Catalyst layer dynamics simulations

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. 

A theoretical understanding of the diffusion of hydrocarbons through the porous catalyst layer (see Fig. 2.45) may be obtained by simulations using semi-classical molecular dynamics (as in Fig. 2.3). Such calculations have been performed for the penetration of various hydrocarbons through AljOj catalysts with and without Pt insertions (Szczygiel and Szyja, 2004). As indicated in Fig. 2.46, it is found that fuel transport depends on both cavity structure and the adsorption on internal catalyst walls. 

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. 

Coarse-grained molecular dynamics (CGMD) simulations have become a viable tool to unravel self-organization phenomena in complex materials and to analyze then-impact on physicochemical properties (Malek et al., 2007 Marrink et al., 2007 Peter and Kremer, 2009). Various MD simulations to study microstructure formation in catalyst layers will be discussed. The impact of structures obtained on pore surface wettability, water distribution, proton density distribution, and Pt effectiveness will be evaluated. 

Xiao, Y., Yuan, J. Sundn, B. Process based large scale molecular dynamic simulation of a fuel cell catalyst layer. J. Electrochem. Soc. 159 (2012), B251-B258. 

These models were developed for single PEM fuel cell taking into account interfacial kinetics at the Pt/ionomer interface, gas-transport and ionic-conductivity limitations in the catalyst layer, and gas-transport limitations in the cathode backing. These models have been improved for dynamic simulation of fuel cell stack with additional feature on anode fuel flow and cathode flow, and membrane hydration air supply models (Pukrushpan, 2003). 

Figure 8.3. Side view of the MD simulation system used in [112] at the maximum Nafion content. The graphite layer is represented by the two parallel planes shown at bottom. (Reprinted from Elecfrochimica Acta, 51.26, Lamas EJ, Balbuena PB. Molecular dynamics studies of a model polymer-catalyst-carbon interface, 5904-11, 32006, with permission from Elsevier.)...

The dynamic behavior of the individual catalyst pellets can then be simulated by solving Eq. (5) for the four different layers shown in Fig. 1. The boundary conditions for Eq. (5) are... 

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