The Surface Adsorption of Hydride Ions and Hydrogen Atoms on Zn Studied by Electrochemical Impedance Spectroscopy with a Non-Equilibrium Thermodynamic Formulation

Abstract

We show that non-equilibrium thermodynamics theory for surfaces combined with electrochemical impedance spectroscopy can be used to derive the excess surface concentrations of reactants and products of an electrochemical reaction at an electrode. We predict the equivalent circuit for a postulated reaction using this theory, and derive expressions for the excess surface concentrations. The method is illustrated with experimental data for the following hydride reaction to hydrogen at a Zn anode in a molten eutectic mixture of LiCl and KCl at 673 K:The results support a two-step mechanism for hydrogen evolution via the hydrogen atom. We calculate the excess surface concentrations of the hydride ions and the hydrogen atoms at the metal surface, and find that the hydride ions cover a fraction of the surface while the hydrogen atoms are present in large excess. The excess surface concentration of the hydride ions varies largely with the polarized state of the surface, and so does its mean activity coe‰cient at the surface. The results contribute to a better understanding of the system in question. The method is general and is expected to give similar information for other electrodes.Brought to you by | University of Arizona Authenticated Download Date | 6/3/15 11:55 AM 232H. Nakajima et al.

Electrochemical Impedance Spectroscopy Study of the Mass Transfer in an Anode-Supported Microtubular Solid Oxide Fuel Cell

“…The resistances in the equivalent circuit can be written as the derivatives of the anode, Ohmic, and cathode overpotentials because EIS measures voltage drops in an infinitesimal interval of the current (Konomi & Saho, 2006;Nakajima et al, 2005). The resistances are non-Ohmic resistance and depend on current density (Nakajima et al, 2006). Overpotentials are hence calculated by integrating each resistance with respect to current as follows.…”

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“…The resistances are non-ohmic resistance and depend on current density (12). Overpotentials are hence calculated by integrating each resistance with respect to current as follows, provided that the current density is uniform.…”

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“…The resistances are non-Ohmic resistance and depend on current density (12). Overpotentials are hence calculated by integrating each resistance with respect to current density as follows (3).…”