Intermediate temperature solid oxide fuel cells using LaGaO₃ electrolyte II. Improvement of oxide ion conductivity and power density by doping Fe for Ga site of LaGaO₃

Effects of small amounts of Fe doping for Ga site in LaGaO₃-based oxide on oxide ion conductivity is investigated in this study. It is found that doping a small amount of Fe is effective for improving the oxide ion conductivity in La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM). The highest oxide ion conductivity was exhibited at x = 0.03 in La_0.8Sr_0.2Ga_0.8Mg_0.2-xFe_xO₃ among the Fe-doped samples. Electron spin resonance (ESR) measurements suggest that Fe is trivalent in LaGaO₃ lattice. The application of the Fe-doped LaGaO₃-based oxide for the electrolyte of solid oxide fuel cell was further investigated. Power density of the solid oxide fuel cell was increased by using Fe-doped LSGM for electrolyte. This can be explained by the decrease in electrical resistance loss by improving the oxide ion conductivity. A maximum power density close to 700 mW/cm² was obtained at 1073 K on the cell using 0.5 mm thick La_0.8Sr_0.2Ga_0.8Mg_0.17Fe_0.03O₃ (LSGMF) and O₂ as the electrolyte and the oxidant, respectively. Therefore, close to the theoretical open-circuit potential was exhibited by the LSGMF cell. On the other hand, the power density was slightly smaller than that of the cell using Co-doped LSGM as electrolyte, especially, at temperatures lower than 973 K. This may result from the large activation energy for ion conductivity. However, the power density of the LSGMF cell was higher than that of the LSGM cell. Therefore, LSGM doped with a small amount of Fe is a promising electrolyte similar to Co-doped LSGM for the intermediate solid oxide fuel cell.