ELECTROCHEMICAL STUDIES OF THE FACTORS INFLUENCING THE CYCLE STABILITY OF PRUSSIAN BLUE FILMS

Some of the factors leading to long term cycle stability for Prussian Blue (PB) films are determined. The pH of the conversion electrolyte was found to be the overwhelming factor in film stability; cycle lifetimes in excess of 100 000 were easily achieved in solutions of pH 2-3. Concurrent with the increase in stability at lower pH was a considerable increase in switching kinetics. Surprisingly, the films grown from chloride-containing solutions were found to be slightly more stable, in terms of cycle life, compared to those grown from chloride-free solutions. In addition, we found that there occurs at least some conversion of PB form during the first cycle, and thus, form conversion kinetics cannot be evoked as a mechanism for film stability. Unfortunately, the films grown electrochemically on ITO glass exhibited a white haze which we attribute to light scattering caused by grain size. Unless it can be eliminated, this haze effect will be a serious drawback in the eventual use of PB in an electrochromic filter. Finally, the films were stable in mixed solvents that provide an expanded temperature range, and preconditioned films were stable to drying and immersion in boiling water.