Effects of La doping on electrical conductivity, thermal expansion and electrochemical performance in LaxSr1–xCo0.9Sb0.1O3–δ cathodes for IT–SOFCs

Abstract

Perovskite oxides LaxSr1–xCo0.9Sb0.1O3–δ (LSCSbx, x=0.0–0.8) are investigated as IT–SOFC cathodes supported with La0.9Sr0.1Ga0.8Mg0.2O3–δ (LSGM) electrolyte. All LSCSbx oxides have a tetragonal distorted perovskite structure with s.g. P4/mmm, while a La2Co2O5 impurity phase was observed within La doping levels at x=0.6–0.8. The LSCSb0.4 has a good chemical compatibility with LSGM electrolyte for temperatures up to 1050°C. XPS examinations indicate the existence of Co3+/Co4+ mixed valence states in LSCSbx. The conductivity increases with La doping and the LSCSbx with x=0.4 exhibits the highest electrical conductivity (e.g., 673–1637Scm−1 at 300–850°C). The thermal expansion coefficient (TEC) decreases from 25.89×10–6 K–1 for x=0.0 to 18.5×10–6 K–1 for x=0.6 at 30–900°C. Among the LSCSbx compositions, the LSCSb0.2 exhibits the lowest polarization resistance (Rp), which is merely 0.069Ωcm2 at 700°C. The maximum power density of the cell with LSCSb0.2 cathode on 300µm thick LSGM electrolyte attains 564mWcm–2 at 850°C, which is higher than that of SrCo0.9Sb0.1O3–δ (SCSb) cathode. All of the results indicate that LSCSb0.2 is a promising material for application in IT–SOFCs cathodes.