Balancing Iridium Speciation in Ir/NiO Single-Atom Catalysts for Overall Water Splitting

Electrocatalysts capable of driving overall water splitting at high current densities and different pH remain elusive. Herein, we demonstrate that iridium single atoms supported on NiO nanosheets (Ir_x-NiO) can serve as promising bifunctional OER/HER catalysts in alkaline, neutral, and acidic media. An Ir_8%-NiO catalyst required low overpotentials of 177, 275, and 174 mV for the oxygen evolution reaction (OER), and 29, 54, and 24 mV for the hydrogen evolution reaction (HER), at 10 mA cm^-2 in 1.0 m KOH, 0.1 m PBS, and 0.5 m H₂SO₄, respectively. When applied as the anode and cathode catalysts in an anion exchange membrane water electrolyser (AEMWE), Ir_8%-NiO achieved 1.0 A cm^-2 at 2.02 V with good operational stability. Spectroscopic and theoretical investigations revealed that the coexistence of embedded and surface-adsorbed Ir atoms in the Ir_8%-NiO catalyst underpinned its bifunctional OER/HER activity. Embedded Ir promoted Ni²⁺ → Ni³⁺ oxidation and NiOOH formation during OER, while surface-adsorbed Ir atoms provided sites for ^*H adsorption, enabling efficient HER. The dual Ir sites optimize ^*OH and ^*H binding energies, resulting in exceptional bifunctional activity. These findings highlight the importance of controlling Ir speciation and metal-support interactions to achieve overall water splitting across a wide pH range.