Operando spectroscopy for the study of reaction mechanisms in electrochemistry

The progressive lack of fossil fuels together with the growing awareness of the environmental problems has led to an extensive research on renewable and sustainable energy sources. In this sense, electrochemistry covers a major role, considering that many processes for sustainable energy production are based on electrochemical reactions. While much attention has been devoted to the development of new electrode materials, a comprehensive understanding of the mechanism of electrochemical reactions is still necessary to rationalize the choice of the electrodes and to design new ones, with the final aim of reaching higher efficiencies. Operando X-Ray absorption spectroscopy (XAS) is a powerful and versatile technique which allows information regarding the electronic and structural properties of a given material, while the material is operating under conditions of reaction. In this thesis, operando XAS was carried out on electrodes involved in (photo)electrocatalytic reactions. Attention was especially paid to photosystems for the photocatalytic water splitting, where operando XAS permitted to study the generation and the fate of the photogenerated carriers (recombination/charge transfer) and to clarify the role of the overlayer in composite electrodes. While operando XAS experiments with hard X-rays (i.e. highly energetic X-rays) are well-established, the same cannot be said for operando XAS with soft X-rays, since their low penetration depth and the severe vacuum limitation have hindered a parallel development. In this work, a pioneering experiment of operando soft-XAS at ambient pressure was performed to study the reactivity of SnO2 towards reducing gases. Finally, dynamic multi-frequency analysis (DMFA) was employed in the field of electrocatalysis as a new strategy for the study of reaction mechanisms. This technique, which consists in acquiring dynamic impedance spectra while cycling the electrode under investigation, allowed information to be gained regarding the mechanism of the HER reaction on Pt and relevant kinetic parameters, through directly following their trend with the potential.