Macrohomogeneous model/theory

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. ... 

Effectiveness Factor of Single Agglomerates Macrohomogeneous electrode theory, described so far, has been successfully explored in fuel cell diagnostics and optimization [17, 122-126], Nowadays, finer details of structure and electrocatalytic mechanisms in CLs and model nanoparticle electrocatalysts are moving into focus [127]. 

The simplification Inherent in the 1-D macrohomogeneous model is that of the microstructure. For the model to be useful In optimising electrode microstructure, the parameter must be related to microstructural characteristics such as pore size and porosity. There are various techniques available from percolation theory to accomplish this and relate Am nd other model parameters to empirical pore-size distribution and total pore volume. 

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 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. 

Combination of the macrohomogeneous approach for porous electrodes with a statistical description of effective properties of random composite media rests upon concepts of percolation theory (Broadbent and Hammersley, 1957 Isichenko, 1992 Stauffer and Aharony, 1994). Involving these concepts significantly enhanced capabilities of CL models in view of a systematic optimization of thickness, composition, and porous structure (Eikerling and Komyshev, 1998 Eikerling et al., 2004). The resulting stmcture-based model correlates the performance of the CCL with volumetric amounts of Pt, C, ionomer, and pores. The basis for the percolation approach is that a catalyst particle can take part in reaction only if it is connected simultaneously to percolating clusters of carbon/Pt, electrolyte phase, and pore space. Initially, the electrolyte phase was assumed to consist of ionomer only. However, in order to properly describe local reaction conditions and reaction rate distributions, it is necessary to account for water-filled pores and ionomer-phase domains as media for proton transport. 

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