Interfacial exchange flux

In many experimental situations, a steady-state nonequilibrium condition between two phases (A and B) is sustained by electrical work being done on the system. For instance, the net interfacial electron flux Ja b is measured as a function of the difference in the electrochemical potential p Me,B- F " sufficiently small departure from equilibrium, it is observed that the net flux is proportional to the exchange flux and increases with increasing p A Me,B -... 

It caused HPi to be recombined and H consumed, so that H is no longer available to pass through the interface t hus preventing the interfacial exchange reaction. The flux of C as C. Cl becomes predominant and the interfacial tension decreases wnile the interfacial convection occurs again as in the beginning The development of eddies produces a hydrodynamical agitation which would restore the bulk concentration of all species on... 

This condition has been recently used in a vaporization-exchange model for water sorption and flux in phase-separated ionomer membranes. The model allows determining interfacial water exchange rates and water permeabilities from measurements involving membranes in contact with flowing gases. It affords a definition of an effective resistance to water flux through the membrane that is proportional to... 

The factors 4 and 4 accormt for the heterogeneity of the interface. The interfacial flux conditions. Equations (6.56) and (6.57), can be straightforwardly applied at plain interfaces of the PEM with adjacent homogeneous phases of water (either vapor or liquid). However, in PEFCs with ionomer-impregnated catalyst layers, the ionomer interfaces with vapor and liquid water are randomly dispersed inside the porous composite media. This leads to a highly distributed heterogeneous interface. An attempt to incorporate vaporization exchange into models of catalyst layer operation has been made and will be described in Section 6.9.4. 

They are necessary to estimate either the local nonconvective fluxes caused by conduction of heat or diffusion of moisture or the interfacial fluxes exchanged either between two phases or through system boundaries (e.g., heat losses through a wall). The first are usually expressed as... 

Fbr a single cation-exchange extraction, such as reaction (8.2-3), concentratkm profiles near the in-terfece are rqitesemed by Eifi- 8.4-1. The fluxes at the interface of all four species are related by the reaction stokhiometry. The interfacial concentrations depend on the bulk-solution concentrations, die in-terfacial metal flux, and the respective mass transfer resistances. Requiring the interfaciai concentrations to be in equilibrium accordii to Eq. (8.3-1) yields the follovring equation for the metal-extraction rate ... 

The correlation between the behaviour of U, U and v, t-curves found in Ref. [52] may be the basis for developing quantitative methods controlHng the hydrodynamic flux structure near the electrode surfaces, as well as methods for influencing the velocity of the heterogeneous processes by means of controlled changing of the hydrodynamic conditions of interfacial mass exchange. 

In the past, it has been a common albeit dubious practice to adopt an equihb-rium sorption isotherm for the relation between Xm and This approach demands an infinite rate constant of vaporization exchange. It is problematic for two reasons. First, the relative importance of interfacial water exchange grows with decreasing membrane thickness. Below a critical thickness, interfacial kinetics, rather than bulk transport, will limit the net water flux, implying an out-of-equilibrium condition. Second, if gases adjacent to the membrane are moving, water may be convected away from its surfaces. It is inherently contradictory to assume equilibrium in the presence of any kinetic or convective process. 

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