Effect of Fe substitution on the structure and properties of LnBaCo2−xFexO5+δ (Ln = Nd and Gd) cathodes

The effect of Fe substitution for Co on the crystal chemistry, thermal and electrical properties, and catalytic activity for oxygen reduction reaction of the layered LnBaCo2−xFexO5+δ (Ln = Nd and Gd) perovskite has been investigated. The air-synthesized LnBaCo2−xFexO5+δ samples exhibit structural change with increasing Fe content from tetragonal (0 ≤ x ≤ 1) to cubic (1.5 ≤ x ≤ 2) for the Ln = Nd system and from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) for the Ln = Gd system. The thermal expansion coefficient (TEC) and electrical conductivity decrease with increasing Fe content in LnBaCo2−xFexO5+δ. While the substitution of a small amount of Fe (x = 0.5) for Co leads to slightly improved performance in solid oxide fuel cells (SOFC), larger Fe contents (x ≥ 1.0) deteriorate the fuel cell performance. In the Ln = Gd system, the better performance of the x = 0.5 sample is partly due to the improved chemical stability with the LSGM electrolyte at high temperatures. With an acceptable electrical conductivity of >100 S cm−1 at 800 °C, the x = 0.5 sample in the LnBaCo2−xFexO5+δ (Ln = Nd and Gd) system offers promising mixed oxide-ion and electronic conducting (MIEC) properties.