The electronic structure of iridium and its oxides

Iridium-basedmaterials are among themost active and stable electrocatalysts for the oxygen evolution reaction. Amorphous iridium oxide structures are found to bemore active than their crystalline counterparts. Herein, we combine synchrotron-based X-ray photoemission and absorption spectroscopies with theoretical calculations to investigate the electronic structure of Ir metal, rutile-type IrO2, and an amorphous IrOx. Theory and experiment show that while the Ir 4f line shape of Ir metal is well described by a simple Doniach–Šunjić function, the peculiar line shape of rutile-type IrO2 requires the addition of a shake-up satellite 1 eV above themain line. In the catalyticallymore active amorphous IrOx, we find that additional intensity appears in the Ir 4f spectrum at higher binding energy when compared with rutile-type IrO2 along with a pre-edge feature in the O K-edge. We identify these additional features as electronic defects in the anionic and cationic frameworks, namely, formally O and Ir, which may explain the increased activity of amorphous IrOx electrocatalysts. We corroborate our findings by in situ X-ray diffraction as well as in situ X-ray photoemission and absorption spectroscopies. Copyright © 2015 John Wiley & Sons, Ltd.