Characterization of Sr2.7Ln0.3Fe1.4Co0.6O7 (Ln = La, Nd, Sm, Gd) intergrowth oxides as cathodes for solid oxide fuel cells

Perovskite-related intergrowth oxides Sr2.7Ln0.3Fe1.4Co0.6O7 − δ (Ln = La, Nd, Sm, and Gd) have been investigated as cathode materials for solid oxide fuel cells (SOFC). With decreasing size of the Ln3+ ions, the unit cell volume, oxygen content, thermal expansion coefficient (TEC), and total electrical conductivity decrease from Ln = La to Gd. The decreasing unit cell volume and oxygen content is attributed to the decreasing size of Ln3+ ions from Ln = La to Gd and a consequent preference for lower coordination numbers. While the decrease in the ionicity of the Ln–O bonds from Ln = La to Gd causes a decrease in the TEC, the increasing amount of oxygen vacancies leads to a decrease in electrical conductivity arising from a thermally activated semiconducting behavior. The cathode polarization conductance (Rp− 1) measured using the ac-impedance spectroscopy and the catalytic activity for the oxygen reduction reaction in SOFC decrease from Ln = La to Gd partly due to the decrease in electrical conductivity.