Surface path

One limit of behavior considered in the models cited above is an entirely bulk path consisting of steps a—c—e in Figure 4. This asymptote corresponds to a situation where bulk oxygen absorption and solid-state diffusion is so facile that the bulk path dominates the overall electrode performance even when the surface path (b—d—f) is available due to existence of a TPB. Most of these models focus on steady-state behavior at moderate to high driving forces however, one exception is a model by Adler et al. which examines the consequences of the bulk-path assumption for the impedance and chemical capacitance of mixed-conducting electrodes. Because capacitance is such a strong measure of bulk involvement (see above), the results of this model are of particular interest to the present discussion. 

Inconsistency of performance with a bulk path at low vacancy concentration. A quantitative comparison between predictions of the Adler model and impedance data for LSC shows the poorest agreement (underprediction of performance) at low temperatures, high F02. and/or low Sr content. These are the conditions under which the bulk vacancy concentration (and thus also the ionic conductivity and surface exchange rate of oxygen with the bulk) are the lowest. These are exactly the conditions under which we would expect a parallel surface path (if it existed) to manifest itself, raising performance above that predicted for the bulk path alone. Indeed, as discussed more fully in section 5, the Adler model breaks down completely for LSM (a poor ionic conductor at open-circuit conditions), predicting an... 

Since the mid-1990 s, several models which consider both a bulk and surface path have been proposed, including the Svensson models as well as a more recent model by Coffey and co-workers. Flowever, these models have not been definitive in addressing the bulk vs surface question for at least... 

The results of this analysis are summarized in Figure 37. Like prior workers studying thin films, the authors conclude that dense films without a TPB under small or cathodic polarizations operate primarily by a bulk path since the surface path is blocked. (Interestingly, they found that dense films under anodic polarization appear to operate under a mixed regime, although it is not clear how much nucleation and transport of O2 along the solid—solid interface contributes to the apparent surface path current.) In contrast, as the porosity is increased (microelectrode diameter is decreased), the surface path be-... 

Figure 37. Qualitative summary of current contributions from the surface path (Is) and the bulk path (7b) for LSM disk microelectrodes on YSZ based on i— measurements in air at 800 °C. (Reprinted with permission from ref 228. Copyright 2002 Elsevier.)...

Surface path at low overpotential. Qualitative and quantitative analysis of impedance data, tracer studies, as well as various studies of thin-film electrodes suggest that under low-overpotential LSM operates primarily via a surface-mediated mechanism (like Pt). This conclusion appears to be consistent with the properties of LSM, which is fully oxygen stoichiometric under ambient Pq. However, little is known about how far the active region of reduction extends beyond the solid/solid interface (via surface diffusion) or the relative importance of chemical steps (on the LSM surface) vs electrochemical kinetics at the solid/solid interface. 

As a general rule, the rates of those reactions that are endoergic for ground-state reactions are dramatically enhanced when excited-state reactants are employed. Electronic excitation and vibrational excitation may or may not have similar effects. In the case of vibrational excitation, the reactions may occur on a single electronic potential surface. In the case of electronic excitation, a single-surface path between electronically excited reactants and ground-state products may not exist because of the prevailing symmetries. 

A third path, namely the ionization of the oxygen on the electrolyte surface followed by a direct incorporation into the electrolyte, can also not be excluded. In this case the electronic charge carriers, which are required in the oxygen reduction reaction, have to be supplied from the electrolyte. In solid electrolytes with very low electronic conductivity (e.g. zirconia), it can therefore be expected that the active zone is restricted to a region very close to the three-phase boundary. Hence, this path is, from a geometrical point of view, similar to the surface path discussed above. 

In the particular case considered here, it can therefore be deduced that in the anodic regime the surface path determines the overall current, since all steps of the... 

There should be no warm surface path connected to the cold area. The trap should maintain a low coolant consumption rate and adequate reservoir capacity. 

FIGURE 3.2.6 Sketch of possible reduction paths at fuel cell cathodes (a) surface path (b) bulk path. 

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