Brownmillerite-Type Ca₂FeCoO₅ Ultrathin Layers as Anodic Catalysts for Water Electrolysis

Brownmillerite-type Ca2FeCoO5 (CFCO) is a remarkable electrocatalyst for oxygen evolution reactions (OER) under alkaline and neutral conditions. In our recent study, crystalline CFCO nanoparticles with single-nanometer dimensions were formed on TiO2 nanoparticles using a reverse-micelle (RM) deposition method. In this study, we report the RM deposition synthesis of ultrathin CFCO layers with varying interactions between the CFCO precursors, which were hydroxides formed in RM, and oxide supports during the loading process: CFCO ultrathin layers with several atomic thicknesses were formed on ZnO, Al2O3, and PbO2 with positive surface charges, whereas CFCO nanoparticles with diameters of ∼10 nm were formed on TiO2 and SnO2 with negative surface charges. The CFCO precursors formed in the RM had a negative charge, indicating that the CFCO precursors were uniformly loaded on ZnO, Al2O3, and PbO2 via strong interactions between the positive and negative charges, leading to ultrathin CFCO layers after calcination. The electrocatalytic activities of the ultrathin CFCO layers for the OER under concentrated alkaline conditions were higher than those of the CFCO nanoparticles. It is speculated that the calcium defects in the CFCO ultrathin layers easily and rapidly change to active cobalt iron species for the OER under concentrated alkaline conditions.