Bulk reduction path

Role of the bulk transport path. In section 3 we saw that for Pt the dissociation of oxygen and transport of reactive intermediates to the electrode/ electrolyte interface is confined to the material surface. With mixed conductors, it is possible for oxygen reduced at the surface to be transported through the bulk of the material to the electrode/ electrolyte interface. If bulk transport is facile, this path may dominate, extending both the accessible surface for O2 reduction as well as broadening the active charge-transfer area from the TPB to include the entire solid—solid contact area. 

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

Fig. 19.4 The mechanism for electrochemical C02 reduction catalyzed by CoPc confined in a poly(4-vinylpyridine) membrane. Possible paths (I and II) are shown. An equilibrium with respect to protons between the polymer matrix and the bulk phase exists.

SOI and strained silicon transistors are comparable to or smaller than the phonon s mean free path (which, for silicon, has been estimated as 300 nm at 300K) [53], In this limit, the film surfaces alter the phonon dispersion relations [76], and the phonon-surface scattering may become the predominant scattering mechanism [3, 53], Since phonons are the main carriers of thermal energy in silicon, these effects alter the thermal conductivity, which differs from that of bulk silicon [10, 36, 77], Measurements of the thermal conductivities of silicon films of thicknesses down to 74 nm found a reduction of 50% with respect to the bulk value at 300K [53], This reduction depends on the temperature and the thickness of the film [3, 53],... 

In the past an attempt was made to describe the reduction process in terms of temperature, time, diffusion-path lengths, and oxide bulk density, based on theoretical diffusion equations [3.16], but such models are not used in practice due to the complexity of parameter interactions. Industrial production today is still based on extensive empirical knowledge, supported by in-depth understanding of the basic aspects governing the reduction process, aceompanied by a feel for the matter. 

The simplest reactions involve only mass transfer of a reactant to the electrode, heterogeneous electron transfer involving nonadsorbed species, and mass transfer of the product to the bulk solution. A representative reaction of this sort is the reduction of the aromatic hydrocarbon 9,10-diphenylanthracene (DPA) to the radical anion (DPAt) in an aprotic solvent (e.g., 7V,A-dimethylformamide). More complex reaction sequences involving a series of electron transfers and protonations, branching mechanisms, parallel paths, or modifications of the electrode surface are quite common. When a steady-state current is obtained, the rates of all reaction steps in a series are the same. The magnitude of this current is often limited by the inherent sluggishness of one or more reactions called ratedetermining steps. The more facile reactions are held back from their maximum rates by... 

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