Morphological and chemical dynamics upon electrochemical cyclic sodiation of electrochromic tungsten oxide coatings extracted by in situ ellipsometry.

The sodiation-desodiation process of sputtered amorphous electrochromic tungsten oxide coatings in an aqueous-based medium was simultaneously monitored over 99 cycles by cyclic voltammetry and in situ spectroscopic ellipsometry. This allowed extracting the evolution of optical and geometrical parameters upon cycling. The resulting electrochemical coloring-bleaching process was dynamically fitted in the 1.8-2.8 eV optical range with a four-phase model including a constrained spline parametrization of the dielectric function. This allows real time access to thickness, surface roughness, and dielectric function of ${{\rm Na}_x}\!{{\rm WO}_3}$NaxWO3. The temporal evolution of the latter in the fully colored state was used to highlight the porosity extent of the probed coating of opened morphology. The designed spectroelectrochemical approach was applied to map the temporal evolution of the $\rm Na$Na content (${x}$x in ${{\rm Na}_x}\!{{\rm WO}_3}$NaxWO3) during and between cycles, taking into account the intricate interplay between charge density, thickness, and electrolyte uptake.