Brownmillerite-Type\nCa<sub>2</sub>FeCoO<sub>5</sub> Ultrathin Layers as Anodic Catalysts\nfor Water Electrolysis

Brownmillerite-type\nCa<sub>2</sub>FeCoO<sub>5</sub> (CFCO)\nis a\nremarkable electrocatalyst for oxygen evolution reactions (OER) under\nalkaline and neutral conditions. In our recent study, crystalline\nCFCO nanoparticles with single-nanometer dimensions were formed on\nTiO<sub>2</sub> nanoparticles using a reverse-micelle (RM) deposition\nmethod. In this study, we report the RM deposition synthesis of ultrathin\nCFCO layers with varying interactions between the CFCO precursors,\nwhich were hydroxides formed in RM, and oxide supports during the\nloading process: CFCO ultrathin layers with several atomic thicknesses\nwere formed on ZnO, Al<sub>2</sub>O<sub>3</sub>, and PbO<sub>2</sub> with positive surface charges, whereas CFCO nanoparticles with diameters\nof ∼10 nm were formed on TiO<sub>2</sub> and SnO<sub>2</sub> with negative surface charges. The CFCO precursors formed in the\nRM had a negative charge, indicating that the CFCO precursors were\nuniformly loaded on ZnO, Al<sub>2</sub>O<sub>3</sub>, and PbO<sub>2</sub> via strong interactions between the positive and negative\ncharges, leading to ultrathin CFCO layers after calcination. The electrocatalytic\nactivities of the ultrathin CFCO layers for the OER under concentrated\nalkaline conditions were higher than those of the CFCO nanoparticles.\nIt is speculated that the calcium defects in the CFCO ultrathin layers\neasily and rapidly change to active cobalt iron species for the OER\nunder concentrated alkaline conditions.