Operando Direct Observation of Stable Water-Oxidation Intermediates on Ca_{2- x}IrO₄Nanocrystals for Efficient Acidic Oxygen Evolution

We report Ca_2-xIrO₄nanocrystals exhibit record stability of 300 h continuous operation and high iridium mass activity (248 A g_Ir^-1at 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 operando 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 Ca₂IrO₄is 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.