Pulsed enhancement of the rate of coloration for tungsten trioxide based electrochromic devices

The rate of electrochromic coloration within tungsten oxide-based optical devices may be enhanced considerably by applying a progression of potentiostatically-controlled current pulses rather than applying a continuous current. The rate of coloration depends strongly on the pulse length employed, the optimum duration depending on the pulse amplitude. An additional advantage of this pulsing procedure is to enhance the durability of electrochromic devices by decreasing the prevalence of undesirable electrolytic side reactions such as the formation of molecular hydrogen gas. The effects of interrupting the current (pulses) is attributed to the formation of a thin layer of bronze having a large insertion coefficient x on the electrolyte-facing side of the WO3. The relationship between optimum pulse width t for rapid coloration when using a pulse height of V-a are analyzed in terms of the Faughnan and Crandall model of potentiostatic ion movement through WO3, obeying the formulations for electro-bleaching. It is inferred that the rate of bronze dissipation between the voltage pulses dominate the overall kinetic V-a-i-t behavior observed. (C) 1998 Elsevier Science Ltd. All rights reserved.