Brownmillerite-type Ca2Fe0.75Co1.25O5 as a robust electrocatalyst for the oxygen evolution reaction under neutral conditions

Abstract

Towards the development of active and robust electrocatalysts for the oxygen evolution reaction (OER) to realize water splitting and CO₂ electroreduction by renewable energies under practical conditions, brownmillerite-type composite oxide Ca₂Fe_2−xCo_xO₅ was synthesized by a sol–gel method and examined for the OER under neutral conditions. A pure brownmillerite-type Ca₂Fe_2−xCo_xO₅ phase, which was categorized as an oxygen-deficiency-ordered perovskite-type structure, was synthesized in a range of x = 0–1.25. Brownmillerite-type Ca₂FeCoO₅ showed higher activity and durability for the OER in a neutral solution than perovskite-type LaFe_0.5Co_0.5O₃ and SrFe_0.5Co_0.5O_3−δ. Furthermore, the durability of Ca₂Fe_2−xCo_xO₅ was drastically improved by increasing the Co content from x = 1 to 1.25. Ca₂Fe_0.75Co_1.25O₅ was remarkably durable for about 70 h during the OER in a neutral solution whereas Ca₂FeCoO₅ lost activity within 20 h, and perovskite-type oxides lost activity within a few hours. The detailed analysis of structures and compositions of local regions of Ca₂Fe_2−xCo_xO₅ and their changes by the reaction based on electron microscopy clarified the important role of the brownmillerite structure itself in catalytic activity under neutral conditions, while, under alkaline conditions, it has been known that the amorphous phase derived from brownmillerite mainly played the role of active species.