Characterization of GdBa1 − xSrxCo2O5 + δ ( 0 ⩽ x ⩽ 1.0 ) Double Perovskites as Cathodes for Solid Oxide Fuel Cells

Abstract

The effect of Sr2+ substitution for Ba2+ on the crystal chemistry, oxygen content, thermal expansion, electrical conductivity, and catalytic activity for oxygen reduction reaction (ORR) of the double perovskite oxides Gd Ba1-x Srx Co2 O5+δ has been investigated for 0≤x≤1.0. The Gd Ba1-x Srx Co2 O5+δ system exhibits a structural change from orthorhombic (x=0) to tetragonal (0.2≤x≤0.6) to orthorhombic (x=1) with increasing Sr content. The difference in ionic radii between (Ba1-x Srx)2+ and Gd3+ plays a dominant role in determining the oxygen-content value in Gd Ba1-x Srx Co2 O5+δ, and the oxygen content and the oxidation state of cobalt increase with increasing Sr content. The electrical conductivity of the Gd Ba1-x Srx Co2 O5+δ system increases with Sr content due to an increasing oxygen content and a straightening of the O-Co-O bonds as evidenced by the structural change from orthorhombic to tetragonal. Sr substitution also improves the chemical stability of the Gd Ba1-x Srx Co2 O5+δ cathodes in contact with the Ce0.9 Gd0.1 O1.95 and La0.8 Sr0.2 Ga0.8 Mg0.2 O2.8 electrolytes at 1100°C. The x=0.2 and 0.6 samples with a tetragonal structure exhibit higher catalytic activity for ORR in single-cell solid oxide fuel cell than the x=0 and 1.0 samples with an orthorhombic structure.