Ionic conductivity basic principles

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

A more advantageous method is a.c.-impedance spectroscopy, which has become a standard method for the measurement of ionic conductivities in general. The basic principles have been described many times and the interested reader may refer to the excellent review by Gabrielli. A small applied potential difference allows measurements close to thermodynamic equilibrium. The accessibility of an extended frequency range (typically 1-10" s ) allows the separation of impedance contributions from the sample itself and from the electrode/electrolyte interface using equivalent circuits to assist the interpretation of the data obtained. Unfortunately interpretation is unambiguous only for simple circuits and the different... 

The concept of SPE dates back to 70s, when Armand firstly proposed a new ion conductor based on a lithium salt properly complexed by a polar and aprotic polymer matrix without the use of any liquid component (additives or liquid electrolytes) [65]. At the beginnings, the research on SPEs was exclusively focused on poly(ethyleneoxide) (PEO) as the complexing polymer [66]. Ever since, a lot of polymer/salt systems were deeply investigated, such as those based on PMMA, PAN, PVDF [66-69]. In principle, SPEs must satisfy some basic requirements (i) ionic conductivity higher than 10 " S/cm at room temperature, (ii) good thermal, chemical and mechanical stability, (iii) lithium transport number close to the unity, and (iv) compatibility with the electrodes and consequently wide electrochemical windows [67]. 

Let us think of ionized molecules in a fluid media, where in principle there are two possible mechanisms of conduction, i.e., ionic and electronic. Ionic conduction, in which the ionized molecules themselves drift in a given electric fleld, is favored in a less-viscous medium, basically because the ionized molecules experience friction from the medium during drift. On the other hand, electronic conduction, in which charges on the ionized molecules migrate from molecule to molecule, is favored in a viscous media, because the ionic conduction is suppressed. 

From the foregoing discussion of electric field effects In Ionic equlibria It Is clear that a solution of a weak electrolyte shows a non-linear behavior In conductance (or resistance) at high field strengths. With an Interdisciplinary look at the field of electronics we note that such nonlinearities are at the heart of all modern electronic circuits and devices. We therefore can use a solution of a weak electrolyte subjects to high electric fields as an electronic device, which Is the basic Idea of the Field Modulation Tecnnlque, the general principles we will discuss now. 

In Chapter 1 we explain the motivation and basic concepts of electrodeposition from ionic liquids. In Chapter 2 an introduction to the principles of ionic liquids synthesis is provided as background for those who may be using these materials for the first time. While most of the ionic liquids discussed in this book are available from commercial sources it is important that the reader is aware of the synthetic methods so that impurity issues are clearly understood. Nonetheless, since a comprehensive summary is beyond the scope of this book the reader is referred for more details to the second edition of Ionic Liquids in Synthesis, edited by Peter Wasserscheid and Tom Welton. Chapter 3 summarizes the physical properties of ionic liquids, and in Chapter 4 selected electrodeposition results are presented. Chapter 4 also highlights some of the troublesome aspects of ionic liquid use. One might expect that with a decomposition potential down to -3 V vs. NHE all available elements could be deposited unfortunately, the situation is not as simple as that and the deposition of tantalum is discussed as an example of the issues. In Chapters 5 to 7 the electrodeposition of alloys is reviewed, together with the deposition of semiconductors and conducting polymers. The deposition of conducting polymers... 

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