In situ crystallization of high performing WO ₃-based electrochromic materials and the importance for durability and switching kinetics

A chemical self-assembled synthesis was used to prepare disordered porous semicrystalline WO ₃-based cathodic electrochromic films. The resulting films were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N ₂ physisorption, Fourier transform infrared spectroscopy (FTIR) and near-edge X-ray absorption fine structure (NEXAFS). The electrochromic performance was evaluated in a Li-ion electrolyte (i.e., 1 M LiClO ₄ dissolved in propylene carbonate) with cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL) and chronoamperometry (CA) techniques. It is demonstrated that the crystallinity of WO ₃ thin films can be readily tuned with the variation of annealing temperatures and TiO ₂ addition. The results demonstrated excellent stability and durability (i.e., 1500 GCPL cycles in 32 days) for WO ₃ electrode annealed at 350 °C, ultrafast switching kinetics for WO ₃-TiO ₂ electrode (i.e., bleaching and coloration times are 5.5 s and 4.2 s, respectively) and excellent charge reversibility (%R ≈ 100%). Electrochemical, TEM and Raman spectroscopy studies suggest that a change in degree of crystallinity in WO ₃ occurs during the extended durability test, which then influences the durability and switching kinetics.