Understanding the Effect of Ni-Substitution on the Oxygen Evolution Reaction of (100) IrO2 Surfaces

The electrolysis process of water is impeded by the slow kinetics of the oxygen evolution reaction (OER). While iridium oxide (IrO2) is considered one of the most efficient metal catalysts for the OER, the adsorption of the OER intermediates at the IrO2 surface is not optimal, the *OH intermediate being too strongly adsorbed. The substitution of iridium by another metal cation is then a common strategy to improve the catalytic activity. A combined computational and experimental approach was followed to elucidate the fundamental effect of nickel on the OER activity of (100)-oriented IrO2. To achieve this, (100)-oriented Ir1–x Ni x O2 model surfaces were synthesized by pulsed laser deposition. Detailed structural and chemical characterizations were performed by X-ray diffraction, transmission electron microscopy, atomic force microscopy, and X-ray absorption spectroscopy. The composition of the film was varied between 0 and 15 at %, which is the solubility limit for the substitution of iridium by nickel atoms in (100)-oriented Ir1–x Ni x O2. All electrochemical characterizations were performed in an alkaline electrolyte, in which nickel dissolution is not observed by X-ray photoelectron spectroscopy. The current density recorded at +1.6 V (vs RHE) was increased from 35 to 348 μA cmox –2 between IrO2 to Ir0.85Ni0.15O2. Density functional theory calculations showed that the energy diagram of the OER intermediates was modified by the substitution of iridium with nickel atoms through a ligand effect. Also, it was found that iridium is the active site for low nickel content, whereas nickel is the most active site when its concentration reaches 15 at %.