Iron Oxides Model Electrocatalysts for the Hydrogen Evolution Reaction

Iron oxides show promising performances as electrocatalysts for the electrochemical water splitting, which is considered a sustainable method for the production of green hydrogen. However, iron oxides undergo complex structural and chemical changes during electrochemical processes and the exact role played by iron species in electrocatalysis and specifically in the hydrogen evolution reaction (HER), has not been fully understood yet. Moreover, in alkaline conditions, given the scarcity of protons, also the dissociation of water should be carefully considered, therefore the full comprehension down to the atomic level of the HER requires the acquisition of detailed information about the local water structure and the hydroxyl binding energy. This thesis systematically investigates the water adsorption and dissociation in different experimental conditions as well as the redox transitions of iron (oxides). To tackle these problems, we applied the Surface Science method based on planar model systems, which are designed with atomic scale precision, and investigated through a reductionistic approach based on sophisticated techniques. To this aim, we prepared epitaxial iron oxides ultrathin films supported on Au(111) and investigated their interaction with water vapors and with the electrolytic environment. By combining electrochemical scanning tunneling microscopy, operando Raman spectroscopy, with cyclic voltammetry, we provided a comprehensive interpretation of all the surface phenomena taking place on iron surfaces in alkaline media in a wide electrochemical range from the HER up to 1.5 V vs reversible hydrogen electrode (RHE). The surface species that are stable in different conditions were identified and characterized at the atomic level. This information indicated that at the onset of the HER, an interfacial hydroxide FeOH(ads.), is significantly stable and shows higher catalytic activity than the metal. Moreover, by comparing the experimental data of planar ultrathin films with those of thicker films and even AuFe alloy nanoparticles (NPs), it was possible on the one hand to identify the specific phenomena related to special nature of ultrathin films, and on the other hand to confirm the large applicability of the knowledge elaborated on model systems even of practical systems. Finally, this study not only demonstrated the feasibility and relevance of operando studies of the model systems, but also suggests novel strategies to improve the performances of iron oxides-based catalysts.