Various oxides were investigated as a ceramic anode of solid oxide fuel cells ͑SOFCs͒ using LaGaO 3 -based oxide electrolyte. It was found that the cell using Sr-doped LaMnO 3 for anode exhibits a fairly large power density among the examined oxides for anode. Because the anodic overpotential as well as the electrical resistance decreased, the power density of the cell increased with increasing the amount of Sr doped for La site in LaMnO 3 , and the highest power density of the cell was obtained by using La 0.5 Sr 0.5 MnO 3 for anode. On this cell, the maximum power density was achieved to a value of 0.64 and 0.25 W/cm 2 at 1273 and 1073 K, respectively. After exposure of the cell to air at 1273 K for 24 h, almost the same power density was recovered by changing air to H 2 . X-ray diffraction measurement suggests that the perovskite phase of La 0.5 Sr 0.5 MnO 3 anode partially decomposed into La 2 MnO 4 , MnO 2 , and SrMnO 3 after the power generation measurement; however, small power density can only be achieved on the cell using MnO 2 , SrMnO 3 , La 2 MnO 4 , and LaSrMnO 4 for anode. Therefore, LaMnO 3 perovskite phase doped with Sr is active as the anode of SOFCs, which is highly tolerant against oxidation.
The effects of water addition to an oxidant on the cathodic overpotential of La-doped BaCoO 3 and Sm 0.5 Sr 0.5 CoO 3 cathodes were investigated in this study. It was found that the overpotential of La-doped BaCoO 3 decreased by the addition of H 2 O to oxygen. In particular, the decrease in the overpotential by addition of H 2 O was greater with the decreasing operating temperature of the cell. Therefore, it becomes clear that the maximum power density increased by the addition of H 2 O to the oxidant. Positive effects of water on the cathodic overpotential are also observed on Sm 0.5 Sr 0.5 CoO 3 . Therefore, the positive effects of water on cathodic reaction are not limited to La-doped BaCoO 3 , but are also expected on general oxide cathodes. The positive effects of a humidified oxidant were sustained stably over 50 h, and X-ray diffraction measurement suggests that there is no change in the crystal structure of an La-doped BaCoO 3 cathode or an LaGaO 3 -based oxide electrolyte. The improved cathodic activity by addition of H 2 O was studied further by using 18 O surface exchange measurement. Although the diffusion coefficient was independent of the coexisting water, the surface exchange coefficient was greatly increased by the addition of water. Therefore, water existing in a cathode seems to assist the dissociation of oxygen into an oxide ion. Therefore, it is considered that a humidified oxidant is one of the effective methods to suppress the cathodic overpotential at decreased temperature.The development of the intermediate temperature solid oxide fuel cell is an important subject at present. In order to achieve a high power density at intermediate temperature, the development of an electrolyte with low electrical resistance and an active electrode is essential. 1-3 In our previous study, it was found that the LaGaO 3 -based oxide exhibits high oxide ion conductivity, 2,4 and high power density can be achieved by using the LaGaO 3 -based oxide for the electrolyte at intermediate temperature. 4,5 The development of active cathodes is another important subject, because the apparent activation energy of the cathodic reaction on the conventional LaMnO 3 -based oxide is much larger than that of Ni-ZrO 2 doped with a Y 2 O 3 cermet anode. Therefore, with decreasing operating temperature, the cathodic overpotential becomes significant. From this point of view, the development of active cathode catalyst is an important subject for the development of the intermediate temperature solid oxide fuel cell ͑IT-SOFC͒. In our previous study, a Co-based oxide was mainly investigated for the active cathode for IT-SOFC and it was found that La-doped BaCoO 3 is highly active for the cathodic reaction. 6 In particular, Ba 0.6 La 0.4 CoO 3 ͑BLC͒ exhibits a small overpotential, less than 50 mV at 1 A/cm 2 , at 873 K. Therefore, this BaCoO 3 oxide doped with La is expected to be the new cathode for IT-SOFC using an LaGaO 3 -based oxide electrolyte. 6 It is reported that the similar Co-based oxide of Ba͑Sr͒Co͑Fe͒O 3 is the active ...
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