Resistive losses, electroactive

Figure 10. Kleitz s reaction pathway model for solid-state gas-diffusion electrodes. Traditionally, losses in reversible work at an electrochemical interface can be described as a series of contiguous drops in electrical state along a current pathway, for example. A—E—B. However, if charge transfer at point E is limited by the availability of a neutral electroactive intermediate (in this case ad (b) sorbed oxygen at the interface), a thermodynamic (Nernstian) step in electrical state [d/j) develops, related to the displacement in concentration of that intermediate from equilibrium. In this way it is possible for irreversibilities along a current-independent pathway (in this case formation and transport of electroactive oxygen) to manifest themselves as electrical resistance. This type of chemical valve , as Kleitz calls it, may also involve a significant reservoir of intermediates that appears as a capacitance in transient measurements such as impedance. Portions of this image are adapted from ref 46. (Adapted with permission from ref 46. Copyright 1993 Rise National Laboratory, Denmark.)...

Solid electrolytes can withstand higher temperatures than liquids which is important for the Carnot efficiency of a thermo-galvanic cell. In the case of devices featuring a liquid electrolyte and a redox couple, the electroactive species diffuse from one electrode to the other. To have high steady state current, diffusion gradient should be as steep as possible which means bringing the electrodes close to each other. There results an increase in thermal loss by conduction. On the other hand, with a solid electrolyte such as 3-A 20s the electroactive species migrate in the electrolyte where it is the only possible current carrier. Consequently the current will not be limited by mass diffusion but by heat diffusion in metallic electrodes or by the electrical resistance of the solid electrolyte. 

Although the ceramic phase of the composite makes the material electroactive, many of the important properties of the material are derived primarily from the properties of the polymer. Also, the choice of polymer can determine whether the best ceramic sensitivity can be realized. The electrical properties to be considered are the resistivity p, the relative permittivity (dielectric constant) the dielectric loss, the dissipation factor D, the power factor F and the dielectric strength. The variation of these properties with changes in the likely environment should also be considered, since many of them vary with temperature, frequency and humidity. 

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