Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbons

Recent 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

Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration

In 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

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

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

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

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

Preparation of SOFC Anodes by Electrodeposition

Anodes 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

Defect-Mediated Adsorption of Methanol and Carbon Dioxide on BaTiO\u3csub\u3e3\u3c/sub\u3e(001)

The surface chemistry of single crystal barium titanate (BaTiO3) has been studied using temperature programmed desorption (TPD). TPD measurements were performed with several probe molecules, including methanol and carbon dioxide. The role of oxygen vacancies in the adsorption and reaction of these molecules was examined by annealing the crystal under oxidizing or reducing conditions prior to performing TPD. It is shown that the adsorption and reaction of methanol and carbon dioxide are enhanced on BaTiO3(001) by annealing the crystal under reducing conditions

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

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