In-situ X-ray absorption spectroscopy on (photo )electrocatalysts: new methods and innovative techniques towards new insights on reaction mechanisms

Electrochemical in-situ and in-operando X-ray absorption spectroscopy represents one of the most powerful available tools to study the fine structure and the behaviour of electrode materials. This serves to better elucidate important reaction mechanisms and to better define structure/activity relations. In this context, the Fixed Energy X-Ray Absorption Voltammetry (1) represents a novel 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. FEXRAV allows to quickly map the variation of the oxidation states of the element under consideration in a desired potential window. As a result, 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. This led, for example, to a better understanding of the mechanism of iridium oxide as catalyst for water oxidation: for the first time the co-existence of more than one Ir oxidation state at E >1.23V (RHE) was directly observed and this is consistent with the role of Ir as center of a catalytic cycle (2). More recently, studies by time-resolved energy dispersive XAS have been carried out with the aim of studying the time-dependence of interfacial phenomena. (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