Tungsten oxide (WO 3 ) electrochromic devices have attracted a lot of interest in the energy conservation field and have shown a preliminary application potential in the market. However, it is difficult to quantitatively direct experiments with the existing electrochromic theoretical models, which can restrict the further development of electrochromism. Here, an electrochromic physical simulation model of WO 3 films was built to solve the above problem. Experimentally, the actual electrochromic kinetics of WO 3 in the LiClO 4 /propylene carbonate electrolyte was determined as a continuous electron-transfer process by cyclic voltammetry measurement and X-ray photoelectron spectroscopy analysis. Theoretically, the continuous electron-transfer process, Li + -ion diffusion process, and the transmittance change process were described by a modified Butler−Volmer equation, Fick's law, and charge versus coloration efficiency/bleaching efficiency coupling equation, respectively. The comparisons between theoretical and experimental data were conducted to verify this model. The shape of the simulated current curves was basically consistent with that of experiments. Besides, the difference of transmittance between the simulation and experiments was less than 8%. The difference between theory and experiment was attributed to the influence of the electric double layer and the actual reaction interface. The success of the simulation was attributed to the accurate description of the electrochromic process by continuous electron-transfer kinetics. This model can be applied in the research of electrochromic mechanisms, experimental result prediction, and novel device development due to its clear physical nature.

Tungsten trioxide (WO3) is a wide band gap semiconductor material that is used as an important electrochromic layer in electrochromic devices. In this work, the effects of the annealing temperature on the optical band gap of sol-gel WO3 films were investigated. X-ray Diffraction (XRD) showed that WO3 films were amorphous after being annealed at 100 °C, 200 °C and 300 °C, respectively, but became crystallized at 400 °C and 500 °C. An atomic force microscope (AFM) showed that the crystalline WO3 films were rougher than the amorphous WO3 films (annealed at 200 °C and 300 °C). An ultraviolet spectrophotometer showed that the optical band gap of the WO3 films decreased from 3.62 eV to 3.30 eV with the increase in the annealing temperature. When the Li+ was injected into WO3 film in the electrochromic reaction, the optical band gap of the WO3 films decreased. The correlation between the optical band gap and the electrical properties of the WO3 films was found in the electrochromic test by analyzing the change in the response time and the current density. The decrease in the optical band gap demonstrates that the conductivity increases with the corresponding increase in the annealing temperature.

Nickel oxide (NiO) is a wide band gap semiconductor material that is used as an electrochromic layer or an ion storage layer in electrochromic devices. In this work, the effect of annealing temperature on sol-gel NiO films was investigated. Fourier transform infrared spectroscopy (FTIR) showed that the formation of NiO via decomposition of the precursor nickel acetate occurred at about 300 °C. Meanwhile, an increase in roughness was observed by Atomic force microscope (AFM), and precipitation of a large number of crystallites was observed at 500 °C. X-ray Diffraction (XRD) showed that the NiO film obtained at such a temperature showed a degree of crystallinity. The film crystallinity and crystallite size also increased with increasing annealing temperature. An ultraviolet spectrophotometer was used to investigate the optical band gap of the colored NiO films, and it was found that the band gap increased from 3.65 eV to 3.74 eV with the increase in annealing temperature. An electrochromic test further showed that optical modulation density and coloring efficiency decreased with the increase in crystallite size. The electrochromic reaction of the nickel oxide film is more likely to occur at the crystal interface and is closely related to the change of the optical band gap. An NiO film with smaller crystallite size is more conducive to ion implantation and the films treated at 300 °C exhibit optimum electrochromic behavior.

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Even though electrochromism has been around for more than 50 years, it still has several issues. Multi-layered films, high manufacturing costs, and a short lifetime are present in existing electrochromic devices. We demonstrate a unique high-performance device with a basic structure and no solid electrochromic sheets in this work. In this device, the electrolyte layer is also avoided. The device uses an electrochromic solution prepared from a mixture of ammonium metatungstate and iron (II) chloride solution as a functional layer with reversible redox properties. The tungstate ions on the electrode surface are reduced when the device is colored, and the Fe2+ on the electrode surface is oxidized on another electrode surface. The generated Fe3+ in the mixed functional layer oxidizes the previously reduced tungstate ions as the device fades. We determined the ΔT (transmittance modulation) and response time among ammonium metatungstate ratios, iron (II) chloride ratios, and driven current density using DOE (design of experiment) trials. Using 0.175 mol/L ammonium metatungstate and 0.30 mol/L iron (II) chloride, a device with outstanding ΔT (more than 57% at 700 nm), a short response time (less than 10 s), and high coloring efficiency (160.04 cm2/C at 700 nm) is demonstrated.