1. Field of the Invention
The present invention relates to an electrochromic display device. More particularly, it relates to an electrochromic display device which utilizes tungsten oxide as an electrochromic compound and an activated carbon fiber cloth as a counter electrode material.
2. Description of the Prior Art
A typical electrochromic display device comprises a display electrode comprising a transparent substrate, a transparent electrode formed on an inner surface of the transparent substrate and an electrochromic material layer for display made of tungsten oxide and formed on the transparent electrode; a counter electrode comprising a substrate, a conductive layer formed on an inner surface of the substrate and a counter electrode material; and a spacer provided between the display electrode and the counter electrode with their inner surfaces facing to each other to shield them to define, between them, an electrolyte chamber which is filled with an electrolyte.
Such an electrochromic display device utilizes the coloring of the electrochromic material by applying voltage between the the display electrode and the counter electrode through the electrolyte.
Generally, tungsten oxide (WO.sub.3) is used as an electrochromic material, and a lithium salt is used as an electrolyte. In such a combination of the electrochromic material and the electrolyte, tungsten oxide turns to blue according to the following reaction: ##STR1##
Usually, as the counter electrode material, carbon fiber cloth, WO.sub.3 powder or MnO.sub.2 powder is used. Among them, the carbon fiber cloth is preferably used because of its large injected charge.
However, it was found that the injected charge gradually decreases during storage when the carbon fiber cloth is used as the counter electrode material. Then, a study was made by the present inventors to determined how to prevent such degradation of the injected charge.
It is known to add an acid to the electrolyte to prevent tungsten oxide from dissolving in the electrolyte (cf. for example, Japanese Patent Kokai Publication No. 138720/1980). However, since the electrochromic display device of said patent application does not utilize the carbon fiber cloth, its injected charge is small, which is far from the desired property of an electrochromic display device.