The effects of doping Co for the Ga site on the oxide ion conductivity of La 0.8 Sr 0.2 Ga 0.8 -Mg 0.2 O 3 have been investigated in detail. It was found that doping Co is effective for enhancing the oxide ion conductivity. In particular, a significant increase in conductivity in the low-temperature range was observed. The electrical conductivity was monotonically increased; however, the transport number for the oxide ion decreased with an increasing amount of Co. Considering the transport number and ion transport number, an optimized amount for the Co doping seems to exist at 8.5 mol % for Ga site. The theoretical electromotive forces were exhibited on H 2 -O 2 gas cell utilizing the optimized composition of La 0.8 Sr 0.2 -Ga 0.8 Mg 0.115 Co 0.085 O 3 . The diffusion characteristics of the oxide ion in La 0.8 Sr 0.2 Ga 0.8 Mg 0.115 -Co 0.085 O 3 were also investigated by using the 18 O tracer method. Since the diffusion coefficient measured by the 18 O tracer method was similar to that estimated by the electrical conductivity, the conduction of La 0.8 Sr 0.2 Ga 0.8 Mg 0.115 Co 0.085 O 3 is concluded to be almost ionic. On the other hand, an oxygen permeation measurement suggests that the oxide ion conductivity increased linearly with an increasing amount of Co. Therefore, specimens with Co content higher than 10 mol % can be considered as a superior mixed oxide ion and hole conductor. The UV-vis spectra suggests that the valence number of doped Co was changed from +3 to +2 with decreasing oxygen partial pressure; the origin of hole conduction can thus be assigned to the formation of Co 3+ . Since the amount of dopant in the Ga site was compensated with Mg 2+ , the amount of oxygen deficiency was decreased by doping Co. Therefore, it is likely that the improved oxide ion conductivity observed by doping with Co is brought about by the enhanced mobility of oxide ion.

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1999 electric properties, superconductors, semiconductors electric properties, superconductors, semiconductors D 8000 -013Improved Oxide Ion Conductivity in La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 by Doping with Co.-Doping with Co on the Ga site is found to increase the oxide ion conductivity of the title compound. Considering the transport number and the ion transport number, the optimal doping level appears to be 8.5 mol% Co. The electrical conductivity of La 0.8 Sr 0.2 Ga 0.8 Mg 0.115 Co 0.085 O 3 is almost independent of the O 2 partial pressure. On the other hand, oxygen permeation measurements show a linear increase of the oxide ion conductivity with Co content. Therefore, samples with an Co content ¿10 mol% can be considered as superior mixed oxide ion and hole conductors. UV/VIS spectra suggest that the valence state of Co changes from +3 to +2 with decreasing O 2 partial pressure; the origin of the hole conduction can thus be assigned to the formation of Co 3+ . -

Abstract. LaGaO3-based perovskite oxide doped with Sr and Mg exhibits high ionic conduction over a wide oxygen partial pressure. In this study, the stability of the LaGaO 3 based oxide was investigated. It became clear that LaGaO 3 based oxide is very stable for reduction and oxidation. SOFCs utilizing LaGaO3-based perovskite type oxide for electrolyte were further studied for the decreased temperature solid oxide fuel cells. The power generation characteristics of cells were strongly affected by the electrode, both anode and cathode. It became clear that Ni and LnCoO 3 (Ln: rare earth) are suitable for anode and cathode, respectively. Rare earth cations in the Ln-site of Cobased perovskite cathode also have a great effect on the power generation characteristics. In particular, high power density could be attained in the temperature range from 973 to 1273 K by using doped SmCoO3 for the cathode. The electrical conductivity of SmCoO3 increases with increasing Sr amount doped for the Sm site and attained the maximum at Smo.5Sr0.5CoO 3. The cathodic overpotential and the internal cell resistance exhibit almost opposite dependence on the amount of doped Sr. Consequently, the power density of the cell reaches a maximum when Smo.5Sr0.5CoO 3 is used for cathode. On this cell, the maximum power density is as high as 0.58 W/cm 2 at 1073 K, although a 0.5 mm thick electrolyte is used. Therefore, this study reveals that the LaGaO3 based oxide for electrolyte and the SmCoO3 based oxide for cathode are promising for solid oxide fuel cells at intermediate temperature.

Oxide ion conductors are important functional materials which can be used as electrolytes of fuel cells, oxygen sensors, and films separating oxygen from air. 1 The development of an oxide ion conductor with high electrical conductivity is a highly demanding subject. At present, tetravalent oxides with fluorite structure such as ZrO 2 or CeO 2 are being extensively investigated as the oxide ion conductors and some of them, in particular, Y 2 O 3 -stabilized ZrO 2 , are practically used as the electrolyte of oxygen sensors for a combustion control. However, the number of reports on the oxide ion conductivity of perovskite-type oxides are rather limited in the literature. 2,3 Oxides with perovskite-type structures have advantages in accommodating a variety of elements within their crystal lattice, and it is relatively easy to dope aliovalent cations to form oxygen vacancies. Therefore, there is a high possibility of finding oxides with high oxide ion conductivity among perovskite-type oxides.In the previous studies, the authors investigated the oxide ion conductivity of Ga-based perovskite oxide, and it was found that LaGaO 3 doped with Sr and Mg for La and Ga sites, respectively, exhibits the oxide ion conductivity which is comparable to that of CeO 2 doped with Gd or Sm. 4,5 Following the authors' reports, the high oxide ion conductivity of the doped LaGaO 3 system attracted several groups and has been studied extensively. 6-12 On the other hand, it was also found that the doped NdGaO 3 exhibits high oxide ion conductivity over a wide range of oxygen partial pressures. The oxide ion conductivity of the NdGaO 3 system is comparable to those of Y 2 O 3 -stabilized ZrO 2 . 13,14 The major advantage of the Ga-based oxide systems are their chemical stability in reducing and oxidizing atmospheres. However, the mechanism of oxide ion conductivity has not yet been thoroughly investigated for the Ga-based oxides. It is considered that the oxide ion migrates through the lattice by passing through the opening of a triangle defined by two large A sites and one small B site cations in case of the perovskite-type oxide denoted as ABO 3 . It is also reported that the enlargement of the size of the opening is responsible for obtaining the fast oxide ion migration in perovskite oxides. [15][16][17] In the present study, the oxide ion conductivity of PrGaO 3 -based oxide was investigated in detail. Since the ionic radii of Pr 3ϩ is larger than that of Nd 3ϩ , it is expected that a larger size of the triangular opening would lead to higher mobility of an oxide ion. Hence, it is reasonable to expect a high oxide ion conductivity in PrGaO 3 . In addition, the electrical conduction property of praseodymium oxide consisting of Pr 3ϩ cation has hardly been investigated up to now because the stable valence number of praseodymium cations is ca. 3.7 in single oxide in air, namely, Pr 4ϩ is more stable than Pr 3ϩ in an oxide. Therefore, the acceptor doped PrGaO 3 is of high interest not only for being the fast oxide ion conductor but also f...