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

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

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

An Examination of Sulfur Poisoning on Pd/Ceria Catalysts

The species formed by exposure of a Pd/ceria catalyst to SO2 under various conditions have been studied using temperature-programmed desorption (TPD) and FTIR. For adsorption of SO2 between 298 and 473 K, a molecular SO2 species adsorbs on the surface, possibly as a surface sulfite; and this species converts to a sulfate above 473 K. Exposure of Pd/ceria to SO2 at temperatures above 473 K in the presence of O2 results in the formation of bulk sulfates. These sulfates decompose to form SO2 and O2 upon TPD in He, with O2 and SO2 peaks at 1023 K assigned to Ce+4 sulfates and peaks at 1123 K assigned to Ce+3 sulfates. When H2 is present in the TPD carrier gas, the sulfates are reduced and a significant fraction of the sulfur is removed as H2S, with the rest remaining as Ce2O2S. When CO is present in the TPD carrier gas, all of the sulfates are reduced to Ce2O2S, with the simultaneous formation of CO2. The formation of CO2 from the reduction of the sulfate occurs in the same temperature range as CO2 production from reduction of Pd/ceria, except that more CO2 is formed from the sulfur-poisoned catalyst. The implications of these results for understanding oxygen storage capacity (OSC) in automotive, three-way catalysts is discussed