Activationless Reduction of the Hexacyanoferrate Anion on a Mercury Electrode

Polarization curves for the electroreduction of [Fe(CN)₆]^3– on a mercury electrode in solutions containing different amounts of surface-inactive supporting cations are simulated on the basis of modern theory of charge transfer in polar media combined with quantum-chemical approaches. The conclusions about an activationless nature of the process in the overvoltage range 1.2 to 2.0 V accessible experimentally, which were drawn from the results of earlier calculations made within simpler models, are confirmed. The maximum contribution to the current is shown to be made by energy levels of metal that lie considerably (up to 1 eV) lower than the Fermi level. To establish the reasons for the anomalous behavior of the current in the activationless region at high overvoltages, the effect various factors sensitive to the electrode charge exert on the model curves at high negative charges of the electrode surface is analyzed. In connection with this, the stability of the results of a calculation of the electron overlap metal/reactant to a model of the interface is considered. The plausibility of the model proposed for the reaction layer and the approaches used for computing the activation energy and the preexponential factor is corroborated by good agreement between the temperature effect found for the region of a minimum current and its experimental value.