Oxygen Vacancy and Chemical Ordering Control Oxygen Evolution Activity of Sr_2–xCa_xFe₂O_6−δ Perovskites

We report on the oxygen vacancy effects controlling oxygen evolution activity of Sr2–xCaxFe2O6−δ perovskites. This system shows a tunable surface reactivity toward oxygen evolution reaction (OER) through changing the A-site composition (A = Sr2–xCax) and the oxygen vacancy content, δ. We use solid-state synthesis to control the structural ordering and the content of oxygen vacancy. Samples with chemical compositions of x ≥ 0.75 crystallize in the brownmillerite-type structure, while those with low calcium content (x < 0.75) crystallize in either perovskite or brownmillerite structure. Our electrochemical analysis shows that compounds with mixed A-site compositions outperform Ca2Fe2O6−δ and Sr2Fe2O6−δ and reveal several compounds with higher activity than LaCoO3−δ (measured at 2 mA cm–2). The oxygen vacancy content (δ) of the compounds was quantified using X-ray photoelectron spectroscopy. The δ vs A-site composition trend shows a transition point where the structural transition was also observed, i.e., x ≃ 0.75. Interestingly, electrochemical activity correlates with δ, and compounds with δ → 1 tend to show higher activity. Our results show how short-range and long-range structural ordering and oxygen vacancy content can be employed to tune catalytic properties of Fe-based perovskites.