Green Electrochromism in the System of p-Cyanophenylviologen and Potassium Ferrocyanide

A high-performance green-electrochromic display has been developed in a system of p-cyanophenylviologen (p-CV) and potassium ferrocyanide in the presence of sodium hypophosphite and potassium chloride. The main difficulty was in the fact that p-CV reacts with potassium ferrocyanide to form a scale-like green precipitate even in the absence of any applied voltage when the concentration of p-CV exeeds only 0.001 M. At this concentration, only a poor write-erase contrast ratio can be obtained. The addition of an excess of potassium chloride, however, dissolves the precipitate, which enabled a concentration as high as 0.035 M to be attained and hence a high contrast ratio. Sodium hypophosphite facilitates the erasing of the reduced p-CV (“green”) and thus contributes to a long working lifetime. The electrochromic characteristics obtained are as follows: driving voltage of 0.4–0.9 V, contrast ratio of 50:1 and write-erase cycle of about 3 × 10 7 .