A Diffusional Study of Electrochromical Effect and Electrointercalation of Li + Ions in WO 3 Thin Films

This paper presents a study of physical and electrochemical properties relevant to the lithium ion diffusion process governing WO3 thin films electrochromic phenomenon. The films were prepared from a solution obtained by the sol–gel process and deposited via dip-coating. They were characterized by chronocoulometry, chronoamperometry and in and ex situ ultraviolet–visible spectroscopy during coloring/bleaching of the films. To analyze the experimental data, a mathematical model was applied considering a reaction kinetics controlled by the diffusion of ion pairs. The effects of the intercalation of Li+, Na+ and K+ ions was compared. Theoretical and experimental results for the current, charge density and absorbance profiles in function of time are included in this paper. The highest values obtained were $$ \eta = 29.98\,{\text{C/cm}}^{2} $$ for electrochromic efficiency, ΔT = 61.6% for optical modulation, $$ Q_{i} = 81.78\,{\text{mC/cm}}^{2} $$ for charge density, and reversibility of coloring/bleaching in the range of 75–90%. X-Ray diffractometry measurements confirmed the amorphous structure of the films. Fourier transform infrared spectroscopy was employed to analyze the functional groups present in the films structure, while their topology was studied via atomic force microscopy. The collected results confirmed a relationship between the electrochromic and electrochemical reactions, definitively associating color change with cations injection and the reduction of W6+ to W5+.