The role of electrochemical impedance spectroscopy (EIS) in the global characterisation of the reduction kinetics of hexacyanoferrate on anodised titanium

Abstract

To quantify the charge transfer ( ct) step during the reduction of hexacyanoferrate, it is studied at a galvanostatically anodised polycrystalline Ti electrode, for which a ct controlled current can be observed over a large potential domain. In particular, the present study aims the characterisation of the electrode semiconducting properties (flat band potential E fb and donor density N D), in contact with the hexacyanoferrate system. Hereto, the use of EIS is essential for the determination of the space charge capacitance ( C sc) of the TiO 2 layer, as a function of the applied potential E. A methodology is proposed to determine C sc in a convenient way. In the high frequency range of the EIS measurements, only the solid state properties are being probed, and a simple series R- C sc equivalent circuit becomes suitable. The validity of this method is checked through the analysis of different equimolar redox solutions. Finally, the values of E fb and N D are determined from the obtained non-linear Mott–Schottky plots ( C −2 sc vs E). In particular, the donor density N D is computed numerically from the instantaneous slope, by solving a finite difference equivalent of the relation for different discrete N D( E) values.