Oxygen surface exchange

In general, however, even at equilibrium, there may also be a double layer, called space charge, next to the surface extending into the oxide interior (Mott-Schottky layer). The width of the space charge layer will be of the order of the 

A simple yet valuable criterion for candidate membrane material selection is the characteristic membrane thickness [41 ]. In the theory, the transport equations for diffusion in the solid, and for the surface exchange are linearized. It should therefore strictly be used when small Po -gradients are imposed across the membrane. In the next section, methods for measuring are briefly discussed. 

Equation (10.19) may be simplified for predominant electronic conduction, assuming that the classical Nemst-Einstein relationship can be represented as 

In the second part of Eq. (10.22) the fact has been used that, if correlation effects can be neglected, the tracer diffusion coefficient, D, is equal to the self-diffusion coefficient, It is important to note once again that Eq. (10.18) is valid in the limit of small Po -gradients only. Since both D and kg for a given material are a function of its specific defect chemistry, in general, will be a function of process parameters Pq and temperature. 

Selected data of D and kg from isotope exchange measurements of 

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FIGURE 3.4 Measured effective surface oxygen exchange coefficient of the LSM/YSZ composites as a function of the LSM content. (From Ji, Y. et al., Solid State Ionics, 176 937-943, 2005. With permission.)... 

Fig. 10.6. Data for the surface oxygen exchange rate, normalized to air, of 25 mol% erbia-stabilized bismuth oxide (BE25) from (a) isotopic exchange and (b) oxygen permeation measurements. Reprinted...

Tabata et al. [177] and Seyama [178] both described significant differences in the chemical composition of the surface, due to Sr segregation, compared with the bulk composition in a series of powders Lai. Sr CoOs. This indicates a behaviour of the surface different from that of the bulk in these compoimds. Not only can this account for a number of observations made in the total oxidation of CO and CH4, as discussed by the authors, but it is also considered to be an important factor when one tries to correlate the composition of a perovskite with its activity in surface oxygen exchange. 

In the first place our understanding of factors that control and limit the interfacial kinetics is still rudimentary, and therefore should be a fruitful area for further investigation. The apparent correlation between the surface oxygen exchange coefficient and the tracer diffusion coefficient D for different classes of oxides, the fluorite-related and the perovskite-related oxides, as noted by Kilner et al. [73], clearly indicate the potential of isotopic exchange. 

However, a problem remains how to relate the observations (at equilibrium) from isotopic exchange to the conditions met during membrane operation. In chemical relaxation experiments, the oxide is studied after perturbation of the equilibrium state. These methods are thus complementary and probably their combined application, whenever possible together with spectroscopic techniques, such as FT-IR, UV and EPR, has a great capacity to elucidate the kinetics of surface oxygen exchange. 

B. A. Boukamp, K.J. de Vries, A.J. Burggraaf, Surface oxygen exchange in bismuth oxide based materials, in J. Nowotny, W. Weppner (Eds.), Non-Stoichiometric Compounds, Surfaces, Grain Boundaries and Structural Deffcts, Kluwer, Dordrecht, 1989, pp. 299-309. 

Upon exposure to higher p02 at SOFC temperatures, the cmiductivity decreases slowly [96]. This is observed in Fig. 3.19 as the significant decrease in conductivity for all of the samples as the /7O2 is increased above 10 " atm. The slow kinetics associated with this may be related to slow cation diffusion [101] and/or low oxygen mobility in these materials [103]. Slow reoxidation kinetics are beneficial to application in the SOFC anode where accidental and occasional exposure of the anode material to air may be expected. Of course, this assumes that a suitable and cost-effective cell synthesis procedure can be derived to initially form these highly conductive states. Furthermore, slow reoxidation may indicate slow surface oxygen exchange and low hydrocarbon oxidation activity. 

The surface exchange coefficient or oxygen transfer coefficient k, which describes the kinetics of surface oxygen exchange at the gas-solid interface. 

Boukamp, B.A., Vinke, I.C., de Vries, K.J., and Burggraaf, A.J., Surface oxygen exchange properties of bismuth oxide-based solid electrolytes and electrode materials. Solid State Ionics, 1989, 32/33, 918-23. 

Fig. 7.3 Enhancement of surface oxygen exchange rate on (La, Sr)Co03 around the deposited...

Figure 5.5 Relationship between isotope oxygen diffusion coefficient (D J and surface oxygen exchange rates Ck") (h-planeplots)forselectedperovskites[SO].

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