Gorte, Raymond J
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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.Publication Preparation of SOFC Anodes by Electrodeposition(2007-10-11) Vohs, John M; Jung, Sukwon; Gorte, Raymond JAnodes for solid oxide fuel cells (SOFCs) have been prepared by electrodeposition of either Co or Ni into a layer of porous yttria-stabilized zirconia (YSZ), 60 µm thick. The YSZ, having 65% porosity, was prepared by tape casting with graphite pore formers and was attached to the dense YSZ electrolyte. After adding 10 vol % CeO2 by impregnation of aqueous solutions of CeNO3)3, followed by calcination at 723 K, the porous YSZ was made conductive by exposing it to n-butane at 1123 K to form a coating of carbon. As much as 40 vol % metal could be added to the porous layers, while the carbon could then be removed by exposing the anode to humidified H2 at SOFC operating temperatures. The ohmic losses in cells containing 40 vol % Co or 30 vol % Ni were unaffected by heating to 1173 K. Finally, a cell with 15 vol % Cu and 15 vol % Co was prepared by electrodeposition of Cu onto electrodeposited Co. No carbon formation was observed on the Cu–Co anode following exposure to dry methane at 1073 K.Publication An Examination of SOFC Anode Functional Layers Based on Ceria in YSZ(2007-08-21) 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 Engineering Composite Oxide SOFC Anodes for Efficient Oxidation of Methane(2008-02-14) Kim, Guntae; Corre, G.; Vohs, John M; Irvine, J. T. S.; Gorte, Raymond JCeramic anodes for solid oxide fuel cells SOFCs were prepared by aqueous impregnation of nitrate salts to produce composites with 45 wt % La0.8Sr0.2Cr0.5Mn0.5O3 (LCSM)in a 65% porous yttria-stabilized zirconia (YSZ) scaffold. Scanning electron micrographs indicate that the LSCM coats the YSZ pores following calcination at 1473 K. Composites produced in this manner exhibit conductivities at 1073 K of approximately 1 S/cm in air and 0.1 S/cm in humidified H2. A SOFC with a composite anode composed of 45 wt % LSCM, 0.5 wt % Pd, and 5 wt % ceria exhibited maximum power densities at 1073 K of 1.1 and 0.71 W cm−2 in humidified (3% H2O) H2 and methane, respectively.Publication Analysis of the Performance of the Electrodes in a Natural Gas Assisted Steam Electrolysis Cell(2008-02-01) Gorte, Raymond J; Wang, Wensheng; Vohs, John MThe performance of solid oxide electrolysis (SOE) cells while operating in the natural gas assisted steam electrolysis (NGASE) mode was evaluated. The SOE cells used yttria-stabilized-zirconia (YSZ) as the oxygen ion conducting electrolyte, Co–CeO2–YSZ as the H2–H2O electrode, and Pd-doped CeO2 YSZ source as the CH4-oxidation electrode. The cell electrochemical performance was evaluated as a function of the H2O/H2 ratio and the extent of conversion of CH4. The results of this study provide insight into the factors that control electrode performance and further demonstrate the viability of an NGASE cell for the production of H2.Publication An Examination of Brønsted-Acid Sites in H-[Fe]ZSM-5 for Olefin Oligomerization and Adsorption(2002-08-15) Kresnawahjuesa, Oferi; Kühl, G. H; Gorte, Raymond J; Quierini, C. A.The adsorption and reaction properties of an Al-free H-[Fe]ZSM-5 were examined and compared to an H-[Al]ZSM-5 sample with the same site density. H-[Fe]ZSM-5 was shown to have Brønsted-acid sites in a concentration equal to the framework Fe concentration. Differential heats of adsorption for ammonia and pyridine were shown to be identical to that obtained in H-[Al]ZSM-5, with differential heats of ~150 kJ/mol for ammonia and 200 kJ/mol for pyridine. For H-[Al]ZSM-5, adsorption of either propylene or 1-butene at room temperature results in rapid oligomerization. TPD-TGA measurements of the oligomers in H-[Al]ZSM-5 show evidence for hydride-transfer reactions, in addition to simple oligomer cracking. By contrast, it is necessary to heat H-[Fe]ZSM-5 to 370 K for rapid oligomerization of propylene and oligomerization of 1-butene occurs only slowly at 295 K. TPD-TGA measurements of the oligomers in H-[Fe]ZSM-5 show no evidence for hydride-transfer reactions and H-[Fe]ZSM-5 forms much less coke than H-[Al]ZSM-5 during steady-state reaction in 1-butene at 573 K. Adsorption measurements of 1-butene on D-[Fe]ZSM-5 suggest that the protonated complexes of 1-butene are formed but that these are relatively stable towards reaction, implying that the carbocation transition states are relatively unstable.Publication A Study of Thermal Stability and Methane Tolerance of Cu-Based SOFC Anodes with Electrodeposited Co(2007-01-01) Vohs, John M; Gross, Michael D; Gorte, Raymond JCu-based, solid oxide fuel cell (SOFC) electrodes were modified by electrodeposition of Co. The addition of only 5-vol% Co by electrodeposition significantly improved the thermal stability compared to either Cu-ceria-YSZ, Cu-Co-ceria-YSZ, or Co-ceria-YSZ electrodes prepared only by impregnation with much higher metal loadings, demonstrating that electrodeposited metal layers form metal films with better connectivity. In the absence of Co, SEM showed structural changes in the impregnated Cu after heating to 1173 K in humidified H2 and these changes caused large increases in the ohmic resistance of fuel cells, as measured by impedance spectroscopy. In contrast, the ohmic resistance of a cell with 13-vol% Cu, 9-vol% ceria, and 5-vol% Co increased only slightly after 48 h at 1173 K in humidified H2. While a Co-ceria-YSZ composite was found to form large amounts of carbon upon exposure to dry CH4 at 1073 K for 3 h, the Co-Cu-ceria-YSZ composites did not form measurable amounts of carbon for the same conditions. XPS results for a Cu foil with a 250-nm Co film demonstrated that Cu migrates to the surface of the Co upon heating above 873 K, forming a stable Cu layer that appears to be approximately one monolayer thick. The implication of these results for the development of practical SOFC electrodes for the direct utilization of hydrocarbons is discussed.Publication Development of Intermediate-Temperature Solid Oxide Fuel Cells for Direct Utilization of Hydrocarbon Fuels(2004-11-30) Lu, C.; An, Sungbae; Vohs, John M; Worrell, Wayne L.; Gorte, Raymond JIn this study, we compare the performance of SOFC having composite Cu-based anodes but made with the following low-temperature electrolytes: samaria-doped ceria (SDC), Sr- and Mg-doped lanthanum gallate (LSGM), and scandia-stabilized zirconia (ScSZ). Performance (V-I) curves and impedance spectra were measured using H2 and n-butane fuels at 973 K. The results suggest that the use of electrolyte materials with higher ionic conductivity can lead to improved anodes for direct-utilization SOFC, although the performance of each of the cells in n-butane appears to be at least partially limited by the electrochemical oxidation reaction.Publication Ultra-Thin CeO2 Overlayer on YSZ Studied by X-Ray Surface Scattering(1997) Dmowski, Wotjek; Gorte, Raymond J; Putna, S.; Vohs, John M; Egami, T.Transition metal catalysts such as Rh/Pt used in a three-way automotive catalytic converter have to perform reduction and oxidation functions at the same time. This can be accomplished only in a specific range of oxygen pressure and temperature. In order to maintain a constant partial pressure of oxygen in the vicinity of catalysts mixtures of ceria and zirconia are used. Ceria is an essential component due to its capability of storing oxygen under oxidizing and releasing oxygen under reducing conditions. However, this function deteriorates with time and eventually a catalytic converter stops working properly. It is not well understood why this particular mixture of oxides can achieve the role as a oxygen buffer and why its lifetime is limited. In order to address this issue and to understand the structural interplay at the ceria/zirconia interface, we studied the atomic structure of ultra-thin ceria layers deposited on single crystals of (001) oriented Y-stabilized zirconia (YSZ), in situ, during annealing in air using the synchrotron x-ray surface diffraction technique.Publication The Acylation of Propene by Acetic Acid over H-[Fe]ZSM-5 and H-[Al]ZSM-5(2004-04-01) Gorte, Raymond J; Kresnawahjuesa, Oferi; White, DavidWe have studied the vapor-phase reaction of acetic acid with propene over H-[Al]ZSM-5 and H-[Fe]ZSM-5 molecular sieves between 425 and 600 K, for acetic acid partial pressures of 25 to 50 Torr, with propene:acetic acid ratios between 1 and 4. Reasonable conversions were obtained above 450 K. Both H-[Fe]ZSM-5 and H-[Al]ZSM-5 showed high activities at 450 K, with selectivities to propyl acetates of ~90% using a propene:acetic acid ratio of 2 and ketones formed by dehydration of the acetates making up most of the remaining products. The reaction rate was completely stable at 450 K for at least 20 h, and the product distribution was insensitive to the reactant feed ratio for a propene:acetic acid ratio above 2. On H-[Fe]ZSM-5, dehydration of the acetates became increasingly important at higher temperatures, with the selectivity to ketones approaching 80% at 525 K. At still higher temperatures, reaction pathways that did not involve propyl acetates became important. The high selectivity for forming propyl acetates and the absence of olefin oligomerization is believed to result from having the acid sites occupied (saturated) by acetic acid. Finally, the acid-site density measured on H-[Fe]ZSM-5 after weeks of catalyst testing remained the same as the initial site density, showing that this material has sufficient stability for fine-chemicals synthesis.