Identification of the Active-Layer Structures for Acidic Oxygen Evolution from 9R-BaIrO 3 Electrocatalyst with Enhanced Iridium Mass Activity

Iridium-based perovskites show promising catalytic activity for oxygen evolution reaction (OER) in acid media, but the iridium mass activity remains low and the active-layer structures have not been identified. Here, we report highly active 1 nm IrO x particles anchored on 9R-BaIrO 3 (IrO x /9R-BaIrO 3 ) that are directly synthesized by solution calcination followed by strong acid treatment for the first time. The developed IrO x /9R-BaIrO 3 catalyst delivers a high iridium mass activity (168 A g Ir –1 ), about 16 times higher than that of the benchmark acidic OER electrocatalyst IrO 2 (10 A g Ir –1 ), and only requires a low overpotential of 230 mV to reach a catalytic current density of 10 mA cm –2 geo . Careful scanning transmission electron microscopy, synchrotron radiation-based X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analyses reveal that, during the electrocatalytic process, the initial 1 nm IrO x nanoparticles/9R-BaIrO 3 evolve into amorphous Ir 4+ O x H y /IrO 6 octahedrons and then to amorphous Ir 5+ O x /IrO 6 octahedrons on the surface. Such high relative content of amorphous Ir 5+ O x species derived from trimers of face-sharing IrO 6 octahedrons in 9R-BaIrO3 and the enhanced metallic conductivity of the Ir 5+ O x /9R-BaIrO 3 catalyst are responsible for the excellent acidic OER activity. Our results provide new insights into the surface active-layer structure evolution in perovskite electrocatalysts and demonstrate new approaches for engineering highly active acidic OER nanocatalysts.