Gorte, Raymond J
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Publication An Examination of SOFC Anode Functional Layers Based on Ceria in YSZ(2007-05-22) Vohs, John M; Gross, Michael D.; Gorte, Raymond JThe properties of solid oxide fuel cell (SOFC) anode functional layers prepared by impregnation of ceria and catalytic metals into porous yttria-stabilized zirconia (YSZ) have been examined for operation at 973 K. By varying the thickness of the functional layer, the conductivity of the ceria-YSZ composite was determined to be only 0.015–0.02 S/cm. The initial performance of anodes made with ceria loadings of 40 or 60 wt % were similar but the anodes with lower loadings lost conductivity above 1073 K due to sintering of the ceria. The addition of dopant levels of catalytic metals was found to be critical. The addition of 1 wt % Pd or Ni decreased the anode impedances in humidified H2 dramatically, while the improvement with 5 wt % Cu was significant but more modest. Pd doping also decreased the anode impedance in dry CH4 much more than did Cu doping; however, addition of either Pd or Cu led to similar improvements for operation in n-butane. Based on these results, suggestions are made for ways to improve SOFC anode functional layers.Publication Nano-Socketed Nickel Particles with Enhanced Coking Resistance Grown in situ by Redox Exsolution(2015-01-01) Neagu, Dragos; Oh, Tae-Sik; Miller, David N; Ménard, Hervé; Bukhari, Syed M; Gorte, Raymond J; Gamble, Stephen R; Vohs, John M; Irvine, John T. SMetal particles supported on oxide surfaces are used as catalysts for a wide variety of processes in the chemical and energy conversion industries. For catalytic spplications, metal particles are generally formed on an oxide support by physical or chemical deposition, or less commonly by exsolution from it. Although fundamentally different, both methods might be assumed to produce morphologically and functionally similar particles. Here we show that unlike nickel particles deposited on perovskite oxides, exsolved analogues are socketed into the parent perovskite, leading to enhanced stability and a significant decrease in the propensity for hydrocarbon coking, indicative of a stronger metal-oxide interface. In addition, we reveal key surface effects and defect interactions critical for future design of exsolution-based perovskite materials for catalytic and other functionalities. This study provides a new dimenstion for tailoring particle-substrate interactions in the context of increasing interest for emergent interfactial phenomena.Publication Oxidation Enthalpies for Reduction of Ceria Surfaces(2007-06-15) Zhou, Gong; Shah, Parag R; Montini, Tiziano; Fornasiero, Paolo; Gorte, Raymond JThe thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow-titration and coulometric-titration measurements on high-surface-area ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O2) of 1.6x10-26 atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from -500 kJ/mol O2 at low extents of reduction to near the bulk value of -760 kJ/mol O2 at higher extents. To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm+3-doped ceria (SDC) and Y+3-doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed.Publication A Comparison of LSM, LSF, and LSCo for Solid Oxide Electrolyzer Anodes(2006-09-11) Wang, Wensheng; Huang, Yingyi; Vohs, John M; Jung, Sukwon; Gorte, Raymond JComposite electrodes of yttria-stabilized zirconia (YSZ) with La0.8Sr0.2MnO3 (LSM), La0.8Sr0.2FeO3 (LSF), and La0.8Sr0.2CoO3 (LSCo) were prepared and tested as solid oxide electrolyzer (SOE) anodes and solid oxide fuel cell (SOFC) cathodes at 973 K, using cells with a YSZ electrolyte and a Co-ceria-YSZ counter electrode. The LSM-YSZ electrode was activated by cathodic polarization but the enhanced performance was found to be unstable during electrolysis, with the electrode impedance increasing to near its unenhanced state after 24 h. LSF-YSZ and LSCo-YSZ electrodes exhibited a nearly constant impedance, independent of current density, during both SOE and SOFC operation. The performance of an LSF-YSZ composite for electrolysis current densities above 200 mA/cm2 was unaffected by changing the O2 partial pressure from ~10–2 to 1 atm, while the lower O2 pressure harmed the performance of the LSCo-YSZ composite. The implications of these results for the characterization and optimization of SOE anodes is discussed.Publication A Comparison of Ceria and Sm-Doped Ceria for Hydrocarbon Oxidation Reactions(2004-12-08) Zhao, S.; Gorte, Raymond JThe oxidation of methane, ethane, propane, and n-butane has been studied over CeO2 and Ce0.8Sm0.2Ox (SDC) catalysts. The rates for methane and ethane were found to be indistinguishable over the two catalysts, while the rates for propane and n-butane were much higher on ceria compared to SDC. The difference between n-butane oxidation over ceria and SDC is shown to result from a low-temperature rate process on ceria that is not present on SDC. Measurements using CD4 and C4D10 show that both low- and high-temperature rates exhibit a similar kinetic-isotope effect; however, the low-temperature process is half-order in O2, while the high-temperature process is zeroth-order. Pulse studies demonstrate that oxygen from the bulk becomes accessible for reaction at approximately the same temperature as that at which the high-temperature rate process becomes important. The implications of these results for understanding the effect of doping on reactions over ceria, and for characterization of oxides using standard test reactions, are discussed.Publication Electrodeposition of Cu into a Highly Porous Ni/YSZ Cermet(2006-06-12) Jung, Sukwon; Gorte, Raymond J; Gross, Michael D; Vohs, John MThe electrochemical deposition of Cu into 0.12 cm and 60 µm thick, highly porous 65 vol % Ni/yttria-stabilized zirconia (YSZ) cermets was investigated. An electrochemical cell in which the electrolyte solution was allowed to flow through a porous Ni/YSZ substrate was used to eliminate mass-transfer limitations and to determine the conditions for which the potential drop in the electrolyte solution was minimized and a uniform Cu layer was produced throughout the porous substrate. The conditions determined from these experiments were then used to electrodeposit Cu throughout a thin, porous Ni–YSZ cermet anode layer on a solid oxide fuel cell (SOFC) using a standard nonflow-through setup. This SOFC was found to exhibit stable operation while using methane as the fuel.Publication Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbons(2003-05-01) Gorte, Raymond J; Vohs, John MRecent developments in solid-oxide fuel cells (SOFC) that electrochemically oxidize hydrocarbon fuels to produce electrical power without first reforming them to H2 are described. First, the operating principles of SOFCs are reviewed, along with a description of state-of-the-art SOFC designs. This is followed by a discussion of the concepts and procedures used in the synthesis of direct-oxidation fuel cells with anodes based on composites of Cu, ceria, and yttria-stabilized zirconia. The discussion focuses on how heterogeneous catalysis has an important role to play in the development of SOFCs that directly oxidize hydrocarbon fuels.Publication Novel SOFC Anodes for the Direct Electrochemical Oxidation of Hydrocarbon(2002-04-01) Gorte, Raymond J; Vohs, John M; Kim, H.This paper describes recent developments in solid-oxide fuel cells (SOFC) that use Cu-based cermets as the anode for direct oxidation of hydrocarbon fuels, including liquids such as gasoline, to generate electrical power without the need for first reforming that fuel to H2. Cu-YSZ cermets were found to be stable in hydrocarbon environments, but exhibited low performance for direct oxidation. Reasonable power densities could only be achieved with the addition of a catalytic oxide, like ceria, with the Cu cermet. Electrochemical oxidation studies demonstrated that the initial products for reaction depend on the catalytic oxide. Finally, the effect of sulfur impurities in the fuel is discussed.Publication Anodes for Direct Oxidation of Hydrocarbons in Solid Oxide Fuel Cells(2001-01-01) Park, Seungdoo; Kim, Hyuk; McIntosh, Steven; Gorte, Raymond J; Worrell, Wayne L; Vohs, John MIn this paper we describe the development of Cu/CeO2/YSZ anodes for solid oxide fuel cells (SOFCs) that are active for the direct electrochemical oxidation of dry hydrocarbon fuels. A novel method for synthesizing thin-electrolyte, anode-supported cells is described. This method uses tape-casting of YSZ layers with graphite pore formers, followed by impregnation with aqueous solutions of Cu(NO3)2 and Ce(NO3)2. The performance of model SOFCs with Cu/CeO2/YSZ anodes while operating on a variety of dry hydrocarbon fuels, including methane, butane, decane, and synthetic diesel is reported.Publication The Activity of Fe-Pd Alloys for the Water-Gas Shift Reaction(2004-01-01) Zhao, S.; Gorte, Raymond JThe role of Fe promoters has been investigated on Pd/ceria, Pt/ceria and Rh/ceria catalysts for the water-gas shift (WGS) reaction in 25 Torr of CO and H2O, under differential reaction conditions. While no enhancement was observed with Pt and Rh, the activity of Pd/ceria increased by as much as an order of magnitude upon the addition of an optimal amount of Fe. Similarly, the addition of 1 wt% Pd to an Fe2O3 catalyst increased the WGS rate at 453 K by a factor of 10 over that measured on Fe2O3 alone, while the addition of Pt or Rh to Fe2O3 had no effect on rates. The amount of Fe that was necessary to optimize the rates increased with Pd loading but was independent of the order in which Fe and Pd were added to the ceria. Increased WGS activity was also observed upon the addition of Fe to Pd supported on Ce0.5Zr0.5O2. XRD measurements, performed after running the catalyst under WGS conditions, show the formation of a Fe-Pd alloy, even though similar measurements on an Fe/ceria catalyst showed that Fe3O4 was the stable phase for Fe in the absence of Pd. Possible implications of these results on the development of new WGS catalysts are discussed.