Highly-conductive Cu-substituted brownmillerite with emergent 3-dimensional oxygen vacancy channels

Abstract

Lowering the oxygen content in transition-metal oxides will inevitably result in the formation of oxygen vacancies or novel crystal structures. In this work, we used Cu to substitute for Co in brownmillerite SrCoO_2.5 (BM-SCO) and found that this cation-substitution strategy could effectively reduce the oxygen level. An emergent 3-dimensional channel was formed from the parent 2-dimensional ordered BM-SCO as the Cu concentration reached 33%. The were ordered in both in-plane (IP) and out-of-plane (OOP) directions and were stabilized by forming CoO₅ pyramids in addition to the existing CoO₆ octahedra and CoO₄ tetrahedra. Further doping with Cu would convert more and more CoO₆ octahedra to CoO₅ pyramids. Although the oxygen content was lowered, the electric conductivity improved by 2–3 orders of magnitude by forming an intermediate conduction band, which makes Cu-substituted BM-SCO more conductive than perovskite SCO at room temperature. Cu-substituted SCO exhibited greatly improved OER performances and boosted electrochemical activity, which is much better than that of a number of Co-based catalysts and comparable to IrO₂ a noble metal oxide. Our work shows that Cu-substitution in brownmillerites is an effective route to stabilize high oxygen vacancy concentration with highly-improved conductivity. The emergent 3-dimensional oxygen vacancy channels offer a promising option to improve the catalytic performances of Co-based complex oxides.