In-situ X-ray absorption spectroscopy on (photo-)electrocatalysts for water oxidation: towards new insights on the reaction mechanism

Here we introduce the Fixed Energy X-Ray Absorption Voltammetry (1), a novel in-situ/in-operando X-Ray Absorption Spectroscopy (XAS) technique for fast and easy preliminary characterization of electrodes and photoelectrodes which consists in recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to a core level absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the initial oxidation state determines a variation of the X-ray absorption coefficient. In this work we demonstrate that FEXRAV allows to quickly map the variation of the oxidation states of the element under consideration in a desired potential window. At this purpose, we use high surface area electrodes to attain a high surface/volume ratio (nanoparticles, nanostructures, highly hydrated films) and be more sensible to any chemical phenomena occurring at the surface. We show that FEXRAV gives important information by itself but can also serve as a preliminary screening of the potential window or, more generally, for choosing the best experimental conditions for a better targeted XAS analysis. In fact, this work includes a detailed XAS study aimed to clarify the mechanism of iridium oxide as catalyst for water oxidation: for the first time we directly observed the co-existence of more than one Ir oxidation state at E >1.23V (RHE), that is consistent with the role of Ir as center of a catalytic cycle. This represents a crucial point for a better understanding of water electrolysis and photoelectrochemical (PEC) water splitting (2). We completed this study by time-resolved energy dispersive XAS for better understanding the time-dependence of the interfacial phenomena occurring during pseudocapacitance charge/discharge and during the water oxidation catalysis. (1) Minguzzi, A.; Lugaresi, O.; Locatelli, C.; Rondinini S.; d'Acapito, F.; Achilli, E.; Ghigna, P. Anal. Chem. 2013, 85, 7009-7013. (2) Minguzzi A., Lugaresi O., Achilli E., Locatelli C., Vertova A., Ghigna P., Rondinini S., Chem. Sci. 2014, 5, 3591-3597.