxide materials, this material presents itself as promising catalyst candidate. We began this study by characterizing the structure, morphology, oxidation state, and OER activity of CoTiO₃ in the absence of light. Results show varying effects on the crystallinity and morphology of this material depending on synthesis conditions. Overall, it was determined that this material catalyzes the oxygen evolution reaction with overpotentials nearly comparable to those of RuO₂ and IrO₂, the best known precious metal catalysts for this reaction. Photospectral characterization of this material also revealed synthesis-dependent differences in its optical properties. Limited amounts of photocurrent were measured from this material with the potential to be optimized given appropriate nanostructuring.

Since the first report of the photocatalytic splitting of water in 1972(1), many attempts have been made to increase the efficiency of the reaction, as the oxidation/reduction of water is kinetically very difficult, and thermodynamically unfavourable.(2) Examples include doping of semiconductors, band-gap engineering, nanostructures, heterostructures, surface plasmon resonance enhancements, and many more.(3) Despite these efforts, however, the development of a single semiconductor material that is suitably robust, cheap and efficient, and that absorbs a large portion of the solar spectrum, has been elusive. One of the reasons for this difficulty is the short charge diffusion length of many of these semiconductor materials. As layered materials increase in thickness, the amount of light they absorb increases as well. Unfortunately, this also means that there is a greater path length that the photoinduced charge carrier must travel in order to reach the electrode, resulting in increased recombination and grain boundary losses. One method of circumventing this issue is to construct a three-dimensional transparent conducting oxide (TCO) inverse opal electrode.(4) By depositing the water oxidation catalyst on the surface of this transparent three-dimensional structure, one can maintain a high light absorption by the catalyst while improving the charge collection efficiency of the material. We report our findings on the application of antimony-doped tin oxide (ATO) inverse opal architectures for the photoelectrochemical oxidation of water using WO3 and Fe2O3 catalysts. Polystyrene spheres were synthesized using a free radical polymerization method, and used as templates in the formation of ATO inverse opals using an ATO nanocrystalline solution precursor, followed by calcination at high temperatures. The WO3 and Fe2O3 materials were then deposited using sol precursors, and the materials were tested using linear sweep voltammetry in acidic and basic electrolyte solutions. A significant improvement in activity was observed for the catalysts on the ATO inverse opal electrodes when compared to the respective inverse opal catalysts without ATO and the planar thin film morphologies. Further studies will be required, however, on the optical properties of these materials, in order to avoid confusion over the source of the enhancement in the activity.(5) References 1. Fujishima, A., and Honda, K. (1972) Nature. 238: 37-38. 2. Walter et al. (2010) Chemical Reviews. 110: 6446-6473. 3. Barreca et al. (2011) Advanced Functional Materials. 21: 2611-2623. 4. Yang et al. (2011) ACS Advanced Materials and Interfaces. 3: 1101-1108. 5. Chen et al. (2011) ACS Nano. 5: 4310-4318.

The functionality of energy storage and generation systems like Li-ion batteries or fuel cells is not only based on but also limited by the flow of ions through the device. To understand device limitations and to draw a roadmap to optimize device properties, the ionic flow has to be studied on relevant length scales of grain sizes, structural defects, and local inhomogeneities, i.e. over tens of nanometers. Knowledge of the interplay between the ionic flow, material properties, and microstructure can be used to optimize the device properties, for example to maximize energy density, increase charging/discharging rates, and improve cycling life for Li-ion batteries for applications in electric vehicles and aerospace. Until recently, existing solid-state electrochemical methods were limited to a spatial resolution of ~10 um or greater, well above the characteristic size of grains and sub-granular defects. Our development of Electrochemical Strain Microscopy (ESM) has reduced this resolution limit to length scales down to 100 - 10 nm which allows studies of the local Li-ion flow in electrode materials and across interfaces. Here, we present how ESM can be used to measure ionic transport properties and extract spatially resolved maps of the activation energy in LiCoO2 thin film cathodes for Li-ion batteries. The ionic transport is measured using the coupling between strain and material volume and temperature-dependent ESM measurements allow to extract the activation energy for ionic transport on the nanoscale which will be correlated with the microstructure of the cathode film. Theoretical calculations are shown to support the experimental data and to give insight into the signal generating mechanism. Research was sponsored as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Research was conducted at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy. Part of this research was sponsored by the Vehicle Technologies Program for the Office of Energy Efficiency and Renewable Energy.

Platinum is known to be one of the best catalysts for the oxygen reduction reaction (ORR). Improvements over the catalytic activity of Pt have come by modifying the adsorption strength of ORR reaction intermediates to be slightly weaker than pure Pt, to approach the optimum binding energy. This has been demonstrated with Pt alloy catalysts such as Pt₃Y, Pt₃Ni, and other transition metal alloys. In order to be of practical use in a fuel cell, the catalysts are prepared as nanoparticles to take advantage of the much higher surface area. The drawback in using nanoparticles, however, is the high number of under-coordinated sites that bind oxygen too strongly, reducing the catalytic activity. This follows the general correlation that the lower the coordination number of a surface catalytic site, the stronger the adsorption bond. It has been shown that thin metal overlayers can exhibit properties different from the metal in its bulk form, creating the potential for higher ORR activity and greater tolerance to surface variations. The purpose of this study is to investigate thin films of Pt on Ru supports. We synthesized these structures by a variety of methods including reduction of Pt onto carbon supported Ru nanoparticles, sequential reduction of Ru and Pt by polyol solvents, and electrodeposition. They have been characterized to verify composition and structure and tested for ORR activity. We will discuss the relationship between different structures and their catalytic activities.

We have optimized the synthesis of Pt and Co-Pt alloy nanoparticles (NPs) and investigated their catalytic properties. We have carried out room temperature hydrogenation of octyne as model reactions and ex-situ characterization of the products. We found that our NPs were highly efficient as catalyst compared to conventional Pt particles deposited on carbon or chemically synthesized Pt NPs. We investigated the effect of ligands adsorbed onto the surface of NPs on the catalytic reactions. Despite the common perception about the “bad” influence of surface ligands on the catalytic properties of NPs, we see that certain types of ligands can improve selectivity significantly without decreasing the catalytic activity. In hydrogenation of octyne, we observed that primary amines significantly improved the selectivity for alkene. In the best conditions, we were able to control the selectivity for alkene from zero to > 90% with the conversion yield fixed at around 100%. Note that the blank experiment with amines without NPs ruled out the possibility of direct effect of amines on this reaction. To investigate the effect of amines on the selective hydrogenation, we carried out various experiment to study the relative binding strength of alkyne, alkene, and amines. To investigate the catalytic functionality of NPs mentioned above, DFT computations on the adsorption of various ligands, including molecular and atomic hydrogen, on the surfaces of pure and Co-doped Pt clusters were carried out and the result was compared with the experimental data. Our computations indicate that in the case of Pt/Co nanocatalysts atomic hydrogen binds preferentially to Pt. The energetics of this binding is, however, substantially affected (reduced) by the presence of Co. Work is progress on evaluation of the adsorption characteristics of other ligands, including C4H6 as a prototype of a highly unsaturated hydrocarbon. The goal of this study is to develop the next generation nanocatalyst with high selectivity for a desired degree of hydrogenation.

The efficient electrochemical reduction of CO2 to fuels could be a viable means to store electricity generated by renewable technologies such as solar cells or wind turbines. Almost all metals have the ability to electrochemically reduce carbon dioxide at low temperatures, however, most do so with low current efficiencies for carbon based fuels or at high overpotentials [1]. Gold has previously been shown to produce carbon monoxide with faradaic efficiencies around 90%, as well as formate with ~1% faradaic efficiency [2]. CO2 reduction, as with many electrochemical reactions, is often dependent upon the electrode surface structure and preparation. Thus, this presentation will focus on enhancement of the activity and product selectivity of CO2 reduction on gold by changing the topology of a polycrystalline gold surface. In this study, the surfaces of gold foils were roughened and then tested for the electrochemical reduction of carbon dioxide. Testing was performed in a 3-electrode, 2-compartment compression cell separated by an anion exchange membrane. Gas product analysis was achieved by a gas chromatograph, liquid products by NMR. The gold foils were characterized by their roughness. A variety of methods were employed: surface roughness was calculated with charge transfer measurements and electrochemical activity was studied using cyclic voltammetry in a 3-electrode electrochemical compression cell. CO2 reduction activities were measured with a continuous flow of CO2 in a CO2 saturated 0.1M potassium bicarbonate solution at 23°C. An anion exchange membrane was used to prevent liquid products from being oxidized at the counter electrode which consisted of a platinum foil. A Ag/AgCl reference electrode was used during experimentation. Potentials were adjusted post experimentation to the reversible hydrogen electrode (RHE). Potentials were also adjusted 100% for uncompensated resistance. Hydrogen, carbon monoxide and methanol were the main products formed from the electrochemical reduction of CO2 on gold. This paper will describe our efforts to modify the surface of gold and how such modifications translate to differences in activity and selectivity for CO2 reduction. We will also discuss the physical and chemical properties of the surface that could give rise to such changes. REFERENCES [1] M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, Journal of The Electrochemical Society 137 (1990) 1772. [2] Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrochimica Acta 39 (1994) 1833.

The world’s energy demand has been rapidly increasing over the past few decades; however, traditional sources of energy such as oil and coal are running out and will not be able to keep pace with the demand. One solution to this problem is the development of regenerative fuel cells which are able to use renewable electricity (e.g. wind and solar) to split water and form H2, and when necessary, reverse operation and use the H2 to produce electricity.1 However, current regenerative fuel cells use Pt and Pt/Ir catalyst which are very expensive; hence there is a need to develop alternative oxygen electrocatalysts comprising abundant materials. Manganese oxide (MnOx) electrocatalysts are potential candidates for reversible oxygen catalysis in a regenerative fuel cell due to its high activity, low toxicity and low cost.2 Previously, we have synthesized thin films of nanostrucutred MnOx via electrodeposition, and the activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is comparable to that of the best known precious metal catalysts such as Pt, Ir and Ru.3 However, the MnOx films are deposited onto a glassy carbon (GC) disk and in their current form, are not suitable for use in a regenerative fuel cell. Hence the goal is to translate this active catalyst to a fuel cell environment. The high temperature calcination involved in the synthesis procedure necessitates a support that is heat-resistant. GC particles are appropriate due to their high temperature resistance, high corrosion resistance which is needed due to the harsh alkaline testing environment, and high conductivity to enhance electron transport to and away from the catalyst surface. MnOx was deposited onto GC particles via an impregnation technique followed by calcination, which resulted in a nanostructured surface dominated by Mn2O3 as determined from SEM and XPS analysis. Electrochemical testing in a rotating disk electrode setup revealed that the ORR activity is similar to the MnOx thin films, while the OER activity is only slightly lower. However, the lower OER activity can be overcome via Ni or Co doping. The synthesis procedure was extended to form NiOx-GC and CoOx-GC particles, which exhibited lower ORR activity but much higher OER activity. The active MnOx-GC particles were then loaded onto carbon paper to form a gas diffusion electrode which can be used in an actual fuel cell. References 1. K.A. Burke, International Energy Conversion Engineering Conference. 2003, AIAA 2003-5939 2. J.O.M. Bockris, Int. J. Hydrogen Energ. 1999, 24, 15 3. Y. Gorlin, T.F. Jaramillo, J. Am. Chem. Soc. 2010, 132, 13612

Titanium dioxide (TiO2) has been widely used as a photocatalytic material for solar energy applications. To deposit nanostructured metal oxide films with controlled morphologies, an aerosol-chemical vapor deposition (ACVD) method was developed.1 The system is a simple, one-step process operating at ambient pressure. It has been noted that besides intrinsic properties, the morphology of metal oxide films plays a significant role in determining the efficiency of solar energy applications. Along with the film morphology, surface modification by noble metal contacts can improve electrochemical properties of metal oxide films. The size and number density of noble metal particles and the distance between noble metal particles play a significant role in determining the efficiency of solar energy applications. For this study, the room temperature RF (radio frequency) magnetron sputtering method2 was employed to deposit monodispersed nano-sized platinum (Pt) metals on the columnar TiO2 films. Uniform nanoparticle size could be achieved from 0.5 to 3 nm having a high particle density (>10E12 cm−2) by varying deposition time with constant pressure and power intensity. As-synthesized Pt-TiO2 films were used as photoanodes for splitting water and for photocatalytic conversion of CO2. Pt nanoparticles deposited on the TiO2 film for 20s produced the highest photocurrent and maximized the energy conversion efficiency. The Pt particles, with a size of approximately 1 nm, reduced electron-hole recombination rates. However, as the size of Pt particles increased, more trapping sites for photogenerated electron-hole pairs caused the decreased photoreaction. REFERENCES 1. An, W.-J., Thimsen, E., and Biswas, P. Aerosol-Chemical Vapor Deposition Method for Synthesis of Nanostructured Metal Oxide Thin Films with Controlled Morphology. J. Phys. Chem. Lett. 1, 249-253 (2010). 2. Yun, M., Maruf Hossain, D.W.M., Misra, V., and Gangopadhyay, S. Sub-2 nm Size-Tunable High-Density Pt Nanoparticle Embedded Nonvolatile Memory. IEEE Electron Device Lett. 30, 1362-1364 (2009).

Direct Methanol Fuel Cells (DMFC) are being considered as commercial alternatives to hydrogen/air system in portable and automotive industries. However, high activation energy of methanol oxidation requires high anode catalyst loadings when compared to hydrogen oxidation in PEMFC. While Pt-Ru based materials are well known as anode catalysts for methanol oxidation, promotion of these catalysts with non-noble metal oxides [1, 2] is considered as one of the ways to make DMFC viable. Some metal oxides are known to show electrochemical catalytic activity because of metal ability to switch between different valences [3]. These metal oxides are also known to exhibit metal-like conductivity when partially filled d- and f-bands are available. In the present work, the catalytic activity of metal oxides such as CeO2, TiO2, MoO2, and NiCoO4 dispersed on carbon aerogel was studied in the reaction of methanol oxidation. These oxides were also studied in combination with Pt-Ru/C. A modified sol-gel Pechini method with glycine as complexing agent was used to deposit 10-13 wt. % of metal oxide nanoparticles within carbon aerogel or commercial catalyst structure. These catalyst powders were annealed in air at 350oC and in N2 at 6000C and 9000C and characterized using XRD, HRTEM, H2-chemisorption, BET, RDE cyclic voltammetry, and chronoamperometry, as a function of catalyst loading and annealing temperature. The crystalline structure of 2-5nm metal oxide nanoparticles derived from HRTEM was confirmed by XRD. The total surface area of the CA-metal oxide powders decreased from 800m2/g to 600m2/g in presence of metal oxide. The surface area of the commercial catalyst without and with 10wt.% of metal oxide was about 150-200m2/g. Cyclic voltammetry of metal oxides in 0.1M HClO4 containing 0.1M-3.0M CH3OH demonstrated catalytic activity towards methanol oxidation. A significant synergic effect was observed when ceria and titania were combined with the state-of-the-art PtRu/C Tanaka catalyst. Sintering of metal oxides on Pr-Ru/C at 600oC resulted in a lower activity towards methanol oxidation. Further studies on complex metal oxide nanoparticles, optimization of catalyst structure, and catalytic activity in DMFC will be presented. Reference (1) M. Aulice Scibioh, Soo-Kil Kim, Tae-Hoon Lim, Seong-Ahn Hong, Heung Yong Ha, Pt-metal oxide Anode Electrocatalysts for Direct Methanol Fuel Cells, ECS Transactions, 6 (13) 93-110 (2007). (2) Roderick E. Fuentes, Brenda L. Garci, John W. Weidner, Effect of Titanium Dioxide Supports on the Activity of Pt-Ru toward Electrochemical Oxidation of Methanol, Journal of The Electrochemical Society, 158 (5) B461-B466 (2011). (3) B. Viswanathan, Ch. Venkateswara Rao, U. V. Varadaraju, On the search for non-noble metal based electrodes for oxygen reduction reaction, Energy and Fuel, 43-101(2006).

Doped barium cerate-zirconates [Ba(Ce,Zr)O3] have been widely investigated as high-temperature protonic conductors. The solid solution of barium cerate and barium zirconate enables an optimum balance between the relatively high protonic conductivity characteristic of the cerate and the chemical stability against CO2 characteristic of the zirconate. The material has been used as an electro-ceramic in various applications such as solid-oxide fuel cell electrolytes and hydrogen-separation membranes. However, the catalytic properties of the material have remained largely neglected in the literature. Recently, Suresh et al. [1] reported the catalytic activity of Ba(Ce,Zr,Co)O3-δ towards hydrogen generation from CH3OH partial oxidation, which was attributed to the presence of Co in the material. The aim of the present study is to examine that assumption and, in the process, identify the inherent catalytic activity of undoped Ba(Ce,Zr)O3. Any observable catalytic activity in the undoped material would open a new avenue in the investigation of doped BaCeO3, BaZrO3 and BaCeZrO3, which has thus far been restricted to the field of electrochemistry. Powders of undoped BaCe0.25Zr0.75O3 and BaCe0.25Zr0.75-xCoxO3-δ were synthesized using solid-state reaction and characterized using X-ray Diffraction (XRD) and BET. The materials were then subjected to CH3OH partial oxidation in a packed-bed reactor at different temperatures and O2:CH3OH ratios. A gas chromatograph was used to analyze the reaction products on a dry basis in order to quantify the catalytic activity of the materials. [1] Suresh et al., Journal of Materials Science, 45 (2010) 3215

We report on the preparation of carbon supported Pt containing bimetallic nanoparticles (NPs) with electrocatalytic activity for fuel cell applications. To form the bimetallic NPs, we chose 3d transition metals such as Co and Fe. In order to synthesize the NPs, sonochemical syntheses method was used. Ultrasound irradiation into carbon support, Pt(acac)2 and Fe(acac)3 or Co(acac)2 dispersed polyol solution could generate a reducing condition of the precursors which resulted in the formation of bimetallic NPs on carbon support. The structures of the nanoparticles were characterized by XRD, XPS, IR, SEM-EDS, HRTEM and STEM-HAADF. The NPs show narrow size distribution with averaging size of about 2 nm and no macroscopic phase segregation. Electrocatalytic oxygen reduction reaction (ORR) behavior of the materials was measured by rotating disk electrode (RDE) technique and compared with commercial Pt/C (E-TEK, 20 wt% and TKK, 37.7 wt%). As a result, we obtained an enhanced electrocatalytic ORR activity than commercial Pt/C.

Low cost conversion of gas phase CO2 into CO is an important step in artificial fuel synthesis. Here we investigate theoretically a possibility and mechanisms of such conversion at surfaces of the stoichiometric and oxygen-deficient subnanoporous complex oxide 12CaO.7Al2O3 (C12A7). C12A7 is formed by a positively charged framework consisting of 12 cages per cubic unit cell and compensated by extra-framework oxygen anions. Thermal treatment of C12A7 in reducing conditions results in formation of a so-called electride state, in which extra-framework anions are replaced with electrons. Recent experimental [1] and theoretical [2] studies indicate that intact bulk-like cages are present at or near the C12A7 surface. The results of our density functional calculations suggest that the near surface cages are capable of trapping and releasing electrons and oxide species. In the case of oxygen deficient C12A7, adsorption of a CO2 molecule near an empty cage can result in decomposition of the molecule, so as an oxygen atom is trapped in the cage and the CO fragment is physisorbed at the surface with the overall energy gain of over 1.2 eV per molecule. We have investigated several pathways of such decomposition and found that interstitialcy-like oxygen in-diffusion process has the lowest activation energy of 0.8 eV, which suggests a possibility of “cold” CO2 to CO conversion at such surfaces. [1] Y. Toda, Y. Kubota, M. Hirano, H. Hirayama, H. Hosono, ACS Nano, 5, pp 1907–1914 (2011). [2] P. V. Sushko, A. L. Shluger, Y. Toda, M. Hirano, H. Hosono, Proc. Royal Soc. A 467, 2066-2083 (2011).

Inorganic photocatalysts are currently being intensely studied for their potential use for the production of fuels from H₂O and CO₂. Designing new efficient photocatalysts requires an increased understanding of the link between catalyst microstructure and activity. Transmission electron microscopy (TEM) is a well established and powerful technique for studying the structure of materials at the nanoscale. However, the environment of the TEM, namely vacuum, room temperature, and total darkness, makes it difficult to correlate the structures observed with experimentally determined activities of the catalysts studied. Thus, environmental TEM (ETEM) is sometimes used to more closely mimic the conditions experienced by a material in use. However, while gaseous environments and variable temperatures are common to ETEM work, illumination of the sample by visible, ultraviolet, and infrared light is much less common. A critical experimental condition is therefore usually absent from current TEM studies of photocatalysts. We have installed a variable wavelength light source to irradiate the sample area of an ETEM column. The current design consists of a broadband light source with filters, optical fibers with a vacuum feedthrough, and a manipulator to precisely position the fiber tip with respect to the TEM sample in the microscope. This apparatus is able to illuminate a photocatalyst sample inside the TEM column with up to 3mW/cm²/nm, from 300-800nm, which is several times the average spectral irradiance of the sun on the Earth’s surface over the same range. Intensities above 1mW/cm²/nm are possible down to 200nm. We are using this new capability to study the structure of titania based nanostructured catalysts, such as nanotubes, nanowires and high surface area powders. We are also working on synthesis and characterization of these same materials functionalized with metal nanoparticles.

With their high surface-to-volume ratio, ultra-small platinum nanoparticles exhibit extremely desirable catalytic behavior in low-temperature proton exchange membrane fuel cells (PEMFCs). However, catalyst degradation is a critical issue that limits the lifetime of PEMFCs (1, 2), and these nanoparticles are no exception. Better understanding of each degradation process will allow for better mitigation techniques and the development of new materials. One of the main processes contributing to electrochemically active surface area (ECSA) loss of catalysts is nano-scale Ostwald ripening, a process driven by surface energy differences (1-3). A reliable method of determining these surface energies involves measuring the size-dependent bond strain of the nanoparticles, since this data can be correlated to a surface energy constant (6, 7). However, at the scale of ultra-small nanoparticles—less than 3 nm in diameter—2 major problems appear: the increased local disorder from a high percentage of surface atoms enfeebles traditional XRD analysis techniques, and the emergence of non-bulk properties questions the validity of a surface energy ‘constant’. Here we utilize the pair distribution function (PDF) to analyze high energy x-ray powder diffraction data for carbon supported nanoparticles of varying diameters. The PDF method has accurately extracted structural information in other materials of this diameter scale (8). We report our values for size-dependent internal bond strain and show evidence favoring a size-dependent surface energy for nanoparticles in the sub-3 nanometer regime.

It has been shown that Cu and Cu alloy nanoparticles supported on inactive ceramics are broadly used for many applications such as selective hydrogenation, oxidation, and methanol formation. The catalytic activity of such supported nanoparticles is related to their particle size, morphology, and electronic properties. It is important to understand the interaction between metals in both Au-Cu and Pt-Cu alloy nanoparticles, as well as the interaction between Cu and the inactive ceramic support. Here we use in-situ environmental transmission electron microscopy to characterize the SiO2 supported Cu and Cu alloy nanoparticles and to study the oxidation state of the reduced Cu and Cu alloy nanoparticles. We show that in-situ electron energy loss spectroscopy is a powerful technique to study the valence state and bond formation on these supported metal catalyst.

Porous metallic nanostructures with tunable morphologies have intrigued us in the general synthesis of functional materials owing to their wide variety of applications in catalytic, optical, electronic, magnetic, sensing, photocatalyst, energy storage devices and bio-device ranging from energy storage to drug-delivery carriers. In this context, various synthetic approaches (e.g. template-directed and non-template method) have been intensively explored to construct porous nanostructures in various morphologies (e.g., hollow and yolk-shell) in convenient and efficient ways. Nevertheless, there has not been reported for the fabrication of porous non-oxide metal with tunable structure through the aerosol pyrolysis. Herein, we present a generalized approach to construct 3D porous nanoarchitecture (porous nickel) with tunable structure through aerosol process. Morphological regulation was achieved by 1) control of thermal behavior of precursor’s ligands and inorganic salt and by 2) control of the solubility of precursor. Controlling the thermal behavior of precursor was performed by employing various inorganic salts and organic ligands with different thermal degradation behavior. The solubility of precursor was controlled by changing the ratio of inorganic salt to organic ligands or by addition of acids with different pKa to the precursor solution. After analysis on the morphology, crystalline, and pore structure of porous nickel, the effect of morphological difference on the catalytic property (propylene hydrogenation reaction) was evaluated where porous nickel with different morphologies were employed as catalysts. Catalytic tests exhibit that the catalytic activity of nickel depends on the structural factors (surface area, crystallinity (grain size) and morphology), morphology (hollow/non-hollow) and elemental compositions (nickel/carbon amount) of catalyst. Since this synthetic process can be generalized by employing different chemicals (e.g., metal salt and organic ligand) and aerosol process condition, various species of porous metallic nanosphere can be prepared. The porous metals prepared by current approach are also applicable to broad spectrum of practices. For example, such porous metals can be used as hydrogen storage since they have capability to store and release the hydrogen at enhanced kinetics. We believe that this new strategy would pave the noble way for the fabrication of high performance catalysts in facile and cheaper way.

CuPt alloys have demonstrated improved intrinsic activity as ORR (oxygen reduction reaction) electrocatalysts compared to conventional nanoparticle-based Pt materials. Electrocatalysts possessing shape anisotropy, such as large-area planar surfaces and 1-d nanowires, have also demonstrated enhanced intrinsic activity. In this work, CuPt nanoplates are presented as a novel ORR electrocatalyst with thorough materials and electrochemical characterization. Detailed experimental synthesis and characterization methods will be presented.

The outstanding electronic and optical properties of single-walled carbon nanotubes (SWCNT) make them a promising candidate to lead the creation of a new generation of small, fast and energy-efficient electronic devices. Remarkably, these properties can be controlled, provided that control on structural features of the nanotubes (i.e. chirality) is achieved. However, current synthesis processes do not possess such level of sophistication. Chemical vapor deposition (CVD) is widely regarded as the most viable option for industrial-scale nanotube production. In this process, nanotubes grow on metallic nanoparticles that catalytically decompose the carbon-containing precursor gas, and act as a support of the growing nanotubes. Recently, it has been proposed that since the nanoparticle and the nanotube are in contact during the growth process, it is possible to control the nanotube chirality by controlling the nanocatalyst structure, through a template effect. Here we use computational tools such as density functional theory (DFT) and classical reactive molecular dynamics (RMD) to investigate whether a structural relation between the nanocatalyst and the nanotube at different stages of growth can be detected. Firstly, DFT is used to optimize a set of nanotube caps and nanoparticles revealing that nanoparticles adopt a particular geometry depending on the chirality, but also showing the likelihood of an inverse template effect on an unsupported nanoparticle. Also, DFT optimizations are used to study the impact of the nanoparticle surface structure on the nucleation of carbon structures using model surfaces Co(211) and Co(321), where the preference for armchair (ac) or zigzag (zz) structures is demonstrated to depend on the surface structure. Secondly, RMD is used to simulate the growth of nanotubes (at 1000 K) on supported nanoparticles of several sizes at various nanoparticle/substrate interaction strengths, where the latter is shown to alter the dynamic and structural behavior of the nanoparticle, thus demonstrating the effect of changing the support. On the other hand, the arrangement of the nascent nanotube –and related nascent carbon structures – is found to be continuously influenced by the underlying surface structure of the nanoparticle supporting the template effect hypothesis. However, the effectiveness of such template effect is found to depend on the interaction with the support, and on the nanoparticle size, which are shown to affect factors such as atom mobility, and site occupation among others. At weak interactions, defect annealing is facilitated, but a cooperative inverse/direct template effect is observed. At strong interactions, the inverse template effect is eliminated, but detriment of defect annealing hinders the structural matching between nanotube and nanoparticle. Our results suggest that controlling the nanotube structure via a template effect is plausible, provided that the right synthesis conditions are found.

Carbon-supported Pt electrocatalysts used in a PEMFC catalyst layer are arguably the most significant component affecting cost, performance, robustness and durability of the membrane electrode assembly (MEA). Conventional MEAs currently in use are based on finely dispersed Pt nanoparticles supported on carbon black and dispersed as ink. Corrosion of the carbon support leads to poor durability and unacceptable lifetimes. During ink fabrication, the colloidal solution of carbon/Pt and ionomer self-organizes into phase-segregated regions with interpenetrating percolating phases for the transport of electrons, protons, and gases. The process of microstructure formation depends on the type of catalyst support, the type and amount of ionomer added, the type of dispersion medium used during ink preparation, and the fabrication conditions. Limitations to this approach have been observed in the past few years, suggesting that a new approach will be required to meet the targets set for successful commercialization. For catalyst layers, the main objective is to obtain the highest current density with respect to the desired electrochemical reactions using a minimum amount of the Pt catalyst (DOE target for 2010: 0.29 gPt/kW). This requires a large active surface area with appropriately engineered microstructure, optimal orientation of the Pt crystal facets, small kinetic barriers to bulk transport and interfacial transfer of protons, electrons and reactant gases, and proper management of product water and waste heat. In order to address these challenges, our research is focused on the fabrication of thin, low Pt loaded catalysts by Reactive Spray Deposition Technique (RSDT) [1]. This one-step direct catalyst coated membrane (CCM) process enables a decoupling of all three catalyst layer components (Pt, carbon, and nafion). The ability to introduce components separately into the hot-dry process stream allows for flexibility in manufacturing hereto unavailable via wet processing techniques. Observations on the RSDT catalysts will be presented. The thermal history of the forming catalyst particle is expected to affect the crystal structure of the formed catalyst. Particle size, distribution, and dispersion on carbon support ranges from adatoms to highly crystalline particles due to sublimation and subsequent coarsening in the process stream. Further work has been done to disperse the catalyst onto supports other than Vulcan XC-72R that show more promising durability. In this work the microstructure and oxygen reduction reactivity will be examined on a highly graphitized carbon support from Cabot as well as on Ebonex® which contains sub-oxides of titanium (Magneli phases). Catalyst dispersion, electrochemical activity and electrode formation is discussed. [1] R. Maric, J. Roller, and R. Neagu, J. Thermal Spray Techn., 20(4) (2011) 698.

Bulk oxides are usually robust and stable systems with well-defined crystallographic structures. At the nanoscale, these compounds can exhibit unique physical and chemical properties due to their limited size and a high density of defect sites such as edges, corners and point defects. As for other materials, the process of size reduction is expected to dictate structural, transport and chemical properties. We developed a large scale synthesis process for nanocrystalline ZnO spheres and tetrapods production by chemical vapor synthesis using zinc metal as precursor and air as oxidant. The chemical vapor synthesis of ZnO tetrapods (ZnOT) from zinc metal will be described using a combination of experiments and fluid dynamics modeling.[1] These materials can find applications in the field of energy, for example for dye sensitive solar cells or as catalyst for CO removal in fuel cells. We will show why the anisotropic ZnOT have increased efficiencies for application in dye sensitized solar cells when compared to ZnO nanospheres.[2] Similarly, the anisotropy of the tetrapods is at the origin of the higher performances for the CO oxidation reaction of Au/ZnOT catalysts, compared to gold catalysts deposited on commercial spherical nano-ZnO.[3,4] The epitaxy of gold nanoparticles could explain the high activity obtained. [1] N. Reuge, R. Bacsa, P. Serp, B. Caussat J. Phys. Chem. C 2009, 113 (46), 19845-19852. [2] R.R. Bacsa, J. Dexpert-Ghys, M. Verelst, A. Falqui, B. Machado, W.S. Bacsa, P. Chen, S.M. Zakeeruddin, M. Graetzel, P. Serp Adv. Func. Mater. 19, 2009, 875-886. [3] S.A.C. Carabineiro, B.F. Machado, R.R. Bacsa, P. Serp, G. Drazić, J.L. Faria, J.L. Figueiredo J. Catal. 2010, 273, 191-198. [4] E. Castillejos, R. Bacsa, A. Guerrero-Ruiz, I. Rodríguez-Ramos, L. Datas, P. Serp, Nanoscale 2011, 3 (3), 929-932.

Cobalt oxide (Co3O4) is the most active catalyst among all the transitional metal oxides for the oxidation of hydrocarbons, which is important for energy conversion and emission control. Conventional method supports Co3O4 nanoparticles (NPs) on porous substrates to maximize their catalytic surface areas, but they suffer from problems such as the agglomeration of NPs and chemical reactions between the catalyst and the porous substrates. In contrast, one-dimensional (1-D) nanostructured Co3O4 supported on metal meshes is a promising alternative catalytic geometry, which not only avoids the above issues associated with the supported NPs, but also benefits from large surface-to-volume ratio, small pressure drop, good heat transfer and ease of surface modification of the 1-D nanocatalysts. However, it remains a challenge to controllably grow nanostructured Co3O4 on metal meshes with high coverage density and good repeatability. Herein, we report an improved ammonia-evaporation-induced synthesis method to grow nanostructured Co3O4 on stainless steel (SS) meshes, by introducing Cu2+ ions to facilitate the nucleation and growth process of Co(OH)2. Firstly, the morphology of as-grown Co3O4 on SS meshes evolves from nanowire (NW) bundles, to microwires (MW) and finally to surface coated MWs. The characterizations of the final products illustrate that the main phase is Co3O4, with some Cu in the core region, which means the surface of all the morphologies is mainly covered by Co3O4. Secondly, the mass loading density greatly increases with the presence of Cu2+ ions. The sample with cobalt to copper atomic ratio of 8:2 in the initial solution has the maximum mass loading and largest surface area. Finally, the catalytic activities of those Co3O4 supported on SS meshes with different amounts of Cu2+ ions were tested for the methane oxidation. With smaller grain size and larger surface area, all the Cu-containing Co3O4 samples show higher catalytic activity than the pure Co3O4, and the methane conversion percentage over the optimized Cu-containing Co3O4 sample is about 50% higher than that over the pure Co3O4 NWs in a temperature range of 300 to 400oC. We believe that these results not only demonstrate an elegant method to control the morphologies of 1-D metal oxides synthesized by solution phase methods, but also facilitate the use of 1-D metal oxide nanocatalysts supported on metal mesh as an economical, scalable and yet efficient catalytic structure for hydrocarbon oxidation reactions and other catalytic applications.

H2-based fuel technologies are highly beneficial for reducing the carbon footprint of human activities. Efficient catalysts for water-splitting and H2 evolution reactions are thus actively sought. In particular, large-scale development of these green technologies will require noble-metal free catalysts, robust towards contaminants such as CO and sulfur. Nickel-based nanocatalysts, such as the Raney catalysts, have shown promising activities for H2 activation in the past. However, their use was hampered by the lack of control concerning their preparation (batch effect) and their sensitivity towards sulfur poisoning. To overcome this limitation, we have modified monodispersed Ni(0) nanoparticles with white phosphorus (P4), a cheap and ton-scale available source of phosphorus, to produce a family of sulfur-resistant nickel phosphide nanoparticles: pure Ni2P nanoparticles, core-shell Ni2P/Ni and amorphous Ni-P alloys. The colloidal synthesis was designed at the gram scale and performed in soft conditions (a few hours, T<220°C), allowing an excellent control of the nanoparticles size, composition and surface species. This made them a suitable model for the fine understanding of the catalysis at the molecular scale, as they were systematically compared with Ni(0) nanoparticles (same size and surface species). A model reaction was chosen to probe the H2 activation performances of our catalysts: the industrially-relevant alkyne hydrogenation reaction. Here, we report the strikingly high selectivity of the Ni2P nanocatalysts compared with the Ni(0) one for the selective formation of alkenes.[4] Cis-alkene with limited steric hindrance were obtained with a high conversion (>90%) and a high selectivity (>90%) for a range of substrates. Through dedicated mechanistic studies, including isotope labeling and in situ FTIR spectroscopy, this behavior was shown to take roots into the nature and abundances of active sites for H2 activation on the catalyst. In particular, the Ni2P surface was found to store less active hydrogen species, leading to a better controlled hydrogenation reaction. The phosphorus content was then adjusted to Ni/P=3 and an intermediate behavior was observed, confirming the paramount role of the phosphide moiety on the controlled formation of surface hydrides. Altogether, this model study highlighted the exciting performances of nickel phosphide nanoparticles as a cheap alternative to Pt-based catalysts for the development of smart nanocatalysts in H2-based energy technologies. [1] Carenco S, Boissière C, Nicole L, Sanchez C, Le Floch P, Mézailles N, Chem. Mater. 2010, 22, 1340 [2] Carenco S, Resa I, Le Goff X, Le Floch P, Mézailles N, Chem. Commun. 2008, 2568. [3] Carenco S, Le Goff X, Shi J, Roiban L, Ersen O, Boissière C, Sanchez C, Mézailles N, Chem. Mater. 2011, 23, 2270. [4] Carenco S., Leyva-Pérez A., Concepciòn P., Boissière C., Mézailles N., Sanchez C., Corma A., Submitted Paper.

Truly bimetallic Co(1-x)Cux (x=0.2-0.8) nanoparticles were synthesized by using amine functional solvents and acetylacetonate precursor salts of metals at elevated temperatures. Spherical nanoparticles could be adjusted in the size range between 3 nm and 10 nm by exploiting various amine functional solvents. 10 nm Co80Cu20 nanoparticles with octahedral shapes were obtained from nitrate precursor salts of metals under otherwise identical synthetic conditions. Nanoparticles were characterized by employing an array of in-situ and ex-situ techniques. Powder X-ray diffraction showed that a poorly-crytalline metal oxide phase co-exists with the nanocrystalline bimetallic alloy phase for all the composition range studied. Scanning transmission electron microscopy with energy dispersive spectroscopy and electron energy loss spectroscopy revealed the formation of truly bimetallic nanoparticles with metallic Cu rich cores and mostly oxidized Co rich shells. Ambient pressure X-ray photoelectron spectroscopy exhibited Cu segregation to surfaces corresponding to mean free path of 300 eV photoelectrons under O2, which illustrated the surface dynamics of bimetallic CoCu nanoparticles under redox atmospheres. Additionally nanoparticle colloids resulted in Cu segregation to surfaces and the formation of hollow nanostructures at room temperature when lightly washed and stored in chloroform. Control experiments put forward a combined action of excess amine, chlorine and oxygen on the differential diffusion of Cu into Co and the leaching of Cu. Well-characterized nanoparticles were supported in MCF-17 SiO2 to evaluate their catalytic reactivity for the Co2/H2 F-T synthesis. At 300 degree Celcius and 5.5 bar, bimetallic nanoparticles performed more selectively toward methane than the monometallic Co and Cu nanoparticles.

Currently, catalysis research is focusing on nanosized alloy particles. Such particles, however, show a great deal of structural (1, 2) and chemical order-disorder effects (3) that influence their catalytic activity very substantially. To understand and gain control over the latter a good knowledge of the former is needed. In the talk we will introduce a recent development in resonant high-energy x-ray diffraction (4) as a tool for characterization the atomic arrangement in alloy nanoparticles both with an excellent spatial resolution and chemical specificity. Results from recent experiments on carbon supported 5 nm PtxAu1-x (x=0, 0.2, 0.4, 0.5, 0.77, 1.0) alloy particles will be presented. Questions like i) are really Pt and Au alloying at the nanoscale, ii) is there bond-length contraction with diminishing particle size and alloying, and iii) how those structural characteristics relate to the PtxAu1-x nanoparticle’s catalytic performance will be addressed. References: 1. V. Petkov et al “Periodicity and atomic ordering in nanosized particles of crystals" J. Phys. Chem. C 112 (2008) 8907. 2. V. Petkov, “3D structure of Nanosized catalysts by High-energy X-ray Diffraction“, Synchr. Rad. News 22 (2009) 29. 3. Jin Luo et al “Carbon-Supported AuPt Nanoparticles with Different Bimetallic Compositions”, Chem. Mater., 17 (2005) 3086. 4. V. Petkov and S. Shastri, “Element-Specific View of the Atomic Ordering in Materials of Limited Structural Coherence by High-Energy Resonant X-Ray Diffraction and Atomic Pair Distribution Functions Analysis: A Study of PtPd Nanosized Catalysts”, Phys. Rev. B 81 (2010) 165428.

Phosphorylated mesoporous carbons with homogeneous distributions of phosphate groups were prepared by a “one-pot” soft-template method using mixtures of phosphoric acid with hydrochloric, or with nitric acids in the presence of Pluronic F127 triblock copolymer. Carbons with distinct adsorption, structural and surface acidity properties were prepared by adjusting the various ratios of phosphoric acid used in these mixtures. Nitrogen adsorption at -196°C showed that mesoporous carbons exhibit specific surface areas between 300-600m²/g, and 6 to 13nm mesopore diameters from the calculated pore size distributions (PSDs). Both structural ordering of the mesopores and the final phosphate contents were strongly dependent on the ratios of H₃PO₄ in the synthesis gels, as shown by transmission electron microscopy (TEM), X-ray photoelectron (XPS) and energy dispersive X-ray spectroscopy (EDS). The number of surface acid sites determined from temperature programmed desorption of ammonia (NH₃-TPD) were in the range of 0.3-1.5mmol/g while the active surface areas are estimated to comprise 5-54% of the total surface areas. Finally, the conversion temperatures for the isopropanol dehydration were lowered by as much as 100°C by transitioning to the catalysts with the higher numbers and strength of acid sites, and higher active surface areas (ASA). In addition to the good thermal and chemical stabilities of soft-templated mesoporous carbons, these results show that our phosphorylated materials have potential for the large scale application as heterogeneous acid catalysts. Acknowledgements: Work supported as part of Fluid Interface Reactions, Structures and Transport (FIRST) Center, Energy Frontier Research Center, U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

An on-board fuel reformer has been designed to convert diesel fuel oil into hydrogen-rich reformates in a fixed bed catalytic reactor, known as auxiliary power units (APUs). The reformates comprising primarily of H2, CO2 and CO can directly power solid oxide fuel cell (SOFC). In an APU system, deactivation of catalyst due to the presence of heavy and branched hydrocarbons, aromatics and sulfur (H2S and organic sulfides or thiols) in the fuel is the major obstacle to the commercialization of diesel fuel reformer. An innovative Ni-based catalyst was developed in this work with the aim of tackling these deactivation issues. The catalyst was prepared through a two-step procedure: firstly, nickel boron (Ni-B) alloy particles (~5 nm) were deposited on an activated carbon substrate (C), secondly, the Ni-B/C powder was mixed with a metallo-organic gel of cerium(III) and Gd(III) ions, and then the mixture was subjected to calcination. The resulting Gd-doped ceria (GDC) supported Ni catalyst displayed excellent resistance to carbon deposition and sulfur poisoning in the autothermal reforming of a proxy fuel comprising of 75%-dodecane, 25%-tetralin and 50ppm 3-methyl-benzothiophene. A conversion of the hydrocarbons >95% and selectivity of H2 > 60% underwent no change after a125-h duration of assessment. The catalyst down loaded from the reactor showed no any coke deposition and sintering of Ni namo particles. This paper also provides fundamental interpretation for the unique performance of this catalytic system.

Introduction Shape control of metallic nanocrystals led to well-defined morphologies exhibiting preferential exposition of crystallographic planes and allowing the establishment of structure-reactivity relationships for structure-sensitive reactions like selective hydrogenations. In this respect, shape-controlled colloidal synthesis can be monitored through the selective adsorption of surfactants like cetyltrimethylammonium bromide (CTAB) on specific crystallographic facets leading to well-defined objects. An important issue for improving colloidal techniques is to develop “greener” approaches. The replacement of alkyltrimethylammonium halides by biocompatible agents is required. Therefore, beta-cyclodextrins, nontoxic oligosaccharides, have been for the first time herein used for the preparation of well-defined Ag and Pd NPs. After deposition onto TiO2, the Pd/TiO2 catalysts were tested in the hydrogenation (HYD) of cinnamaldehyde to evaluate their catalytic properties. Materials and Methods Ag or Pd seeds were first prepared by mixing aqueous solutions of AgNO3 or Na2PdCl4 with sodium citrate. NaBH4 was then added to reduce the Ag or Pd salt. Second, Ag and Pd growth solutions were obtained by mixing aqueous solutions of beta-CD, AgNO3 or Na2PdCl4 with ascorbic acid. Different beta-CD/Ag(Pd) molar ratios were used. Finally, 100 µL of Pd or Ag seeds were injected to initiate growth. Pd NPs were then impregnated onto a TiO2 support. Results and Discussion In the case of Ag, well-defined morphologies were obtained by tuning the beta-CD/Ag molar ratio. At a molar ratio of 50, different morphologies (icosahedra, nanodisks, ..) were found. Increasing the molar ratio to 150 led to the selective formation of icosahedra. The main role of beta-CD is to restrain particle growth favoring the evolution of seeds into icosahedra. In the case of Pd, Pd dendrites were formed at beta-CD/Pd = 20 while increasing this ratio to 100 led to the selective formation of multipods. This is related to an oriented attachment mechanism. Primary particles formed from an heterogeneous nucleation are first attached to the growing seeds while surface diffusion was then restrained by beta-CD leading to dendrites exposing {111} facets. After deposition onto TiO2, Pd/TiO2 catalysts were evaluated in the cinnamaldehyde HYD. Activity results showed that beta-CD does not interfere with the hydrogenation properties. Selectivity results were close to those expected from theoretical calculations validating in real experimental conditions the influence of {111} facets on catalytic properties. Conclusion “Greener” synthesis strategies based on the use of cyclodextrins have been developed for the shape-control of Ag or Pd NPs leading to well-defined morphologies for Ag or to dendrites or multipods for Pd. After deposition onto TiO2, evaluation of catalytic properties allows to validate in real conditions theoretical predictions about this structure-sensitive reaction.