No abstract

The structure, oxygen stoichiometry, and chemical and thermal expansion of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) between 873 and 1173 K and oxygen partial pressures of 1 × 10 -3 to 1 atm were determined by in situ neutron diffraction. BSCF has a cubic perovskite structure, space group Pm3 hm, across the whole T-pO 2 region investigated. The material is highly oxygen deficient with a maximum oxygen stoichiometry (3δ) of 2.339(12) at 873 K and a pO 2 of 1 atm and a minimum of 2.192(15) at 1173 K and a pO 2 of 10 -3 atm. Good agreement is obtained between oxygen stoichiometry data determined by neutron diffraction and thermogravimetry. In the range covered by the experiments, the thermal and chemical expansion coefficients are 19.0(5)-20.8(6) × 10 -6 K -1 and 0.016(2)-0.026(4), respectively.

We have examined the changes that occur in the performance of solid oxide fuel cells ͑SOFCs͒ with Cu-ceria-yttria-stabilized zirconia anodes at 973 K following exposure to various hydrocarbon fuels, including methane, propane, n-butane, n-decane, and toluene. For cells with Cu contents of 20 wt % or less, large increases were observed in the power densities for operation in H 2 after the anode had been exposed to any of the hydrocarbons except methane. The increased performance is completely reversible upon oxidation of the anode and subsequent reduction in H 2 . The enhancement decreases with increasing Cu content, implying that the deposits improve the connectivity of the metallic phase in the anode. Impedance spectra taken on cells before and after exposure to hydrocarbon fuels confirm that the conductivity of the anode improves after exposure. Temperature-programmed oxidation and weight changes were used to show that the deposits that enhance performance correspond to ϳ1 wt % of the anode and are probably not graphitic. Measurements of the open-circuit voltages in hydrocarbon fuels suggest that equilibrium is established with partial oxidation products and that the chemical structure of the deposits change upon current flow. Finally, the implications of these results for operation of SOFC on hydrocarbons without added steam and with low copper contents are discussed.

The performance of electrodes in direct-utilization, solid oxide fuel cells ͑SOFCs͒ has been studied on anode-supported and electrolyte-supported cells using impedance spectroscopy, coupled with calculations of the potential distribution in the electrolyte. The cells in these studies were composed of a Cu-ceria-yttria-stabilized zirconia ͑YSZ͒ anode, a YSZ electrolyte, and a Sr-doped LaMnO 3 ͑LSM͒-YSZ cathode and were operated at 983 K using both H 2 and n-butane as fuel. Both calculations and experiments show that three-electrode measurements on anode-supported electrolytes, with the reference electrode opposite the anode, provide no additional information over two-electrode measurements and cannot be used to estimate the performance of individual electrodes. Three-electrode measurements were able to estimate anode and cathode performance on thick, electrolyte-supported cells, with symmetric placement of the working electrodes. However, both experiments and calculations demonstrate that differences in the kinetics of the two electrodes make perfect separation of anode and cathode processes difficult. The cathode performance of LSM-YSZ in these experiments was described by a single arc in the Cole-Cole plot, with a frequency of 2 kHz and a resistance of 0.4 ⍀ cm 2 . The performance of the anode in H 2 was also characterized by a single arc, with a frequency of 4 Hz and a resistance of 0.8 ⍀ cm 2 . While anode performance in H 2 is only weakly dependent on current density, nonlinear processes are observed with n-butane, so that the area-specific resistances depend strongly on the current density.The focus of research on solid oxide fuel cells ͑SOFCs͒ in our laboratory has been on the development of alternative anodes that allow the direct, electrochemical oxidation of hydrocarbon fuels in the absence of added steam or air. Because Ni, the most commonly used metal for SOFC anodes, 1 catalyzes the formation of carbon filaments when exposed to hydrocarbons at SOFC operating temperatures, 2-5 it must be replaced with a different electronic conductor that is not catalytically active for this reaction. We have focused primarily on replacing Ni with Cu, since Cu is a poor catalyst for carbon formation. 6,7 Ceria is included in the anode to enhance anode performance, in part because of the catalytic activity of ceria for oxidation of hydrocarbon fuels. 8 It has been shown that Cuceria-yttria-stabilized zirconia ͑YSZ͒ anodes are capable of direct, electrochemical oxidation of various fuels, including hydrocarbons that are liquids at room temperature. 9,10 While the performance that has been achieved with the Cu-ceria-YSZ anodes is reasonable, further optimization of the directoxidation SOFC requires better monitoring of the anode performance. Separation of the losses associated with the anode from the losses associated with the electrolyte and the cathode is usually accomplished using reference electrodes, but there is no consensus on what electrode geometry gives the best results. Some groups simply place a reference elec...