EIS study of the FeIII/FeII redox couple at a Nafion®-coated Pt electrode

The redox reaction FeIII + e ↔ FeII is studied at a Nafion®-filmed Pt electrode using both steady-state and EIS methods. It is shown that the fitting of the EIS experimental data can be greatly improved by using the new concept of a ‘theoretical–experimental hybrid model’, i.e., by introducing experimental data obtained at a bare electrode in the theoretical model for a filmed electrode. It is also shown that a combined steady-state and EIS study cannot lead to a separate determination of the three key parameters of the system: the diffusion coefficient in the polymer film (Df), the thickness of the film (L), and the partition coefficient between the solution and the polymer (γ), only the two parameters γDf/L and L2/Df being obtained from experimental data by a curve fitting procedure. Finally, an exhaustive theoretical EIS study in quiescent solution is also proposed.