Résumé
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) [1]. 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 [2]. 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 [3]. Ackowledgements This work was supported by the French National Research Agency in the frame of the MOISE project (grant number ANR-17-CE05-0033). References 1. M. Scohy, S. Abbou, V. Martin, B. Gilles, E. Sibert, L. Dubau, F. Maillard, ACS Catal . 9 (2019) 9859-9869. 2. F. Claudel, L. Dubau, G. Berthomé, L. Solà-Hernandez, C. Beauger, L. Piccolo, F. Maillard, ACS Catal . 9 (2019) 4688-4698. 3. S. Abbou, R. Chattot,F. Claudel, L. Dubau, L. Solà-Hernandez, C. Beauger, L. Dubau, F. Maillard, in preparation. Figure 1 . Schematics of the main findings obtained on Ir(hkl) single crystal surfaces: the initial crystallographic orientation and the fraction of high- and low-coordinated atoms influence the nature and the oxidation state of Ir surface atoms, thus determining their initial OER activity. The Janus nature of Ir(III) species is demonstrated: these species are the most active towards the OER but they get easily dissolved, leading eventually to a less active yet more stable surface state. Figure 1