The formation of bilayered nickel-iron, cadmium-iron and cadmium-silver hexacyanoferrates by an electrochemically driven insertion-substitution mechanism

Upon cyclic oxidation and reduction of a parent metal hexacyanoferrate, metal ions from the electrolyte solution can be driven into interstitial positions of the zeolitic structure of the metal hexacyanoferrate. Whereas alkali metal ions are stable on these sites, some other metal ions undergo a substitution reaction with the N-coordinated metal ions, since the latter are only weakly bonded, This results in the formation of a layer of a daughter metal hexacyanoferrate covering the parent metal hexacyanoferrate. A bilayered structure is formed which can be identified by its behaviour in cyclic voltammetry. There is no indication that the electron and ion transfer through the interface of the two layers is hindered. From this and the growth mechanism it is probable that the daughter hexacyanoferrate will form an epitaxial layer on top of the parent hexacyanoferrate, provided that the two hexacyanoferrates do not differ too much in their lattice parameters. Experimental results, for which abrasive stripping voltammetry was used, are reported for a nickel hexacyanoferrate layer on Prussian blue, for a cadmium hexacyanoferrate layer on Prussian blue, and for a cadmium hexacyanoferrate layer on silver hexacyanoferrate.