Surface Exchange Coefficients

Oxygen Diffusion Coefficient (D ), Oxygen Surface Exchange Coefficient (Ar), and Oxygen Ionic Conductivity (tr,) of LaMn03-Based Oxides Measured by SIMS Technique... 

Faraday s constant (96,487 C/mol) overpotential total current current density exchange current density ratio of ohmic constriction to inter-facial resistance surface exchange coefficient volume-specific interfacial resistance in a composite thickness utilization length characteristic length of a porous microstructure... 

For this estimate, values for the surface diffusion coefficient (D) and the surface exchange coefficient (i) in eq 2 were obtained by linearizing Mitterdorfer s rate expressions for surface transport and adsorption/desorption (ref 84) and re-expressing in terms of the driving forces in eq 2. 

The reduction of oxygen to takes place at the interface of the three phase boundary. The mixed cathode which allows both electron and to travel along the surface and bulk extends the interface of the three phase boundary to an active zone for the reduction of O2 to 0 . The extension of the active zone depends on the oxygen diffusion, surface exchange coefficient, the microstructure and porosity of the cathode layer, shown in Fig. 2 (a). ... 

For predominant electronic conductors like, for example, the perovskites Lai xSrxCoi.yFey03 is determined by the ratio of the oxygen self diffusivity and surface exchange coefficient. Both parameters can be measured simultaneously by using isotopic exchange techniques. Calculations show that... 

Tracer-diffusion coefficient D, surface exchange coefficient fe and characteristic thickness for selected perovskite-type oxides... 

Fig. 6.5 Oxygen permeation flux through LSFG/5M membrane versus oxygen partial pressure on air side for different temperatures lep) and corresponding values of the surface exchange coefficient calculated with Eq. 6.3 (right)...

Table 6.1 Surface exchange coefficient k of different membranes...

In order to gain a direct measurement of the oxide ion diffusion and surface exchange coefficients in ceramic materials an isotopic exchange and SIMS analysis technique has to be used. This technique is detailed elsewhere [9] and has been successfully used for a number of SOFC component materials, most notably with electrolytes [10,11] and cathodes... 

Diffusion coefficient and surface exchange coefficient measurements have been reported for the K2MF4 type oxide materials by a number of authors [4-8, 13-19] and have been complemented by electrochemical permeation measurements [20-27] all of which demonstrate the fast oxide ion conduction of hyperstoichiometric K2NiF4 type oxides. Early reports also demonstrate the relatively poor oxide ion mobility in those materials found to be hypostoichiometric [28,29]. Initial reports of the fast oxide ion conduction in La2Ni04+s [4, 6-8] have generated a number of further studies [13-19] regarding the optimization of composition and determination of the effects of anisotropy on the conduction properties of these materials. Each of these features will be discussed in more detail below. 

Kim, S., Yang, Y.L., Jacobson, A.J. and Abeles, B. (1998) Diffusion and surface exchange coefficients in mixed ionic electronic conducting oxides from the pressure dependence of oxygen permeation. Solid State Ionics, 106 (3-4), 189-195. 

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