A series of solid oxide fuel cells ͑SOFCs͒ with anodes containing mixtures of either Cu and Ni or Cu and Co, prepared by impregnation of the metal salts into porous yttria-stabilized zirconia ͑YSZ͒, were examined for operation with both H 2 and n-butane fuels at 973 and 1073 K. The Cu-Ni mixtures formed single-phase alloys; however, anodes prepared with Cu 0.8 Ni 0.2 were found to be unstable in n-butane at 973 K if the alloy was reduced at 973 K, but stable if reduced at 1073 K, indicating a probable change in surface composition with reduction temperature. The addition of Ni to Cu significantly increased the performance of the cells for operation in H 2 , but caused no change in performance for n-butane. Unlike the Cu-Ni system, Cu-Co bimetallics formed two phases. It appears that Cu covers the surface of most of the Co, based on reduced CO adsorption and the fact that Cu 0.5 Co 0.5 mixtures were stable for at least 3 h in n-butane at 973 K. Again, at 973 K, the addition of Co to Cu significantly increased the performance of the cells for operation in H 2 , but caused no change in performance for n-butane. At 1073 K, improved performance was observed for both H 2 and n-butane with anodes based on Cu and Co. The results suggest that bimetallics are worth considering for fuel cells that operate on hydrocarbons.

The deposition, stability, and function of carbonaceous films formed by exposing porous yttria-stabilized zirconia ͑YSZ͒ anodes in YSZ-based solid oxide fuel cells ͑SOFCs͒ to n-butane at elevated temperatures was studied using a combination of four-probe conductivity, impedance spectroscopy, and cell polarization measurements. The carbonaceous deposits were found to have high electronic conductivity and to be relatively stable for steam-to-carbon ratios as high as 3.75. Comparison of the performance of cells in which carbon films were used as the sole current collector in the anode with anodes containing both Cu and carbon films indicated that in the latter case, the carbon layer plays an important role in providing electronic conductivity near the three-phase boundary.Metal-ceramic ͑cermet͒ composites, with Ni as the metal, are the most commonly used materials for solid oxide fuel cells SOFC anodes. 1,2 In these composites Ni provides high electronic conductivity, reasonably good high temperature stability, and high catalytic activity for steam reforming. The latter allows for some internal reforming when methane or syngas is used as the fuel. Unfortunately, Ni also catalyzes the formation of carbon fibers if insufficient amounts of steam are present along with methane or CO. 3-5 The problem of carbon fiber formation is particularly severe for hydrocarbons larger than methane. It is well known from the steamreforming literature that high H 2 O:C ratios must be maintained, 3,5,6 higher even than that predicted from thermodynamic considerations, 6 in order to avoid plugging the reactor with carbon while operating with higher hydrocarbon fuels.While it is theoretically possible to operate an SOFC directly on hydrocarbon fuels, this requires replacement of Ni with other electronically conductive materials that do not catalyze carbon formation. In our laboratory, we have been studying Cu-based cermets 7,8 for this purpose. While Cu-YSZ ͑yttria-stabilized zirconia͒ composites are stable in hydrocarbon fuels, it is necessary to add a catalyst, ceria, to the anode in order to achieve reasonable performance. 9,10 Furthermore, the fabrication of Cu-based anodes has required the development of synthetic methods that are different from those used to produce Ni ceramic-metallic ͑cermet͒ composites, because CuO and Cu 2 O melt at the temperatures required for processing YSZ. 8 Rather than calcining mixtures of CuO x and YSZ, the Cu cermets are fabricated by first producing a highly porous YSZ matrix and then adding Cu to the matrix by impregnation with Cu salts.We have recently shown that exposure of Cu-ceria-YSZ anodes to n-butane at 973 K can lead to a large increase in performance due to the formation of carbonaceous residues within the anode. 11 Based on the fact that the enhancement is large for anodes with low Cu contents and small for anodes with high Cu contents, it was concluded that the carbonaceous residues enhance electronic conductivity within the anodes. Analysis of the compounds formed by passing n-butane over ...

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