Elucidating the Evolution of the Iridium Oxide Catalysts’ Surface: From Activation through Operation to Degradation Modes

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Iridium oxide nanoparticles as self-supported catalysts for the oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWE) have been used for a decade because of their stability in acidic environments. The community has in general accepted that the amorphous and rutile structures of this oxide have distinct intrinsic activity, both in the liquid-electrolyte model system and in membrane electrode assemblies. However, the surface properties that contribute to the higher recorded activity of amorphous iridium oxides are not yet defined. The spectroscopic features of iridium atoms are not very sensitive to changes in the chemical environment, making structural properties difficult to elucidate. The oxygen spectrum, however, provides distinct footprints for amorphous and rutile iridium oxide materials and its features are sensitive to the surface and near-surface environment changes.In this work, oxygen features are used to identify the surface composition of five commercially available materials consisting of amorphous, rutile, or mixed phases. The spectra of the amorphous (IrOx) and rutile (IrO2) structures present distinct features that are associated with chemical moieties identified using soft X-ray adsorption spectroscopy (1). A peak-fitting model is developed to analyze the X-ray photoelectron spectra (XPS) of the oxygen 1s orbitals. This model is used in this work to characterize and understand the modifications of the iridium oxide surfaces upon surface activation.During the electrochemical activation of these catalysts, different surface modifications occur depending on the structure of the pristine material. Such surface evolution, which can occur during the first cycles of polarization, renders the rational design of highly active/durable materials difficult. The analysis of the superfine O1s XPS spectra at different stages of activation allows us to understand the structural evolutions leading to activity differences. This work intends to provide sufficient insights for the identification of the intermediate structures responsible for the reactivity and stability of OER catalysts.(1) Mom, R. V.; Falling, L. J.; Kasian, O.; Algara-Siller, G.; Teschner, D.; Crabtree, R. H.; Knop-Gericke, A.; Mayrhofer, K. J. J.; Velasco-Vélez, J.-J.; Jones, T. E. Operando Structure–Activity–Stability Relationship of Iridium Oxides during the Oxygen Evolution Reaction. ACS Catal. 2022, 12 (9), 5174–5184. https://doi.org/10.1021/acscatal.1c05951. Figure 1