Meso-scale Model of CL Microstructure Formation

Despite reeent progress in developing multiscale modeling approaches [117], enormous challenges remain in bridging between atomistic simulations of realistic structures and continuum models that describe the operation of functional materials for PEFC applications. While full multiscale methods will not be available in the near future, meso-scale simulation techniques can close the gap between atomistic simulations and macroscopic properties of the system. Such simulations provide vital insight into self-organization phenomena and adhesion properties that have to be considered in the fabrication and operation of CL materials for PEFCs [118]. 

To improve the structure-dynamics relationships of CLs, the effects of applicable solvents, particle sizes of primary carbon powders, wetting properties of carbon materials, and composition of the catalyst layer ink should be explored. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules and, therefore, control the catalyst layer formation process. Mixing the ionomer with dispersed Pt/C catalysts in the ink suspension prior to deposition will increase the interfacial area between ionomer and Pt/C nanoparticles. The choice of a dispersion medium determines whether ionomer is to be found in the solubilized, colloidal, or precipitated forms. 

Carbonaceous particles can be coarse-grained in various ways, building upon a new technique called multi-scale coarse graining (MS-CG) [125-127]. In this method, the CG potential parameters are systematically obtained from atomistic-level interactions [127]. Using this technique, a model was built for semispherical carbon particles based on CG nonpolar sites in the C60 system. 

The interactions between non-bonded beads are modeled by the Lennard-Jones (LJ) potential. In this potential, the effective bead diameter is = 0.43 nm for 

This coarse-grained molecular dynamics model helped consolidate the main features of microstructure formation in CLs of PEFCs. These showed that the final microstructure depends on carbon particle choices and ionomer-carbon interactions. While ionomer sidechains are buried inside hydrophilic domains with a weak contact to carbon domains, the ionomer backbones are attached to the surface of carbon agglomerates. The evolving structural characteristics of the catalyst layers (CL) are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2) and water as well as the distribution of electrocatalytic activity at Pt/water interfaces. In principle, such meso-scale simulation studies allow relating of these properties to the selection of solvent, carbon (particle sizes and wettability), catalyst loading, and level of membrane hydration in the catalyst layer. There is still a lack of explicit experimental data with which these results could be compared. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

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