To facilitate water oxidation, we formed a nickel-iron oxyhydroxide co-catalyst on the Ni layer of the front contact. The co-catalyst was formed using a galvanic replacement method with Fe 2+ solution (Supplementary Fig. ). First, we investigated the Fe 2+ treatment on a Nicoated FTO surface. The electrochemical water oxidation kinetics were then evaluated in 1 M KOH solution (pH 14) using cyclic voltammetry (CV). In the CV measurement, the Fe 2+treated Ni electrode exhibited an improved onset potential and current density (Supplementary Fig. ). Based on this result, we also treated a Ni surface of the packaged Cu2O photoanode in the same manner to improve the PEC water oxidation efficiency. XPS analysis confirmed the presence of Fe 2p peaks on the Ni surface after Fe 2+ treatment (Supplementary Fig. ).