Discussion on Inductive Loop Observed in Complex Plane Plot of Electrochemical Impedance of Mg

Abstract

Electrochemical impedance spectroscopy (EIS) is able to discriminate several time constants involved in electrode reactions. The time constant regarding adsorption-desorption relaxation may be detected in the plot of impedance spectrum when the adsorption at liquid/solid interface contributes to the kinetics of electrode reaction. For example, the complex plane plots of the electrochemical impedance of Pt electrode, where oxygen reduction occurs, show an apparent inductive loop, indicating the presence of the reaction intermediate adsorbed on the electrode1). The inductive loop is often observed in the cases of metal dissolutions in aqueous solutions2-4), and the origin has been explained by the adsorption of adsorbed intermediate in dissolution reaction. In this presentation, the author derives the theoretical equations of the Faraday impedance involving the contribution of adsorption process during metal dissolution. A current i, which flows across liquid/solid interface, is a function of electrode potential E and surficial coverage θ of adsorbed species as follows. i = f(E, θ) (1)The Faradaic impedance Z F can be represented from the differential calculus of the equation (1), where, a 1 and a 1 are partial differential coefficients.1/Z F = di/dE = a 1 + a 2(dθ/dE) (2)The equation (2) indicates that the Faradaic impedance is a function of (dθ/dE) and describes a semicircle due to an adsorption relaxation on the complex plane. In these cases, the semicircle in the low frequency range is related to the time constant of adsorption and desorption of the reaction intermediate. The conditions that the electrochemical impedance shows the inductive loop in the low frequency range are discussed on the basis of the numerical simulations.References1) M. Itagaki, H. Hasegawa, K. Watanabe, T. Hachiya, J. Electroanal Chem 557 (2003) 59.2 M. Keddam, O. R. Mattos, H. Takenouti, J. Electrochem. Soc, 128 (1981) 257.3) M. Itagaki, M. Tagaki, K. Watanabe, Electrochim.Acta, 41 (1996) 1201.4) G. Baril, G. Galicia, C. Deslouis, N. Pebere, B. Tribollet, V. Vivier, J. Electrochem. Soc, 154 (2007) C108.