Electrochromic films involving Prussian blue and viologen materials

Abstract

Prussian blue (PB) and viologen materials color by electrode oxidation and reduction reactions, respectively; therefore, it may be possible for these materials to function in a complementary fashion in an electrochromic optical filter. The soluble viologen compounds deposit as a colored film during the cathodic reaction and go back into solution when the current is reversed for the erase cycle. Prussian blue, in contrast, is prepared as a permanent film that can be switched back and forth from the colorless to the blue form. In this study we have determined some of the major factors leading to longterm cycle stability for viologen and Prussian blue films. The pH of the tlectrolyte is a major factor in the film stability for Prussian blue. Life-times in excess of 100,000 cycles were easily achieved for Prussian blue films in solutions of pH =2. In contrast, none of the films tested in neutral solutions showed a lifetime exceeding several hundred cycles. For viologen materials, major factors affecting cycle lifetimes are the molecular structure of the viologen along with the electrolyte and solvent compositions. The use of several mixed electrolytes such as KNO3IK2SO4 as well as the use of mixed solvents can greatly increase the cycle stability of some viologen compounds. In fact, the failure mechanism for viologen films can be completely defeated by the appropriate viologen/electrolyte/solvent selection. The complementary behavior of Prussian blue and viologen films will be investigated at electrolyte and solvent compositions favorable for cycle stability.