A cobalt-free layered oxide as an oxygen reduction catalyst for intermediate-temperature solid oxide fuel cells. (7th December 2015) - Jiang, Shanshan
Abstract: A cobalt-free layered oxide-Sr4 Fe6 O13−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 Sr4 Fe6 O13−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 cm 2 at 700 °C is attained by calcining Sr4 Fe6 O13−δ at 900 °C with ∼66.2 wt% of perovskite phase in the composite material. Graphical abstract: Cobalt-free layered perovskite oxide Sr4 Fe6 O13−δ which shows good phase stability (e.g. retain single phase between room temperature and 1050 °C in air) and low thermal expansion was evaluated as an oxygen reduction component in intermediate-temperature solid oxide fuel cells. We showed that the drawback of this layered oxide in terms of its low oxygen reduction reactivity (due to the restriction of oxygen transport in the ac plane along the layered oxide (FeO5 ) layer) can be circumvented by tuning the phase composition (perovskite to layered oxide phases) through an oxygen deficient calcination process. The optimum performance was attained by calcining Sr4 Fe6 O13−δ at 900 °C; leading to an area specific resistance (ASR) of 0.139 Ω cm 2 at 700 °C. Highlights: Sr4 Fe6 O13−δ cathode exhibits single phase and low thermal expansion between room temperature and 1050 °C. Sr4 Fe6 O13−δ layered structure restricts the oxygen ionic transport in the ac plane along the layered oxide (FeO5 ) layer only. The ORR activity can be optimized by tuning the phase composition of Sr4 Fe6 O13−δ using oxygen deficient calcination process.