Characterization of ion transport in thin films using electrochemical impedance spectroscopy: I. Principles and theory

Abstract

The paper presents the principle and theoretical basis for application of electrochemical impedance spectroscopy (EIS) to study ion transport and partitioning in thin films or membranes supported by a solid electrode and exposed to an electrolyte solution. It is shown that the equivalent circuit is in general composed of two parallel branches: one corresponds to transient charge transfer processes, in which all ions (both electrochemically active and inactive) participate, and the other to the diffusion and reaction of electroactive ions only. Among the experimentally accessible parameters three appear to be of particular relevance to ion transport: (a) the high-frequency resistance of the film directly related to the sum of permeabilities of all mobile ions in the membrane including counter-ions bound to the fixed charges, (b) the diffusion impedance of the electroactive ion that is capable of separately retrieving the values of diffusion and partitioning coefficient of the specific ion and (c) dielectric capacitance of the film, which may yield the effective thickness of the film, particularly interesting if the membrane is not homogeneous. Such information may be highly relevant to analysis of ion exclusion mechanisms in the film and provide inputs to computational models of ion transport in membranes. Experimental examples involving thin polyamide films are provided to partly illustrate the use of equivalent circuit, data analysis and possible artifacts.