PrBaCo2O5+δ-Sm0.5Sr0.5CoO3 Composite Oxide as Active Cathode for Intermediate-Temperature Solid Oxide Fuel Cells

Abstract

From decreasing the cost and the reliability of solid oxide fuel cells (SOFCs), increase in power density is strongly required. In order to achieve these requirements, decrease in operating temperature is the most effective way. However, reduced operating temperature leads to significant decrease in electrode activity, particularly cathode. Therefore, the development of novel and highly active cathode is strongly required for intermediate temperature (873-1073 K). In this study, we studied cathodic performance of double perovskite oxide of PrBaCo2O5 as active cathode of SOFC. Because of delamination of PrBaCo2O5 cathode is easily occurred because of larger thermal expansion coefficient. Mixing effects of PrBaCo2O5 with another perovskite oxide were investigated using a LaGaO3-based electrolyte. The electrochemical performance of the cell was measured using electrolyte-supported single cell at 873, 973 and 1073 K and humidified H2 was used as fuel and O2 as oxidant.Among the oxides mixed with PrBaCo2O5 (PBCO), it was found that mixing Sm0.5Sr0.5CoO3 (SSC)shows the smallest cathodic overpotential and the most suitable for increase in cathodic activity. Therefore, composite oxide of PBCO with SSC is active as cathode for SOFC. However, the cells with composite cathodes show lower performance at 1073 K comparing with PBCO cathode and this could be explained by small cathodic internal resistance at 873 K. On the other hand, the cell with PBCO-SSC (1:1) shows small overpotential at 873 K and much larger power density. Therefore, it was found that mixing PBCO with SSC shows the positive effects on the oxygen dissociation on the cathode at decreased temperature and suitable for low temperature operation SOFC. The optimization of SSC content on the cathodic activity of PBCO-SSC mixed oxide was further studied. The cathodic overpotential at 50 mA/cm2 was decreased with increasing amount of SSC and the smallest overpotential was achieved at 1:1 composition at all temperature studied. Positive effects of mixing PBCO and SSC could be assigned to the increased three phase boundary and increased activity to oxygen dissociation. Since cathodic overpotential was decreased significantly by mixing PBCO-SSC, power density of the cell using PBCO-SSC cathode at 1:1 composition shows 232 mW/cm2 at 873 K which is two times larger than that of the cell using PBCO single oxide cathode. Application of PBCO-SSC composite oxide for anode of steam electrolyser was also studied and it was found that PBCO-SSC is also active to oxygen association under applied voltage. Therefore, this study reveals that PBCO-SSC composite is potential air electrode material for intermediate-temperature solid oxide cells.