Ionic transference number and

When there is the same electrolyte MA with different concentrations on the two sides of the interface, Eu (a) may be described by a relatively simple equation [41-43] and is dependent on ionic transference numbers and the electrolyte concentration in both solvents. Obviously, if the activity of the electrolyte in both solvents is identical, then (a) drops to zero. 

Since the electronic conductivity of nanocrystalline ScSZ becomes significant in reducing atmosphere (Fig.3), its contribution to the electrical transport should be considered. This can be discussed based on the dependence of the ionic transference number, t,- = cr,- / (oxygen activity. Such information is important for the development of ScSZ solid electrolyte for Solid Oxide Fuel Cells. Figure 4 presents the relationship between the ionic transference number and oxygen activity, which has been determined based on the presented conductivity measurements and the defect model [13]. 

Complete characterization of an electrolyte requires various parameters, especially when charge transport is described or modeled. An electrolyte with n species needs n(n— )l2 concentration-dependent transport properties, where n is the number of independent species [1], In hterature, conductivity of electrolytes receives the majority of attention because it is easy to measure. But for a fuU characterization of the electrolyte, other properties such as ionic transference numbers and ion mobUities, structure formation by ion association, and solvation, as well as bulk properties such as viscosity, are required as well [2-7]. 

Holz, M Lucas, O Muller, C, NMR in the Presence of an Electric Current, Simultaneous Measurements of Ionic Mobilities, Transference Numbers, and Self-Diffusion Coefficients Using an NMR Pulsed-Gradient Experiment, Journal of Magnetic Resonance 58, 294, 1984. Hooper, HH Baker, JP Blanch, HW Prausnitz, JM, Swelling Equilibria for Positively Ionized Polyacrylamide Hydrogels, Macromolecules 23, 1096, 1990. 

Hence the decrease of AgN03 concentration within the catholyte is exactly equal to its increase within the anolyte, which for this symmetrical type of cell is to be expected therefore, only one of the two needs to be measured in order to determine the transference numbers. From the transference numbers and the limiting equivalent conductivity A0, one obtains the equivalent ionic conductivities Aq = tg A0 and Aq = tg A0. 

The chemical diffusion coefficient may be expressed by the diffusivity of the mobile species i, the transference number and the variation of the activity of the mobile component as a function of its concentration according to Eqns (8.26) and (8.27), if the transference numbers of other ionic species are negligibly small. 

From Eqns. (8.56) and (8.57), we can obtain the electrical conductivity of the crystal and also the ionic transference number. 

Conductivity The materials must have an ionic transference number close to unity i.e., the electronic conductivity in the electrolyte must be sufficiently low in order to minimize internal shorting and provide high energy conversion efficiency. The electrolyte materials should also possess high oxygen ion conductivity to minimize the ohmic losses in the cell. 

Type of Boundary and Liquid Junction Potential.—When the two solutions forming the junction contain different electrolytes, the structure of the boundary, and hence the concentrations of the ions at different points, will depend on the method used for bringing the solutions together. It is evident that the transference number of each ionic species, and to some extent its activity, will be greatly dependent on the nature of the boundary hence the liquid junction potential may vary with the type of junction employed. If the electrolyte is the same in both solutions, however, the potential should be independent of the manner in which the junction is formed. In these circumstances the solution at any point in the boundary layer will consist of only one electrolyte at a definite concentration hence each ionic species should have a definite transference number and activity. When carrying out the integration... 

Fig. 10.7. (a) Defect and (b) conductivity diagram for ceria-doped YSZ at 1000°C. The relevant parameters to construct the diagrams are given in Ref. [119]. The theoretical dependence of ionic transference number tionand oxygai permeability /02 are given in (c). Dashed lines in (c) refer to YSZ. Fm (YzrO represents the aliovalent dopant used. Reproduced (slightly adapted) from Marques et al. 

The electrolyte dissociation from the lower partial pressure side does not initially result in the decrease of em/for cell (1.20) because the average ionic transference number plays the crucial role and can be given as follows [26] ... 

As long as we have an equilibrium between electrons on the local levels and electrons in the conductivity zone, that is, F = the ionic transference number (t ) can be described by the foUowing equation ... 

These reactions determine the apparent potential of the zirconia-based sensor. Since the raw exhaust gas at automotive and combustion applications constitutes a nonequilibrium gas mixture, thermodynamic equilibrium has to be achieved at the SE surface of the zirconia-based sensor before monitoring the potential. Consequently, such sensors contain catalytically active materials and are operated at temperatures above 600°C, when the average ionic transference number % > 0.99. For less active materials or temperatures below 600°C the apparent em/starts to deviate significantly from the value under equilibrium conditions due to insufficient catalytic activity. Earlier research of the YSZ-based sensors was focused on the electrode materials with high exchange currents and high catalytic activity for the desired electrode reactions. Pt electrodes were found to be the most suitable for this type of application. 

FIGURE 4.7 Ionic transference number as a function of oxygen concentration for the sensor based on a zirconia single crystal at400°C. (From Zhuiykov, S., Zirconia single crystal analyser for low-temperature measurements, Proc. Control and Quality 11 (1998) 23-37. With permission.)... 

Let s consider 4, within the above-mentioned ranges of temperatures and pressures. The true ionic transference number based on [10] can be expressed according to the following equation ... 

As has been shown above, the average ionic transference number t at the above-mentioned ranges of oxygen pressures and temperatures is equal to 1, and conse-qnently, this error, stipnlated by the nonionic component of condnctivity (8Pi92(I))e2. can also be ignored. 

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