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 H₂ 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) Ir⁴⁺ doping and surface metallic Ir nanoparticles loaded onto the NiO. Owing to the bonding of the loaded Ir with surface-exposed Ni²⁺ , the nearby Ni atoms exist in the +3 valence state, that is, the surface-loaded Ir particles behave like a stabilizer for the Ni³⁺ sites. Under the synergistic effect of SA Ir⁴⁺ and high-valance-state Ni³⁺ , 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 IrO₂ (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 Ni³⁺ , which adsorb and activate water molecules to form Ni³⁺ -*OH at low voltage, the intermediate of Ni⁴⁺ -•O is then formed at a relatively high bias voltage, and then the •O is transferred to the SA Ir⁴⁺ sites to generate Ir⁴⁺ -O-O with OH at increased voltage. This work can help design more SA-based highly active OER materials.

Keywords: Ir 4+; Ni 3+; NiO; high-valance state; oxygen evolution reaction; single-atom doping.