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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 ...

The performance of solid-oxide fuel cells ͑SOFCs͒ with Cu-ceria yttria-stabilized zirconia anodes in n-butane at 973 K has been studied as a function of fuel conversion. In order to simulate the local anode environment at high fuel utilization, n-butane was oxidized with O 2 in a separate reactor before sending the fuel to a small 0.45 cm 2 cell. When n-butane oxidation was carried out over a ceria catalyst at 973 K, the main oxidation products were CO 2 and H 2 O. By comparing the performance of the cell in the partially oxidized fuel to the performance of the cell in n-butane diluted in He, it was demonstrated that CO 2 and H 2 O have only minimal effect on cell performance other than to dilute the fuel. This dilution of the fuel results in a significant decrease in performance at higher fuel conversions. When n-butane oxidation was carried out over a Pd-ceria catalyst, significant amounts of H 2 were generated by steam reforming of hydrocarbons with the steam generated by hydrocarbon oxidation. As a result, the maximum power density generated by the cell increased with fuel conversions up to 30% and remained very high at fuel conversions up to at least 70%. Based on these results, it is suggested that the inclusion of a steam-reforming catalyst within the anode compartment of direct-conversion SOFC should improve their performance at high fuel utilization.Work in our laboratory has recently demonstrated the direct electrochemical conversion of a variety of hydrocarbon fuels in solidoxide fuel cells ͑SOFCs͒ using Cu-based anodes. 1-4 Direct conversion, without the addition of steam or air to internally reform the fuel, is made possible by the fact that Cu, unlike Ni, does not catalyze the formation of carbon in the presence of dry hydrocarbons at 973 K. 5,6 Fuel cells that can produce electricity by direct electrochemical conversion of hydrocarbons could have significant efficiency advantages over conventional systems that require the fuel to first be reformed, especially when using hydrocarbons larger than methane. Because steam reforming of higher hydrocarbons requires the use of high H 2 O:C ratios to avoid coke formation, 7-9 it is usually necessary to do autothermal or oxy-reforming of these fuels. 8,9 In addition to the loss of fuel efficiency that occurs with oxidation, the addition of air to the fuel results in dilution of the fuel by N 2 . For example, even ideal oxy-reforming of C 4 H 10 to CO and H 2 with air results in a fuel that has a mole fraction of N 2 that is 47%. Since the ultimate fuel utilization that can be achieved is limited by the concentration of fuel remaining at the stack exit, this N 2 dilution lowers overall cell efficiency.However, fuel dilution can also be a problem in directconversion fuel cells because each fuel molecule produces a larger number of product molecules. For example, in the case of butane, 9 mol of CO 2 or H 2 O are formed for each mole of butane that reacts. These product molecules dilute the fuel so that the concentration of butane is only 10% at 50% fuel convers...

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The morphology and reducibility of vapor-deposited ceria films supported on yttria-stabilized zirconia (100) (YSZ(100)) and α-Al 2 O 3 (0001) single crystals were studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The results of this study show that the gas environment has a significant effect on the structure of the ceria films on both substrates. CeO 2 films on α-Al 2 O 3 (0001) were found to be stable in a reducing environment at temperatures up to 1000K, but underwent agglomeration and reaction with the support to form CeAlO 3 upon annealing at 1273 K in air. Heating CeO 2 /YSZ(100) in air at 1273 K caused the ceria thin film to agglomerate into bar-shaped features which were re-dispersed by subsequent annealing in vacuum. Interactions at the CeO 2-YSZ interface were also found to dramatically enhance the reducibility of ceria films supported on YSZ(100).