“In Operando” X-ray Absorption Spectroscopy Analysis of Structural Changes During Electrochemical Cycling of WO3 and WxSiyOz Amorphous Electrochromic Thin Film Cathodes

This work reports a X-ray absorption spectroscopy (XAS) study under “in operando” conditions of the structural and chemical changes undergone by WO 3 and W x Si y O z thin films used as electrochromic cathodes. The electrochromic films were prepared by magnetron sputtering deposition at oblique angles and then characterized by a large variety of techniques. The voltammograms and chronoamperometric diagrams in both aqueous and organic electrolyte media revealed a total reversibility of the electrochromic behavior, a low response time, and a high coloration efficiency for the two types of thin films. The “in operando” X-ray absorption study of the films working in aqueous solutions revealed that when they were electrochemically cycled the average W–O distances reversibly varied by a Δ d of 0.06 and 0.08 Å for, respectively, WO 3 and W x Si y O z . These changes are discussed by assuming the reduction of W 6+ cations and the transformation of W═O double bonds into single W–O bond structures during the electrochemical cycling of the films.