Yellow tungsten oxide, or tungsten trioxide, is a cathode discoloration material. The color contrast of tungsten trioxide is still relatively low compared to a composite electrode complementary electrochromic device assembled using WO3 and nickel oxide. Wherein nickel oxide is the anode coloring material preferred by researchers, which includes nickel oxide of ideal ratio (light green) and nickel oxide of non-ideal ratio (gray black).
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The NiO of non-ideal ratio is very chemically active. And it can be converted between Ni2+ and Ni3+, resulting in a color change. The initial state of the nickel oxide film is grayish black. When HClO4- is injected in the nickel oxide film, the nickel oxide film is colored. When the HClO4- is removed from the nickel oxide film, the film is bleached. The nickel oxide film in bleached state can store more cations than the cathode coloring material tungsten oxide in colored state. Therefore, the nickel oxide film can be used not only as an electrochromic layer, but as an ion storage layer of a device, so it provides sufficient cations for coloring the electrochromic layer. Moreover, it complements the cathode electrochromic layer to further enhance the optical modulation range of the electrochromic device.
