Gorte, Raymond J
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Publication A Comparison of Molten Sn and Bi for Solid Oxide Fuel Cell Anodes(2010-01-19) Jayakumar, Abhimanyu; Vohs, John M; Lee,; Gorte, Raymond J; Hornés,Molten Sn and Bi were examined at 973 and 1073 K for use as anodes in solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolytes. Cells were operated under "battery" conditions, with dry He flow in the anode compartment, to characterize the electrochemical oxidation of the metals at the YSZ interface. For both metals, the open-circuit voltages (OCVs) were close to that expected based on their oxidation thermodynamics, ~0.93 V for Sn and ~0.48 V for Bi. With Sn, the cell performance degraded rapidly after the transfer of approximately 0.5-1.5 C/cm2 of charge due to the formation of a SnO2 layer at the YSZ interface. At 973 K, the anode impedance at OCV for freshly reduced Sn was approximately 3 Ω cm2 but this increased to well over 250 Ω cm2 after the transfer of 1.6 C/cm2 of charge. Following the transfer of 8.2 C/cm2 at 1073 K, the formation of a 10 µm thick SnO2 layer was confirmed by scanning electron microscopy. With Bi, the OCV anode impedance at 973 K was less than 0.25 Ω cm2 and remained constant until essentially all of the Bi had been oxidized to Bi2O3. Some implications of these results for direct carbon fuel cells are discussed.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.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 Polarization-Induced Hysteresis in CuCo-Doped Rare Earth Vanadates SOFC Anodes(2012-09-14) Adijanto, Lawrence; Gorte, Raymond J; Padmanabhan, Venu Balaji; Vohs, John MThe physical and electrochemical properties of strontium substituted cerium vandates in which a portion of the cerium cations have been substituted with transition metals (Ce0.8Sr0.1Cu0.05TM0.05VO4−0.5x, TM = Ni or Co) were investigated and their suitability for use in solid oxide fuel cell (SOFC) anodes was assessed. Upon reduction at elevated temperature, Cu and Co or Cu and Ni were exsolved from the electronically conductive Ce1−xSrxVO4 lattice to produce Cu-Ni and Cu-Co catalytic nanoparticles. The Ce0.8Sr0.1Cu0.05Co0.05VO3 appears to have high activity and relatively high hydrocarbon tolerance, suggesting that intimate contact between the exsolved Cu and Co and that the majority of the Co nanoparticles must be at least partially coated with the Cu. The electrochemical performance when used in anodes operating on hydrogen has been characterized, and the results demonstrate the exsolution of both metals from the host lattice; but observed dynamic changes in the structure of the resulting metal nanoparticles as a function of SOFC operating conditions complicate their use in SOFC anodes.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 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 Doped-Ceria Diffusion Barriers Prepared by Infiltration for Solid Oxide Fuel Cells(2010-05-24) Küngas, Rainer; Bidrawn, Fred; Vohs, John M; Gorte, Raymond JTo stabilize solid oxide fuel cells cathodes prepared by infiltration of La0.8Sr0.2CoO3 (LSCo) into porous yttria-stabilized zirconia (YSZ), a coating of Sm-doped ceria (SDC) was first deposited onto the YSZ scaffold. The dense SDC coating was prepared by infiltration with aqueous solutions of SM(NO3)3 and Ce(NO3)3, followed by calcination to 1473 K. The SDC coating prevented ~ 20 mΩ cm2, at 973 K, with acceptable degradation after heating to 1373 K.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.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 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).