Vohs, John M
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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 TPD study of the reaction of CH3CH2SH and (CH3CH2)2S2 on ZnO(0001) and ZnO(2006-10-01) Halevi, B.; Vohs, John MTemperature programmed desorption (TPD) was used to study the reaction of CH3CH2SH and (CH3CH2)2S2 on the (0001) and surface of ZnO. The interaction of these molecules with ZnO was found to be structure-sensitive. Both the thiol and disulfide adsorbed dissociatively on ZnO(0001) forming adsorbed ethylthiolate intermediates, while only molecularly on ZnO. This result indicates that exposed cation-anion site pairs and exposed cations are the active sites for dissociative adsorption of CH3CH2SH and (CH3CH2)2S2, respectively. Decomposition to produce ethylene and adsorbed sulfur atoms was the only reaction pathway observed for adsorbed ethylthiolates on the (0001) surface. This is in contrast to adsorbed methylthiolates which undergo coupling to produce dimethylsulfide and oxydesulfurization to produce aldehydes and adsorbed carboxylates.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 Reactivity of monolayer V2O5 films on TiO2(110) produced via the oxidation of vapor-deposited vanadium(2003-03-01) Wong, Gordon Sek-Yin; Concepcion, M. R; Vohs, John MThe growth, and reactivity of monolayer V2O5 films supported on TiO2(110) produced via the oxidation of vapor-deposited vanadium were studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Oxidation of vapor-deposited vanadium in 10-7 Torr of O2 at 600 K produced vanadia films that contained primarily V+3, while oxidation in 10-3 Torr at 400 K produced films that contained primarily V+5. The reactivity of the supported vanadia layers for the oxidation of methanol to formaldehyde was studied using TPD. The activity for this reaction was found to be a function of the oxidation state of the vanadium cations in the film.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 SOFC Anodes Based on Infiltration of La0.3Sr0.7TiO3(2008-09-22) Lee, Shiwoo; Vohs, John M; Gorte, Raymond J; Kim, GuntaeComposites formed by infiltration of 45 wt % La0.3Sr0.7TiO3 (LST) into 65% porous yttria-stabilized zirconia (YSZ) were examined for application as solid oxide fuel cell (SOFC) anodes. Although LST does not react with YSZ, the structure of the LST deposits was strongly affected by the calcination temperature. At 1373 K, the LST formed loosely packed, 0.1 µm particles that filled the YSZ pores. The conductivity of this composite depended strongly on the pretreatment conditions but was greater than 0.4 S/cm after heating to 1173 K in humidified (3% H,2O) H2. Following calcination at 1573 K, the LST had sintered significantly, decreasing the conductivity of the composite by a factor of approximately 5. The addition of a catalyst was critical for achieving reasonable electrochemical performance, with the addition of 0.5 wt % Pd and 5 wt % ceria increasing the power density of otherwise identical cells from less than 20 to 780 mW/cm2 for operation in humidified (3% H2O) H2 at 1073 K. Electrodes prepared from LST deposits calcined at 1373 K were found to exhibit a much better performance than those prepared from LST deposits calcined at 1573 K, demonstrating that the structure of the composite is critical for achieving high performance.