(Invited) Unveiling Changes in Surface Chemistry of Iridium Single Crystals and Metal Oxide Supported IrO_x Nanoparticles in Oxygen Evolution Reaction Conditions

摘要

Gaining fundamental insights into the formation and the stability of iridium (Ir) surface oxides is pivotal to efficient and sustainable oxygen evolution reaction (OER) electrocatalysis, a key reaction in the field of power-to-fuels and power-to-chemicals. However, proton-exchange membrane water electrolysers (PEMWE) systems currently suffer from their high investment and operational costs, which are partly due to the use of micrometer-sized iridium oxide (IrO_x) particles to electrocatalyze the OER. A passage to IrO_x nanoparticles would be highly desirable but these nanomaterials face time-dependent changes in structure and chemical composition in OER conditions. To shed fundamental light into the mechanisms at stake, we used electrochemical techniques in combination to X-ray photoelectron spectroscopy (XPS), and inductively coupled mass plasma spectrometry (ICP-MS). Experiments on Ir(111), Ir(210) and nanostructured Ir(210) surfaces showed that the oxy-hydroxides layers forming in the pre-OER region feature mixed Ir oxidation states (presence of Ir(0), Ir(+III) and Ir(+IV) species), and that the fraction of each oxidation state depends on the crystallographic orientation. In the OER region, Ir(+III) species progressively dissolve leading to an enrichment of the surface and near-surface regions of the single crystals into Ir(+IV) species, and resulting in a decrease of their intrinsic activity towards the OER. The results indicate a convergence towards a more stable but less active surface state, which does not depend neither on the initial arrangement of surface atoms (crystallographic orientation, proportion of high- and low-coordinated atoms) nor on their oxidation state (initial state vs. electrochemically-activated Ir(hkl) surfaces). These findings were confirmed on more technologically relevant materials such as IrO_x nanoparticles supported onto antimony-doped (ATO), niobium-doped (NTO) or tantalum-doped (TaTO) tin oxide aerogels. Moreover, the combination of identical-location transmission electron microscopy (IL-TEM) and in situ ICP-MS helped in identifying which potential ranges are critical to the stability of IrO_x nanocatalysts and their supports, and provided practical guidelines for the development of more active and more stable PEMWE anodes.