Ultrafine iridium nanoparticles prepared without a surfactant for the acidic oxygen evolution reaction†

Proton exchange membrane (PEM)-based water electrolysis currently requires the use of iridium (Ir) as the anodic catalyst. Among the various iridium-based electrocatalysts, ultrafine metallic Ir nanoparticles have gained considerable attention due to their high acidic oxygen evolution reaction (OER) activity. Although recent progress has enabled the preparation of metallic Ir nanoparticles using surfactants, which can block the active sites of catalysts, the preparation of metallic Ir nanoparticles without surfactants is uncommon. Herein, we report an ultrafine metallic iridium electrocatalyst (UF-Ir/IrO_x) prepared via a surfactant-free hydrothermal reaction. During the OER, UF-Ir/IrO_x undergoes significant structural reconstruction, which is clearly revealed by X-ray photoelectron spectroscopy (XPS) and in situ Raman characterization. The amorphous IrO_x layer generated during the OER displays outstanding acidic OER activity and stability. We uncovered that the catalysis of UF-Ir/IrO_x follows the adsorbate evolution mechanism (AEM).