The paper is concerned with the small signal ac impedance of porous film electrodes in contact with solution. An overview is presented of the standard transmission line model with two transport channels and a crosswise element. The simplest configurations are discussed: a single resistance in one of the channels, and either an interfacial capacitor or a RC transfer circuit at the pore's wall. The resulting relaxation functions are classified in terms of two characteristic frequencies: one for coupled transport and interfacial polarization and another one for the interfacial reaction. Subsequently, these models are extended in order to describe porous electrodes where the interfacial polarization displays complex properties, i.e., frequency dispersion. The capacitive element is described by a constant-phase element (CPE), and it is shown that the fractionary exponent provides an additional and measurable degree of freedom in the parameter space of the relaxation function, whose determination can be exploited as a supplementary tool for analysis. The analysis of impedance measurements of TiO 2 nanoporous photoelectrodes in negative bias voltage shows that the suggested approach is capable of identifying two characteristic relaxation frequencies in a frequency-resolved measurement on this system.

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Electrochemical impedance spectroscopy was used in the study of lithium insertion in a WO3 film. Spectra obtained at different potentials showed the characteristic shape of a diffusion process, with a Warburg regime at high frequencies and a capacitive branch at low frequencies arising from the charge storage associated with the intercalation of Li+. But the low frequency wing showed, as in many other reports, a pronounced dispersion which could not be accounted for solely by the diffusion capacitance. The possibility of an additional kinetic process was investigated. A number of models accounting for such effect were considered, including the constant phase element restricted diffusion model that differentiates diffusion from blocking effects and another model with fast and slow charging processes. The evolution of the impedance with charge-discharge cycles was considered as well. It was found that a suitable interpretation of the experimental results is provided by a new model that considers the coupling of diffusion and the solid-state reaction whereby inserted ions are incorporated in fixed sites of the lattice. This model could be further elaborated on the basis of a diffusion and multiple trapping scheme, which provides an ordinary or Warburg diffusion region in the range of highest frequencies, and additional components at lower frequencies which reflect the kinetics of the ion immobilizing process, and which in some cases show an anomalous diffusion pattern. This explanation was found consistent with the results of complementary techniques of measurement such as the electro-optical transfer function. © 2001 The Electrochemical Society. All rights reserved.

A number of approaches to the determination of water content of soil use a dielectric permittivity measurement, which is accurate but involves expensive and complex devices. We explore the use of indium tin oxide, a wide-band gap semiconductor with high conductivity, long-term stability, and low cost, to build a cheap sensor of low humidity values for automatic irrigation systems. The electrical resistance determined by an ac impedance method discounts the effects of electrode contacts and provides a reliable measure of the water content, even when the chemical conditions of water vary widely. The reason for this is that, at low values of humidity, the conductivity is controlled by a surface conduction mechanism that is largely insensitive to the electrolytic properties of added water. The results appear promising for applications that require the detection of very low levels of humidity in soil.