Thin films of ruthenium purple (RP) KFeRu(CN)6 or NH4FeRu(CN)6 are shown to be electrochromic.The compounds are reduced to a colorless state at 216 and 245 mV in 1 N K2S04 and (NH4)2S04, respectively.These reduction reactions are analogous to the reduction of Prussian blue (PB) but there is no corresponding oxidation to the analogue of Berlin green.The kinetics of the reduction of ruthenium purple and Prussian blue are examined in detail.It is shown that the scan rate dependence of voltammetric peak current and peak separation can be explained in terms of a concentration-dependent film resistance.The kinetic response to a potential step is explained in terms of migration and diffusion of charge carriers within the film.A simple transport model, which involves the assumption of electroneutrality within the film, leads to a special form of the Nernst-Planck equation which is solved for the current at the film-solution interface.
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Rajan et al. (1982) studied this question.