Single‐Atom Doping and High‐Valence State for Synergistic Enhancement of NiO Electrocatalytic Water Oxidation

Abstract The NiO‐based electrocatalytic oxygen evolution reaction (OER) of water splitting is recognized as a promising approach to produce clean H 2 fuel. However, the OER performance is still low, and especially, the overpotential is larger than 200 mV at the current density of 10 mA cm −2 . Herein, an Ir@IrNiO sample is prepared with single‐atom (SA) Ir 4+ doping and surface metallic Ir nanoparticles loaded onto the NiO. Owing to the bonding of the loaded Ir with surface‐exposed Ni 2+ , the nearby Ni atoms exist in the + 3 valence state, that is, the surface‐loaded Ir particles behave like a stabilizer for the Ni 3+ sites. Under the synergistic effect of SA Ir 4+ and high‐valance‐state Ni 3+ , the Ir@IrNiO nanostructure effectively reduces the overpotential to 195 mV at a current density of 10 mA cm −2 . Moreover, it gives an Ir‐content‐normalized current density of 0.0457 A mg Ir −1 , 72.1 times higher than that of the best commercialized IrO 2 (6.33 × 10 −4 A mg Ir −1 ), under the condition of 1.5 V versus reversible hydrogen electrode. Operando Raman and X‐ray absorption fine‐structure (XAFS) measurements reveal that there are more surface‐active species of Ni 3+ , which adsorb and activate water molecules to form Ni 3+ –*OH at low voltage, the intermediate of Ni 4+ –•O is then formed at a relatively high bias voltage, and then the •O is transferred to the SA Ir 4+ sites to generate Ir 4+ –O–O with OH at increased voltage. This work can help design more SA‐based highly active OER materials.