How Oxygen Deficiency and Its Ordering Control Electrical Conductivity in Sr_2–<i>x</i>Ca_<i>x</i>Fe₂O_6−δ Perovskites as Related to Water Oxidation Electrocatalysis

We report on the analysis of oxygen vacancies (OVs) content and ordering of Sr_2-<i>x</i> Ca _<i>x</i> Fe₂O_6-δ perovskites and explain how OVs change the electrical conductivity and oxygen evolution catalytic activity of these compounds. The structure and OV content are tuned by controlling the <i>A</i>-site composition and the reaction atmosphere. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) are used to identify the crystal structures and to quantify bulk oxygen vacancy contents. Our analysis shows that OV content and crystal structure govern the electronic transport properties of the Sr_2-<i>x</i> Ca _<i>x</i> Fe₂O_6-δ system. The electrical conductivity of oxygen-deficient perovskites (ODPs) is significantly higher than those in brownmillerites (by at least 2 orders of magnitude). Seebeck coefficient measurements identified that the Sr-rich ODPs and brownmillerites are <i>p</i>-type semiconductors, while Ca-rich brownmillerites are either insulators (within the experimental temperature range) or p-type semiconductors at lower temperatures (<750 K). Electrical conductivity of <i>p</i>-type semiconductors (Sr-rich compounds) reduces with higher OV content, and in brownmillerites with <i>x</i> ≥ 1.25, a transition to <i>n</i>-type semiconductor is observed at temperatures above 750 K. Our analysis shows that the hole and electron concentrations are similar in these brownmillerites, indicating major contributions from ionic transport. Finally, we show how oxygen deficiency alters the electrical conductivity and catalytic activity of the Sr_2-<i>x</i> Ca _<i>x</i> Fe₂O_6-δ system in noncomplementary ways.