LaGaO,-based perovskite oxides doped with Sr and Mg exhibit high ionic conductivity over a wide range of oxygen partial pressure. In this study, the stability of LaGaO,-based oxide was investigated. The LaGaO,-based oxide was found to be very stable in reducing, oxidizing, and CO2 atmospheres. Solid oxide fuel cells (SOFCs) using LaGaO,-based perovskite-type oxide as the electrolyte were studied for use in intermediate-temperature SOFCs. The power-generation characteristics of cells were strongly affected by the electrodes. Both Ni and LnCoO, (Ln:rare earth) were suitable for use as anode and cathode, respectively. Rare-earth cations in the Ln site of the Co-based perovskite cathode also had a significant effect on the power-generation characteristics. In particular a high power density could be attained in the temperature range 973-1273 K by using a doped SmCoO, for the cathode. Among the examined alkaline earth cations, Srdoped SmCoO, exhibits the smallest cathodic overpotential resulting in the highest power density. The electrical conductivity of SmCoO3 increased with increasing Sr doped into the Sm site and attained a maximum at Sm0 ,Sr,,CoO3. The cathodic overpotential and internal resistance of the cell exhibited almost the opposite dependence on the amount of doped Sr. Consequently, the power density of the cell was a maximum when Sm0 ,Sr,,CoO, was used as the cathode. For this cell, the maximum power density was as high as 0.58 W/cm2 at 1073 K, even though a 0.5 mm thick electrolyte was used. This study revealed that a LaGaO,-based oxide for electrolyte and a SmCoO,-based oxide for the cathode are promising components for SOFCs operating at intermediate temperature.
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.
Solid oxide fuel cells (SOFC) provide a new and clean electric power generating system At present, Y 2 O 3 -stabilized ZrO 2 (YSZ) is commonly used as the electrolyte of a solid oxide fuel cell. Since the oxide ion conductivity of YSZ is insufficient for the electrolyte of fuel cells, a thin electrolyte film without gas leakage and an excessively high operating temperature such as 1273 K are essential for the high power density of SOFCs when YSZ is used as electrolyte. On the other hand, all the advantages of SOFC such as a high efficiency and a variety of usable fuels can be obtained at decreased temperatures such as 1073 K. Furthermore, the choice of the materials for cell construction becomes wider; in particular, cheap refractory metals such as a stainless steel will be usable by decreasing the operating temperature to 900 K. Consequently, decrease in operating temperature is an important subject for the development of the cheap but the reliable SOFCs. 1 In order to develop the intermediate temperature SOFC, an active electrode, in particular, the cathode electrode catalyst, and an electrolyte with a low resistance are essential. Ceria doped with Gd or Sm is considered for the candidate electrolyte of the intermediate temperature SOFC. 2 However, ceria-based oxide exhibits a significant electronic conduction in the reducing atmosphere, 2 which causes various problems for the application of SOFC. 3-5 On the other hand, preparation of a very thin YSZ film (less than 10 m thick) is another solution method for decreasing the operating temperature, and various methods are now under investigation. 6 Except for making a very thin YSZ film, it is of great importance for the intermediate temperature SOFCs to develop new electrolyte materials which exhibit a high oxide ion conduction over a wide range of oxygen partial pressures. In our previous study, the oxide ion conductivity in the perovskite oxide was investigated, and it was found that the LaGaO 3 -based perovskite-type oxide exhibits high oxide ion conductivity, 7,8 which is comparable to that of CeO 2 -based oxide. In particular, LaGaO 3 doped with Sr for La and Mg for Ga sites (denoted as LSGM) exhibits a high oxide ion conductivity stably over a wide range of oxygen partial pressure. 9-12 It is generally believed that doping with a transition metal cation is undesirable for the ionic conductor due to the appearance of electron or hole conduction. 13 However, it was found that the oxide ion conductivity was also improved by doping Co for Ga site of LSGM, although hole conduction appeared slightly in the high oxygen partial pressure range. Furthermore, it became clear that the application of Co-doped LSGM for the electrolyte of SOFC greatly improves the power density of the cells at low temperature. On the other hand, it was found that doping Fe also has similar positive effects on the electrical conductivity in LaGaO 3 -based oxide. In the present study, therefore, effects of Fe doping on the oxide ion conductivity in LSGM were studied systematically. In addit...
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