Evaluation of Sr- and Mn-substituted LaAlO 3 as potential SOFC anode materials

Abstract

Lanthanum–aluminate-based oxides, (La 0.8Sr 0.2) 1− y Al 1− x Mn x O 3− δ ( x = 0, 0.3, 0.5; y = 0 or 0.06) (LSAM), were synthesized and evaluated in detail as potential anode materials for solid oxide fuel cells (SOFCs). The electrical conductivity of LSAM (Mn ≥ 30 mol%) is dominated by p-type electronic conduction and can be treated as a diluted system of lanthanum manganites, (La,Sr)MnO 3. At 810 °C, the electrical conductivity of (La 0.8Sr 0.2) 0.94Al 0.5Mn 0.5O 3− δ (LSAM8255b) reaches 12 S/cm in air and 2.7 S/cm in humidified Ar/4% H 2 ( p(O 2) ≈ 10 − 18 bar). The thermal expansion coefficients of LSAM8255a and LSAM8255b match YSZ very well and no chemical reaction was observed between these two perovskite materials and YSZ up to at least 1400 °C. Fairly good electrochemical performance was observed for an LSAM8255b–YSZ composite anode. At 850 °C, the polarization resistances are only 0.34 and 0.50 Ω cm 2 in wet (∼3% H 2O) Ar/20% H 2 and wet Ar/20% CH 4, respectively. In addition, an exposure to Ar/20% CH 4/3% H 2O for 35 h did not cause any apparent carbon deposition on the electrode. However, the chemical stability of LSAM8255a and LSAM8255b in a typical anode environment under open circuit conditions does not seem sufficient, leading to performance degradation with time in wet Ar/20% H 2 or wet Ar/20% CH 4. Furthermore, relatively large chemical expansion (0.3–0.5%) was observed when the atmosphere was switched from air to wet Ar/4% H 2, which might cause intolerable stress on the thin film electrolyte layer for a large-area anode-supported planar SOFC, but which might be tolerable for small geometries or electrolyte-supported SOFCs.