An electrochemical study of cation permeability into polypyrrole-containing [Fe(CN)₆]^4-

Polypyrrole [Fe(CN)₆]^4-, (PPFCN), prepared potentiostatically from aqueous solution, is shown to exhibit a "breaking-in" effect during continuous cycling past the formal potential of the redox species within the layer or by holding the layer at a potential where the [Fe(CN)₆]^4- is oxidised. Ideal thin layer cyclic voltammetric behaviour was not obtained for the redox couple in these layers, due to the presence of double layer charging associated with the polypyrrole itself. The formal potential of [Fe(CN)₆^4- within the layer was found to be 100mV more negative than [Fe(CN)₆]^4- solution species while the p[K_a for [Fe(CN)₆]^4- within the layer was similar to that of solution species. Slopes of E^f_p vs. log (cation concentration), where E^f_p is the formal potential of the redox species within the polymer, were found to increase with hydrated cation radius, with Cs⁺ showing the closest to ideal behaviour. Non-ideality at low cation concentration was narrowed down to a cation preconcentration effect evident in the position of the anodic peak of the couple. H⁺ behaved differently from other cations since there was a competition between it acting as a cation for charge neutrality and a protonating species for the [Fe(CN)₆]^4-. The layer also shows an ability as a probe for examining ion movement through overlaying layers.