Direct Observation of Stable Water-Oxidation Intermediates on CaIrO Nanocrystals for Efficient Acidic Oxygen Evolution.

We report CaIrO nanocrystals exhibit record stability of 300 h continuous operation and high iridium mass activity (248 A g at 1.5 V) that is about 62 times that of benchmark IrO. Lattice-resolution images and surface-sensitive spectroscopies demonstrate the Ir-rich surface layer (evolved from one-dimensional connected edge-sharing [IrO] octahedrons) with high relative content of Ir sites, which is responsible for the high activity and long-term stability. Combining infrared spectroscopy with X-ray absorption spectroscopy, we report the first direct observation of key intermediates absorbing at 946 cm (Ir═O site) and absorbing at 870 cm (IrOO- site) on iridium-based oxides electrocatalysts, and further discover the Ir═O and IrOO- intermediates are stable even just from 1.3 V. Density functional theory calculations indicate the catalytic activity of CaIrO 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.