Indium hexacyanoferrate films, voltammetric and impedance characterization

Charge transfer processes within indium hexacyanoferrate (InHCF) films have been studied by using mainly cyclic voltammetry and faradaic impedance methods. Cyclic voltammetry reveals the existence of two types of InHCF films deposited from solutions of high and low potassium concentrations, respectively. The difference in the electrochemical behavior of these films disappears after their cyclic processing in the range of electrode potentials from 0.0 to 1.3 V with respect to Ag/AgCl electrode. The obtained data are strongly dependent on the nature of supporting electrolyte cations. However, in all the cases, the obtained impedance spectra are qualitatively the same. In the region of high a.c. frequencies, they contain a semi-circular part, the parameters of which depend on both the substrate nature and the mechanical pre-treatment (polishing) of the used substrate (platinum, glassy carbon). This allows one to assign that part of impedance spectra to the film/electrode interface. The next part of the spectra corresponds to the Warburg impedance that is somewhat masked by the subsequent increase of the imaginary part of the film impedance under decreasing a.c. frequency. Effective diffusion coefficients calculated from the Warburg dependence for different supporting electrolytes change their values in the series: DK+≥DNa+≫DLi+. Such a great difference between DLi+ and DK+≥DNa+ is confirmed by the results of chronoamperometric measurements. It is assumed that the above difference originates from different binding of the compared counter-ions with anion film fragments. This assumption is in agreement with the results derived from low frequency parts of the impedance spectra.