Simulation model for electrochromic process in WO3 electrodes by a current step and its application to the smart window systems

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Abstract
Voltage-time curves, concentration profiles and surface concentration of lithium for the electrochromic process at WO3 electrodes have been calculated using a model based on constant current charge injection into electrochromic WO3. The simulated data agree well with the experimental data for both sol-gel and sputter deposited WO3 films. Parameters such as diffusion coefficient, series resistance and ion mobility can be estimated from theoretical fitting to the experimental data. The results are comparable to those obtained by the AC impedance measurements. The model predicts a higher concentration of Li+ ions in the region near the electrolyte/WO3 interface than in the region far from the interface. For higher diffusion coefficients, a more homogeneous lithium distribution is established in the film at the conclusion of the coloration part of the cycle. This result has significant implications for the long term testing of electrochromic devices, as testing to reflect real device conditions should ensure that bleaching starts when the charge distribution in the device is the same as in a window which has been colored for several hours. It is well known that LixWO3 is not reversible for x greater than 0.4. The contours of diffusion coefficient in charge density-current density plane obtained from the model show that for a given current density a lower diffusion coefficient corresponds to a smaller allowable amount of charge injection for a reversible cycle, while for a given diffusion coefficient a smaller current density allows a larger amount of charge injection. This gives a quantitative way to determine appropriate operational limits for electrochromic devices.

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