FIXED ENERGY X-RAY ABSORPTION VOLTAMMETRY (FEXRAV) : PROOF OF CONCEPT AND APPLICATION TO A WATER OXIDATION CATALYST

Fixed Energy X-Ray Absorption Voltammetry represents a novel in-situ X-Ray absorption technique for fast and easy preliminary characterization of electrode materials and consists in recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to an X-Ray absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the original oxidation state determines a variation of the 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 windows. To this end, we use highly hydrated iridium oxide films. Proof of concept of FEXRAV. left: Normalized XANES spectra of IrO2 (a) and IrCl3 (b). The arrow marks the energy chosen for the FEXRAV measurements (11221.0 eV). Right: CV (empty arrow, left axis) and FEXRAV signal (full arrow, right axis) of an IrOx film. We show that FEXRAV gives important information by itself but can also serve as a preliminary screen of the potential window or, more generally, for selecting the best experimental conditions for a more detailed X-Ray Absorption Spectroscopy (XAS) analysis. In fact, this work includes a detailed XAS study aimed to clarify the mechanism of iridium oxide as catalyst for water oxidation. Acknowledgements: BM08 (GILDA) @ESRF is acknowledged for provision of beamtime, travel and accommodation expenses (Exp. CH-3511). Financial support by Fondazione Cariplo (2010-0506) is gratefully acknowledged. C.L. is grateful to Università degli Studi di Milano for a post-doc fellowship