Composites of yttria-stabilized zirconia ͑YSZ͒ with Sr-doped LaFeO 3 ͑LSF͒ were studied for application as high-performance cathodes for solid oxide fuel cells ͑SOFCs͒. The composites were formed by aqueous impregnation of porous YSZ with La, Sr, and Fe salts, followed by calcination at various temperatures. X-ray diffraction measurements showed that the LSF perovskite phase had formed by 1023 K and that solid-state reaction with the YSZ did not occur below approximately 1223 K. The electronic conductivity of the 40 wt % LSF-YSZ composite was maximized by calcination at 1123 K. SOFCs prepared with a 40 wt % LSF-YSZ cathode showed improved performance over SOFCs prepared with conventional LSM-YSZ cathodes at 973 K, although the performance of cells made with cathodes having lower LSF content did not perform as well. Based on measurements with a reference electrode on an electrolyte-supported cell, the impedance of the 40 wt % LSF-YSZ cathode is approximately 0.1 ohm cm 2 in air at 973 K. Finally, a cathode-supported cell was fabricated from a 40 wt % LSF-YSZ cathode and shown to perform well in H 2 .

Composites of La 0.8 Sr 0.2 MnO 3 ͑LSM͒ and yttria-stabilized zirconia ͑YSZ͒ were prepared by impregnating porous YSZ with salts of Sr, La, and Mn to 40 wt % LSM. The effect of calcination temperature on cathode performance at 973 K was then studied using both symmetric cells and solid oxide fuel cells ͑SOFCs͒. Following calcination at 1123 K, the LSM in the composite remained porous. In symmetric-cell tests, LSM͑1123͒-YSZ electrodes exhibited a low loss, 0.5 ⍀ cm 2 , which was unchanged by polarization; however, in fuel cell tests, the performance was poor at current densities above 0.2 A/cm 2 . Following calcination at 1323 or 1523 K, LSM formed a dense layer coating on the YSZ. In symmetric-cell tests, the composite exhibited an ASR greater than 4 ⍀ cm 2 , but this decreased, reversibly, upon cathodic polarization or upon heating in humidified H 2 at 973 K. In fuel cell tests, the performance of composites calcined at 1323 K was superior to ones at 1523 K, primarily because the ohmic resistance of the cell was 0.25 ⍀ cm 2 higher after 1523 K, probably due to solid-state reactions with the electrolyte. The optimal conditions for forming impregnated LSM-YSZ cathodes and the reasons for the observed hysteresis in cathode polarization are discussed based on these results.

Porous yttria-stabilized zirconia ͑YSZ͒ was impregnated with aqueous solutions of the nitrate salts of Sr, La, and Co to obtain composites of La 0.6 Sr 0.4 CoO 3Ϫ␦ ͑LSCo͒ and YSZ. X-ray diffraction showed that the perovskite phase formed after heating to 973 K and conductivities as high as 7 S/cm were achieved at 973 K with only 30 wt % LSCo. Coefficient of thermal expansion ͑CTE͒ measurements of the composites with as much as 50 wt % LSCo indicated that the CTEs were much lower than the weighted average of the YSZ and LSCo. Electrochemical measurements using symmetric cells in air indicated that the area-specific resistance ͑ASR͒ of the composites could be as low as 0.03 ⍀ cm 2 at 973 K and 0.01 ⍀ cm 2 at 1073 K. These low ASR were verified in a cell with an LSCo-YSZ cathode and a Co-ceria-YSZ anode, which exhibited a total electrode polarization loss of 0.25 ⍀ cm 2 at 973 K and 0.1 ⍀ cm 2 at 1073 K for operation in humidified H 2 . However, changes in electrochemical performance with time showed that some electrode degradation occurred over a period of 250 h, even at 973 K.

Composite electrodes of yttria-stabilized zirconia (YSZ) with La 0.8 Sr 0.2 MnO 3 (LSM), La 0.8 Sr 0.2 FeO 3 (LSF), and La 0.8 Sr 0.2 CoO 3 (LSCo) were prepared and tested as solid oxide electrolyzer (SOE) anodes and solid oxide fuel cell (SOFC) cathodes at 973 K, using cells with a YSZ electrolyte and a Co-ceria-YSZ counter electrode. The LSM-YSZ electrode was activated by cathodic polarization but the enhanced performance was found to be unstable during electrolysis, with the electrode impedance increasing to near its unenhanced state after 24 h. LSF-YSZ and LSCo-YSZ electrodes exhibited a nearly constant impedance, independent of current density, during both SOE and SOFC operation. The performance of an LSF-YSZ composite for electrolysis current densities above 200 mA/cm 2 was unaffected by changing the O 2 partial pressure from ~10 -2 to 1 atm, while the lower O 2 pressure harmed the performance of the LSCo-YSZ composite. The implications of these results for the characterization and optimization of SOE anodes is discussed.

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