Macrohomogenous concept

Studied electrode geometry, the essentials of the kinetic mechanisms are not totally settled. Each of the key steps (adsorption, surface mobility, charge transfer, and desorption) still constitutes a huge scientific problem involving the application of the macrohomogeneous concept. 

Therefore, the macrohomogeneous concept can also be adequately extended to the whole cell. For instance, a framework for macrohomogeneous modeling of porous SOFC electrodes is possible by taking into account multicomponent diffusion, multiple electrochemical and chemical reactions, and electronic and ionic conduction. The concept applies to both porous anodes and cathodes. The derivation of the model is illustrated by considering different chemical and electrochemical reaction schemes. The framework is general enough so that additional chemical and electrochemical reactions can be accounted for. 

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. 

As the macrohomogeneous electrode theory has proven its worth in electrode diagnostics and design, so the finer details of electroactive layer structure and elec-trocatalytic mechanisms are moving to the fore. A useful concept is to consider agglomerates as structural units of the electroactive layer. Ideal locations of electroactive particles are at the true two- or even three-phase boundary. This approach is capable of and vital for showing that micropores inside agglomerates are filled with liquid water to keep the particles active. Even for well defined and extensively... 

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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