Macrohomogeneous catalyst layer

Main Results of Macrohomogeneous Catalyst Layer Models... 

At macroscopic level, the overall relations between structure and performance are strongly affected by the formation of liquid water. Solution of such a model that accounts for these effects provides full relations among structure, properties, and performance, which in turn allow predicting architectures of materials and operating conditions that optimize fuel cell operation. For stationary operation at the macroscopic device level, one can establish material balance equations on the basis of fundamental conservation laws. The general ingredients of a so-called "macrohomogeneous model" of catalyst layer operation include ... 

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. ... 

In this section, the reactions and general equations for the catalyst layers are presented first. Next, the models are examined starting with the interface models, then the microscopic ones, and finally the simple and embedded macrohomogeneous ones. Finally, at the end of this section, a discussion about the treatment of flooding is presented. 

The final simple macrohomogeneous porous-electrode models are the ones that are more akin to thin-film models. In these models, the same approach is taken, but instead of gas diffusion in the catalyst layer, the reactant gas dissolves in the electrolyte and moves by diffusion and reaction. The... 

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.)...

In Refs. [18,19], the macrohomogeneous theory was extended to include concepts of percolation theory. The resulting structure-based model correlates the performance of the CCL with the volumetric amounts of Pt, C, ionomer, and pores. A detailed review of macroscopic catalyst layer theory can be found in Ref. [17]. A further extension of this theory in Ref. [25] explores the key role of the CCL for the fuel cell water balance. This function is closely linked to the pore size distribution. Major principles of these models will be reproduced here. The details can be found in the literature cited. 

Simple pore models (Srinivasan et al., 1967), thin-film models (Srinivasan and Hurwitz, 1967), macrohomogeneous models, and refined variants of agglomerate models are still being applied and further developed for the present generation of ionomer-bound composite catalyst layers in PEFCs (Gloaguen and Durand, 1997 Jaouen et al., 2002 Karan, 2007 Kulikovsky, 2002a, 2010b Sun et al., 2005). Effectiveness factor approaches have been elaborated as quantitative tools to compare... 

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