Electrochemical properties of Sr2.7-xCaxLn0.3Fe2-yCoyO7-δ cathode for intermediate-temperature solid oxide fuel cells

Abstract

The influence of Ca and Co doping in the Ruddlesden ‒ Popper series of oxide Sr2.7-xCaxLn0.3Fe2-yCoyO7-δ with x = 0 and 0.3, y = 0 and 0.6, and Ln = La and Nd on the electrical and thermal properties, catalytic activity for the oxygen reduction reaction (ORR) in solid oxide fuel cells (SOFC), and phase stability has been investigated. The Co-substituted samples display an increase in oxygen vacancies, electrical conductivity, thermal expansion coefficient, and electrocatalytic activity for both Ln = La and Nd. Although Ca doping in the Sr site slightly reduces the oxygen loss and thermal expansion coefficient, the symmetric cell performance is improved for both lanthanides at high temperatures. Among the cathode materials in this study, the highest catalytic activity for the ORR is achieved with Sr2.7Ca0.3Nd0.3Fe1.4Co0.6O7-δ + Gd0.2Ce0.8O1.9 (GDC) composite cathode with an area specific resistance of 0.034 Ω cm2 at 800 °C. Long-term thermal stability test shows that no impurity forms when Sr2.7-xCaxLn0.3Fe2-yCoyO7-δ oxides were heated at 800 °C for 150 h in air.