Macrohomogeneous model current density

The macrohomogeneous model was exploited in optimization studies of the catalyst layer composition. The theory of composifion-dependent performance reproduces experimental findings very well. - The value of the mass fraction of ionomer that gives the highest voltage efficiency for a CCL with uniform composition depends on the current density range. At intermediate current densities, 0.5 A cm < jo < 1.2 A cm , the best performance is obtained with 35 wt%. The effect of fhe Nation weight fraction on performance predicted by the model is consistent with the experimental trends observed by Passalacqua et al. ... 

Figure 13. Plot of cathode potential as a function of current density for a macrohomogeneous embedded model where the proton conductivity is assumed to be uniform (0.044 S/m), curve a, or varies with water production (changing humidity) across the catalyst layer, curve b. (Reproduced with permission from ref 98. Copyright 2002 The Electrochemical Society, Inc.)...

Figure 26 exhibits the polarization curves in terms of the cathode overpotential variation with current density for the CL27 obtained from the 3-D, single-phase DNS model prediction,25,27 the experimental observation25,27 and the liquid water transport corrected 1-D macrohomogeneous model.27 The polarization curve refers to the cathode overpotential vs. current density curve in the... 

A macrohomogeneous electrode can be established in different dimensional structures and the resulting models, which can present analytical or numerical solutions, could relate the global performance of the cathodic or anodic layer to unmeasurable local distributions of reactants, electrode potential, and reaction rates. These unmeasurable local distributions define a penetration depth of the active zone and suggest an optimum range of current density and electroactive layer thickness with minimal performance losses and highest electroactive effectiveness. In addition, the macrohomogeneous theory can be extended to include concepts of percolation theory. 

In the past, studies of the macrohomogeneous model have explored the effeets of thickness and composition on performance and catalyst utilization. At the outset, it should be noted that these works neglected the effects of liquid water accumulation in pores on performance. The specific effects due to the complex coupling between porous morphology, liquid water formation, oxygen transport, and reaction rate distributions will be discussed in Section 8.5.5. The results presented in this section are only valid at sufficiently small current densities, for which liquid water accumulation in secondary pores is not critical. 

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