Highly Durable Oxygen Evolution Reaction Catalyst: Amorphous Oxyhydroxide Derived from Brownmillerite-Type Ca₂FeCoO₅

Brownmillerite-type Ca2FeCoO5 (CFCO) is a highly active electrocatalyst for the oxygen evolution reaction (OER). In this study, we identified the actual catalytically active phase of this oxide formed via the long-term OER and, moreover, demonstrated that the active phase can persist during the OER for 4 weeks without significant loss of electrocatalytic activity. The long-term durability tests were carried out on CFCO via continuous galvanostatic OER in 4 mol dm–3 KOH aqueous solution for periods ranging from a few hours to 1 month, and the specimens submitted to the tests were characterized by means of electrochemical measurements and structural analysis using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, Auger electron spectroscopy, and X-ray absorption fluorescence spectroscopy. CFCO was readily converted to amorphous cobalt oxyhydroxides with 10% Fe substituents through the OER process, and these compounds had a local rearrangement similar to that of the layered γ-CoOOH-type structure. This transformation involved large morphological changes of the oxide particles because of the extensive dissolution of Ca and Fe, yielding skeletal grains made of oxyhydroxide nanosheet aggregates. The extended durability studies with total polarization charge density of the order of 105 C cm–2 revealed that a (Co, Fe)OOH-like compound is the actual electrocatalytic phase.