Characteristic current density proton transport

In the nonideal case, one has to evaluate the interplay of three effective electrode parameters, viz. volumetric exchange current density P, proton conductivity ap, and oxygen diffusivity D. Instead of considering these parameters, it is more insightful to evaluate and compare three corresponding characteristic current densities. These include the current conversion capability f = PIcl, defined above, the characteristic current density due to proton transport... 

Fig. 13 Dimensionless plot of polarization characteristics of the catalyst layer and profiles of current density and pressure along the thickness coordinate Here, these properties are depicted in the mixed regime, when proton and oxygen transport limitations are of equal importance, / = a b(g = 1). (a) The potential drop rj0 and particular contributions according to Eq. (81)...

Equations 47-49 describe variations of parameters along the y coordinate of the catalyst layer (y = z/lc 1), where z is the catalyst layer thickness coordinate, y = 0 specifies the catalyst layer/gas interface, and y = 1 specifies the catalyst layer/ionomeric membrane interface (see Fig. 44), in which Rc (= /Ci/cr) is the protonic resistance through a unit cross-sectional area of the catalyst layer and 7d (=nFDC /lc ) is a characteristic diffusion-controlled current density in the catalyst layer. The thickness of the catalyst layer disappears from the equations by introducing Rc, ax, and I ). The experimental variables considered include the overpotential 1], the current density /, and the oxygen concentration C, when pox = 1 atm at the catalyst layer/gas interface. The O2 partial pressure, pox, at the catalyst layer/backing layer interface is determined, in turn, by the cathode inlet gas stream composition and stoichiometric flow rate and by the backing layer (GDL) transport characteristics. 

The existence of a maximum thickness beyond which the performance degrades is due to the concerted impact of oxygen and proton transport limitations. Considered separately, each of these limitations would only serve to define a minimum thickness, below which performance worsens due to insufficient electroactive surface. The characteristic thickness of the effective layer, in which the major fraction of the current density is generated, is... 

The main characteristic to consider for a PEM to be used in potential fuel cell is proton conductivity. To achieve good performance of a PEM fuel cell, high proton conductivity is essential, especially at a high current density. To understand proton transport at a molecular level in hydrated polymeric membranes, there are two principal proton transport mechanisms (1) the Grotthus mechanism or proton hopping mechanism, and (2) the vehicular mechanism or diffusion mechanism [243-245]. 

Verbrugge and Hill (1990) presented the theoretical representation and experimental data to characterize the proton and water in perfluorosulfonic acid membrane. The analysis describes the transport of water molecules carried along with the transport of proton across the membrane. Springer et al. (1991) presented a one-dimensional, isothermal model of FEMFC in which detailed considerations of the Nafion-117 membrane characteristics in terms of water content and water transport properties including water drag coefficient and diffusion coefficient are given. Their results demonstrated increased membrane resistance with current density. 

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