Easy Accommodation of Different Oxidation States in Iridium Oxide Nanoparticles with Different Hydration Degree as Water Oxidation Electrocatalysts

In this paper, we present a comprehensive study on low hydration Ir/IrO2 electrodes, made of an Ir core and an IrO2 shell, that are designed and synthesized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir-O active centers. Three materials with different hydration degrees have been deeply investigated in terms of structure and microstructure by means of transmission electron microscopy (TEM) and synchrotron radiation techniques such as high-resolution (HR) and pair distribution function (PDF) quality Xray powder diffraction (XRPD), X-ray absorption spectroscopy (XAS), and for what concerns their electrochemical properties by means of cyclic voltammetry and steady-state I/E curves. The activity of these materials is compared and discussed in the light of our most recent results on hydrous IrOx. The main conclusion of this study is that the Ir core is noninteracting with the IrOx shell, the latter being able to easily accommodate Ir in different oxidation states, as previously suggested for the hydrated form, thus explaining the activity as electrocatalysts. In addition, in operando XAS experiments assessed that the catalytic cycle involves Ir(III) and (V), as previously established for the highly hydrated IrOx material.