<i>Operando</i> Direct Observation of Stable Water-Oxidation Intermediates on Ca_2–<i>x</i>IrO₄ Nanocrystals for Efficient Acidic Oxygen Evolution

Abstract

We report Ca_2-<i>x</i>IrO₄ nanocrystals exhibit record stability of 300 h continuous operation and high iridium mass activity (248 A g_Ir^-1 at 1.5 V_RHE) that is about 62 times that of benchmark IrO₂. Lattice-resolution images and surface-sensitive spectroscopies demonstrate the Ir-rich surface layer (evolved from one-dimensional connected edge-sharing [IrO₆] octahedrons) with high relative content of Ir⁵⁺ sites, which is responsible for the high activity and long-term stability. Combining <i>operando</i> infrared spectroscopy with X-ray absorption spectroscopy, we report the first direct observation of key intermediates absorbing at 946 cm^-1 (Ir⁶⁺═O site) and absorbing at 870 cm^-1 (Ir⁶⁺OO- site) on iridium-based oxides electrocatalysts, and further discover the Ir⁶⁺═O and Ir⁶⁺OO- intermediates are stable even just from 1.3 V_RHE. Density functional theory calculations indicate the catalytic activity of Ca₂IrO₄ is enhanced remarkably after surface Ca leaching, and suggest IrOO- and Ir═O intermediates can be stabilized on positive charged active sites of Ir-rich surface layer.