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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 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 Modification of LSF-YSZ Composite Cathodes by Atomic Layer Deposition(2017-01-01) Rahmanipour, M.; Cheng, Yuan; Onn, Tzia M; Vohs, John M; Donazzi, A.; Gorte, Raymond Jcomposite, Solid-Oxide-Fuel-Cell (SOFC) electrodes of La0.8Sr0.2FeO3 (LSF) and yttria-stabilized zirconia (YSZ) were prepared by infiltration methods and then modified by Atomic Layer Deposition (ALD) of ZrO2, La2O3, Fe2O3, or La2O3-Fe2O3 codeposited films of different thicknesses to determine the effect of surface composition on cathode performance. Film growth rates for ALD performed using vacuum procedures at 573 K for Fe2O3 and 523 K for ZrO2 and La2O3 were determined to be 0.024 nm ZrO2/cycle, 0.019 nm La2O3/cycle, and 0.018 nm Fe2O3/cycle. For ZrO2 and Fe2O3, impedance spectra on symmetric cells at 873 K indicated that polarization resistances increased with coverage in a manner suggesting simple blocking of O2 adsorption sites. With La2O3, the polarization resistance decreased with small numbers of ALD cycles before again increasing at higher coverages. When La2O3 and Fe2O3 were co-deposited, the polarization resistances remained low at high film coverages, implying that O2 adsorption sites were formed on the co-deposited films. The implications fo these results for future SOFC electrode development are discussed.Publication An Investigation of LSF-YSZ Conductive Scaffolds for Infiltrated SOFC Cathodes(2017-01-01) Cheng, Yuan; Oh, Tae-Sik; Gorte, Raymond J; Wilson, Rachel; Vohs, John MPorous compostites of Sr-doped LaFeO3 (LSF and yttria-stabilized zirconia (YSZ) were investigated as conductive scaffolds for infiltrated SOFC cathodes with the goal of producing scaffolds for which only a few perovskite infiltration steps are required to achieve sufficient conductivity. While no new phases form when LSF-YSZ composites are calcined to 1623 K, shifts in the lattice parameters indicate Zr can enter the perovskite phase. Measurements on dense, LSF-YSZ composites show that the level of Zr doping depends on the Sr:La ration. Because conductivity of undoped LSF increases with Sr content while both the iconic and electronic conductivities of Zr-doped LSF decrease with the level of Zr in the perovskite phase, there is an optimum initial Sr content corresponding to La0.9Sr0.1FeO3 (LSF91). Although schaffolds made with 100% LSF had a higher conductivity then scaffolds made with 50:50 LSF-YSZ mixtures, the 50:50 mixture provides the optimal interfacial structure with the electrolyte and sufficient conductivity, providing the best cathode performance upon infiltration of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF).Publication Zirconia-Based Electrolyte Stability in Direct-Carbon Fuel Cells with Molten Sb Anodes(2015-01-01) Zhou, Xiaoliang; Vohs, John M; Oh, Tae-Sik; Gorte, Raymond JDirect carbon fuel cells (DCFC) that use zirconia-based electrolytes and molten Sb anodes have much promise for the efficient conversion of carbonaceous solid fuels into electricity. However, etching of the electrolyte, and ultimately cell failure, has been observed during operation. In this study, we have investigated this etching phenomenon as a function of the electrolyte composition and cell operating conditions and demonstrated that it is not electrochemical in nature, but rather results from reaction between the electrolyte and Sb2O3.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 Multilayer High-Performance Ceramic Anodes(2007-01-01) Gorte, Raymond J; Gross, Michael D.; Vohs, John MA new approach to the design of ceramic anodes that uses a thin catalytically active functional layer that has only modest electronic conductivity sandwiched between the electrolyte and a non-catalytic elecronically conducting ceramic layer that is used as the current collector is described. The anode design is flexible and allows various materials to be used in the functional and current collector layers. Results are presented for anodes with thin functional layers (12 µm) consisting of a porous CeO2/YSZ composite impregnated with 1 wt% Pd to optimize catalytic activity and a 100 µm thick layer of porous La0.3Sr0.7TiO3 (LST) as the current collector. Low anode impedances and excellent overall performance were obtained with cells with these anodes while operating on both humidified hydrogen and hydrocarbon fuels.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 Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration(2011-04-01) Küngas, Rainer; Vohs, John M; Gorte, Raymond JIn this study, the effect of the morphology and ionic conductivity of the electrolyte material in SOFC composite cathodes is systematically studied. The specific surface area of prous yttria-stabilized zirconia (YSZ) scaffolds was varied by almost two orders of magnitude using different pore formers and surface treatment with hydrofluoric acid (HF). The effect of ionic conductivity on the performance of SOFC cathodes was studied for electrodes prepared by infiltration of 35 wt % LSF into 65% porous scandia-stabilized zirconia (ScSZ), YSZ, or yttria-alumina co-stabilized zirconia (YAZ) scaffolds of identical microstructure cathodes.Publication Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration(2011-03-21) Bidrawn, Fred; Küngas, Rainer; Vohs, John M; Gorte, Raymond JA mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2MnO3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10-3 and 10-4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10-7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.