engleski

Kinetics of the [Fe(CN)6]3-/[Fe(CN)6]4- Redox Couple Reaction on Anodically Passivated Fe80B20

The redox reaction of various concentrations (in 1:1 molar ratio) of the {;Fe(CN)_6};^3-/{;Fe(CN)_6};^4- redox couple in the neutral borate-buffered (pH=8.4) electrolyte solution is studied on the Fe80B20 electrode passivated under strongly defined conditions. Impedance measurements in the potentiostatic mode and "quasy" steady-state polarization measurements are used. The results of measurements clearly show the electron transfer reaction occuring as the rate determining reaction in both directions (cathodic and anodic) vs. R_rev of the redox couple. The rates of reactions as well as the value of the exchange current are low, but characteristic of the oxide-covered electrodes. The impedance data analysis reveals the electron transfer reaction impedance to be in parallel with an already existing passive film/solution interface impedance in the blank solution. Redox reaction on the passivated Fe80B20 proceeds via electron transfer between the conduction band and ions in the solution, which is characteristic of the classical semiconductor of n-type. Contribution of either tunnelling of electrons across an electron depletion space-charge layer and/or electron transfer occuring partly via surface states is characteristic of the cathodic side of potentials. At anodic potentials, however, in the conditions of a thick space-charge layer acting as a high surface barrier, an electron transfer proceeds mainly via surface states being insured by ionization of donors within the passive film. In both ranges of potentials, the rate of reaction is determined predominantly by the solid side properties, and not by the properties of the ionic double layer present at the solution side of the passive film/solution interface. Comparison with literature data on (thick) anodic passive films formed on the pure iron points to similar kinetic data of two passive films, but the lower catalytic efficiency of the Fe80B20 passive film surface.

Passivated Fe80B20 ; redox couple reaction ; impedance

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