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

Lowering the oxygen content in transition-metal oxides will inevitably result in the formation of oxygen vacancies (Vo(center dot center dot)) or novel crystal structures. In this work, we used Cu to substitute for Co in brownmillerite SrCoO2.5 (BM-SCO) and found that this cation-substitution strategy could effectively reduce the oxygen level. An emergent 3-dimensional Vo(center dot center dot) channel was formed from the parent 2-dimensional Vo(center dot center dot) O ordered BM-SCO as the Cu concentration reached 33%. The Vo(center dot center dot) O 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.