Operando Direct Observation of Stable Water-Oxidation Intermediates on Ca 2- x IrO 4 Nanocrystals for Efficient Acidic Oxygen Evolution.

We report Ca 2- x IrO 4 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 IrO 2 . Lattice-resolution images and surface-sensitive spectroscopies demonstrate the Ir-rich surface layer (evolved from one-dimensional connected edge-sharing [IrO 6 ] octahedrons) with high relative content of Ir 5+ 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 (Ir 6+ ═O site) and absorbing at 870 cm -1 (Ir 6+ OO- site) on iridium-based oxides electrocatalysts, and further discover the Ir 6+ ═O and Ir 6+ OO- intermediates are stable even just from 1.3 V RHE . Density functional theory calculations indicate the catalytic activity of Ca 2 IrO 4 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.