Determining the Ce\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3eS-CeO\u3csub\u3ex\u3c/sub\u3e Phase Boundary for Conditions Relevant to Adsorption and Catalysis

The interaction of sulfur with ceria under highly reducing conditions was investigated. The phase boundary between CeO1.83 and Ce2O2S was determined for temperatures between 873 and 1073 K. This data was used to derive an empirical equation for ΔGfº of Ce2O2S in this temperature range. This equation along with thermodynamic data for cerium oxides and sulfides obtained form the literature was used to predict Ce-O-S phase diagrams at 873 and 973 K. These phase diagrams provide insight into the mechanism of the deactivation of ceria-based catalysts by sulfur under reducing conditions

TPD and XPS Investigation of the Interaction of SO\u3csub\u3e2\u3c/sub\u3e with Model Ceria Catalysts

The interaction of SO2 with model thin film ceria catalysts was studied using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). During TPD of ceria samples that had been exposed to SO2 at room temperature, SO2 desorbed in peaks centered at 473, 883 and 963 K. The lower temperature peak is associated with molecularly adsorbed SO2, while the higher temperature peaks are due to decomposition of adsorbed sulfates. XPS results show that at room temperature a small fraction of the adsorbed SO2 is oxidized to SO42- using oxygen supplied by the ceria. This reaction also results in partial reduction of the ceria surface. High coverages of surface sulfate species could be produced by exposing the ceria samples to mixtures of SO2 and O2 at 573 K. In addition to producing gaseous SO2, thermal decomposition of surface sulfates at temperatures above 823 K resulted in the formation of an oxy-sulfide (Ce2O2S) on the ceria surface

Recent Developments on Anodes for Direct Fuel Utilization in SOFC

This paper reviews recent work on SOFC anode fabrication at the University of Pennsylvania. In this work, anode fabrication is based on the preparation of a porous YSZ matrix, into which electronic and catalytic components are added by impregnation of the appropriate metal salts. First, the methods used to prepare porous YSZ are described, along with a description of the structures that are obtained. Next, it is demonstrated that cell performance is strongly affected by the methods used to impregnate and pretreat ceria that is added to the porous YSZ. Third, the role of carbonaceous deposits within the anode is discussed. These deposits can lead to improved electronic conductivity that results in improved performance. Finally, the effect of precious-metal dopants, added to ceria to improve the catalytic properties of the anode, is discussed. Pd, Pt, and Rh are shown to give large increases in the performance of the cells, particularly in CH4

Novel SOFC Anodes for the Direct Electrochemical Oxidation of Hydrocarbon

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

Analysis of the Performance of the Electrodes in a Natural Gas Assisted Steam Electrolysis Cell

The 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

Hydrogen Production via CH\u3csub\u3e4\u3c/sub\u3e and CO Assisted Steam Electrolysis

Porous composite anodes consisting of a yttria-stabilized zirconia (YSZ) backbone that was impregnated with CeO2 and various amounts of metallic components including Cu, Co and Pd were fabricated. The performance of these anodes was then tested in a solid oxide water electrolysis cell under conditions where the anode was exposed to the reducing gasses H2, CH4 and CO. The reducing gasses were used to decrease the electrochemical potential of the cell and increase overall efficiency. The results of this study show that Cu-CeO2-YSZ anodes have low catalytic activity for the oxidation of CO and CH4 and are not very effective in lowering the cell potential while operating in the reducing gas assisted mode. The addition of Co to the Cu-CeO2- YSZ anode resulted in a modest increase in the catalytic activity and enhanced the thermal stability of the anode. A Pd-C-CeO2-YSZ anode was found to have the highest catalytic activity of those tested and gave the largest reductions in the operating potential of the solid oxide electrolysis cell

Effect of the Ionic Conductivity of the Electrolyte in Composite SOFC Cathodes

Solid oxide fuel cell (SOFC) cathodes were prepared by infiltration of 35 wt % La0.8Sr0.2FeO3 (LSF) into porous scaffolds of three, zirconia-based electrolytes in order to determine the effect of the ionic conductivity of the electrolyte material on cathode impedances. The electrolyte scaffolds were 10 mol % Sc2O3-stabilized zirconia (ScSZ), 8 mol % Y2O3-stabilized zirconia (YSZ), and 3 mol % Y2O3- 20 mol % Al2O3-doped zirconia (YAZ), prepared by tape casting with graphite pore formers. Each electrolyte scaffold was 65% porous, with identical pore structures as determined by scanning electron microscopy (SEM). Both symmetric cells and fuel cells were prepared and tested between 873 and 1073 K, using LSF composites that had been calcined to 1123 or 1373 K. Literature values for the electrolyte conductivities were confirmed using the ohmic losses from the impedance spectra. The electrode impedances decreased with increasing electrolyte conductivity, with the dependence being between to the power of 0.5 and 1.0, depending on the operating temperature and LSF calcination temperature

An Examination of Lanthanide Additives on the Performance of Cu-YSZ Cermet Anodes

The effect of various lanthanide additives on the performance of Cu-YSZ (yttria-stabilized zirconia), cermet anodes for solid-oxide fuel cells (SOFCs) was investigated at 973 K for H2 and the direct oxidation of butane. In all cases, the lanthanide oxides were added to the SOFC by impregnation of a porous YSZ matrix with aqueous solutions of the nitrate salts, followed by decomposition of nitrate ions by calcination. Ceria was found to be significantly more effective in promoting SOFC performance compared to the other lanthanides, and the performance of the lanthanide additives followed the catalytic activity observed for butane oxidation with 100 torr each of butane and O2. Samaria doping of ceria led to a slight decrease in performance but also decreased the catalytic active of ceria for butane oxidation. Membrane-reactor studies with propylene fed to Cu-molybdena-YSZ anodes at 723 K showed a high selectivity to acrolein, although Cu-ceria-YSZ anodes showed only total oxidation products under these conditions, implying that the catalytic properties of the oxides must be important. Finally, the application of these results to improved SOFC for direct oxidation of hydrocarbons is discussed