A cobalt-free layered oxide as an oxygen reduction catalyst for intermediate-temperature solid oxide fuel cells

A cobalt-free layered oxide-Sr₄Fe₆O_13-δ is synthesized and characterized for application as a cathode of intermediate temperature solid oxide fuel cells. High temperature powder x-ray diffraction, oxygen temperature programmed desorption and electrochemical impedance spectroscopy are employed to evaluate the temperature-dependent crystal structure and oxygen reduction reaction (ORR) activity of Sr₄Fe₆O_13-δ. The oxide exhibits a single phase between room temperature and 1050 °C with low thermal expansion coefficient. The drawback of this oxide lies on its low ORR activity which is likely due to its layered structure which favors oxygen ionic transport two-dimensionally in the ac plane along the layered oxide layer. We show here that the original ORR activity can be improved by adjusting the phase compositions through an oxygen-deficient calcination process. An area specific resistance of 0.139 Ω cm² at 700 °C is attained by calcining Sr₄Fe₆O_13-δ at 900 °C with ∼66.2 wt% of perovskite phase in the composite material.