Self-assembled films from WO3: Electrochromism and lithium ion diffusion

WO3/chitosan and WO3/chitosan/poly(ethylene oxide) (PEO) films were prepared by the layer-by-layer method. The presence of chitosan enabled PEO to be carried into the self-assembled structure, contributing to an increase in the Li+ diffusion rate. On the basis of the galvanostatic intermittent titration technique (GITT) and the quadratic logistic equation (QLE), a spectroelectrochemical method was used for determination of the “optical” diffusion coefficient (Dop), enabling analysis of the Li+ diffusion rate and, consequently, the coloration front rate in these host matrices. The Dop values within the WO3/chitosan/PEO film were significantly higher than those within the WO3/chitosan film, mainly for higher values of injected charge. The presence of PEO also ensured larger accessibility to the electroactive sites, in accordance with the method employed here. Hence, this spectroelectrochemical method allowed us to separate the contribution of the diffusion process from the number of accessible electroactive sites in the materials, thereby aiding a better understanding of the useful electrochemical and electrochromic properties of these films for use in electrochromic devices.