Synthesis of Brownmillerite-type Ca₂Fe_0.5Co_1.5O₅ as a Durable Electrocatalyst for Oxygen Evolution Reaction in Neutral Aqueous Solution (Supporting Information)

<p dir="ltr">Green hydrogen production and carbon dioxide electroreduction in neutral aqueous solutions, are key reactions for zero-carbon energy systems. In both cases, the oxygen evolution reaction (OER) is a counter reaction that requires a high overpotential. Therefore, active and durable OER catalysts at neutral pH are demanded. We previously reported that brownmillerite-type Ca₂Fe_0.75Co_1.25O₅ calcined at 1073 K exhibits higher OER activity and durability than that of perovskite-type iron cobalt oxides and Ca₂Fe₂₋<i>_x</i>Co<i>_x</i>O₅ (<i>x</i> ≤ 1) at neutral pH. Here, we synthesized Ca₂Fe_0.5Co_1.5O₅ at temperatures between 1073 and 1373 K and compared its OER activity and durability at neutral pH with those of Ca₂FeCoO₅ and Ca₂Fe_0.75Co_1.25O₅. X-ray diffraction and chemical composition analysis revealed that pure brownmillerite phase of Ca₂Fe_0.5Co_1.5O₅ was only achieved at 1273 K, while samples calcined at lower and higher temperatures contained Ca₃Co₂O₆ and CaO impurities, respectively. The Ca₂Fe_0.5Co_1.5O₅ exhibited the highest activity and durability for over 100 h, significantly outperforming Ca₂Fe_0.75Co_1.25O₅ without significant changes in morphology or bulk composition.</p>