Temperature-dependent structural evolution and electrochromic properties of peroxopolytungstic acid

Temperature-dependent in situ W L3-edge X-ray absorption spectroscopy has been performed at 20–500 ‡C to investigate the geometric environment and electronic structure of tungsten upon the transformation of electrochromic peroxopolytungstic acid to tungsten trioxide. The temperature-dependent variation of the coordination number and bond distance of the nearest (W–O) sphere reveals that the peroxopolytungstate changes into crystalline tungsten oxide through the gradual removal of peroxo species, zeolitic water, hydroxyl groups, and terminal WLO bonds, which is also confirmed by thermal analysis, powder XRD, and FT-IR spectroscopy. It is especially emphasized that the intermediate structure of peroxopolytungstate at 120–200 ‡C is characterized by a substantial number of oxygen vacancies together with a partially formed oxide lattice. A thin film of peroxopolytungstic acid has been prepared on a transparent conducting substrate and post-annealed at 100–300 ‡C. From cyclic voltammetry and optical density measurements in a LiClO4–propylene carbonate electrolyte, it was found that the voltammetric exchange charges are maximized for films annealed at 150– 200 ‡C, whereas the voltage response of coloration/bleaching is depressed by the increase in annealing temperature. Considering both the electrochromic kinetics and redox capacity, the optimum annealing temperature for the formation of WO3 film was determined to be 120–200 ‡C. Such results could be well explained in terms of the structural evolution of peroxopolytungstic acid on the basis of X-ray absorption spectroscopic analysis.