Effective Catalyst Layer Properties from Percolation Theory

In this approach, discussed in the section Effective Catalyst Layer Properties from Percolation Theory, Tstat corresponds to the statistical fraction of Pt particles at or near the triple-phase boundary of solid carbon/Pt phase (volume fraction Xptc), ionomer phase (A /), and pore space (Xp = 1 — Xptc — Xei). 

Simulations of physical properties of realistic Pt/support nanoparticle systems can provide interaction parameters that are used by molecular-level simulations of self-organization in CL inks. Coarse-grained MD studies presented in the section Mesoscale Model of Self-Organization in Catalyst Layer Inks provide vital insights on structure formation. Information on agglomerate formation, pore space morphology, ionomer structure and distribution, and wettability of pores serves as input for parameterizations of structure-dependent physical properties, discussed in the section Effective Catalyst Layer Properties From Percolation Theory. CGMD studies can be applied to study the impact of modifications in chemical properties of materials and ink composition on physical properties and stability of CLs. 

Figure 4.6b compares calculated plots of Eceii versus jo with experimental data of Uchida et al. (1995a,b) for CCL with different ionomer content, as specified in the legend. Composition-dependent properties were parameterized using the functions proposed in the section Effective Catalyst Layer Properties from Percolation Theory in Chapter 3. The fuel cell voltage was assumed to be of the form... 

As discussed in the section Ionomer Structure in Catalyst Layers Redefined in Chapter 3, a theory of composition-dependent effective properties that incorporates recent insights into stmcture formation in CCLs is yet to be developed. At present, the relations presented in the section Effective Catalyst Layer Properties from Percolation Theory in Chapter 3 do not account for agglomerate formation and skin-type morphology of the ionomer film at the agglomerate surface. Qualitative trends predicted by the simple structure-based catalyst layer theory should be correct, as confirmed by the results discussed in this section. 

At the mesoscopic scale, interactions between molecular components in membranes and catalyst layers control the self-organization into nanophase-segregated media, structural correlations, and adhesion properties of phase domains. Such complex processes can be studied by various theoretical tools and simulation techniques (e.g., by coarse-grained molecular dynamics simulations). Complex morphologies of the emerging media can be related to effective physicochemical properties that characterize transport and reaction at the macroscopic scale, using concepts from the theory of random heterogeneous media and percolation theory. 

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