Nanophase-Segregation and Transport in Nafion 117 from Molecular Dynamics Simulations:  Effect of Monomeric Sequence

Abstract

Nafion polyelectrolyte is widely used in polymer electrolyte membrane fuel cells (PEMFC) due to its high proton conductivity. The properties of hydrated Nafion are attributed to its nanophase-segregated structure in which hydrophilic clusters are embedded in a hydrophobic matrix. However, there has been little characterization of how the monomeric sequence of the Nafion chain affects the nanophase-segregation structure and transport in hydrated Nafion. To study such properties, we carried out molecular dynamics (MD) simulations of Nafion 117 using two extreme monomeric sequences: one very blocky and other very dispersed. Both produce a nanophase-segregated structure with hydrophilic and hydrophobic domains. However, the blocky Nafion leads to a characteristic dimension of phase-segregation that is ∼60% larger than for the dispersed system. We find that the water-polymer interface is heterogeneous, consisting of hydrophilic patches (water contacting sulfonate groups of Nafion) and hydrophobic patches (water contacting fluorocarbon group). The distribution of the hydrophilic and the hydrophobic patches at the interface (i.e., the heterogeneity of interface) is much more segregated for blocky Nafion. This leads to a water diffusion coefficient for the dispersed case that is ∼25% smaller than for the blocky case (0.46 × 10 -5 vs 0.59 × 10 -5 cm 2 /s at 300 K). The experimental value (0.50 × 10 -5 cm 2 /s) is within the calculated range. On the other hand, we find that the vehicular diffusion of hydronium is not affected significantly by the monomeric sequence. These results should be useful in optimizing the properties of Nafion and as targets for developing other membranes to replace Nafion in PEMFC and other applications.

Modelling of morphology and proton transport in PFSA membranes

“…In addition to the variation in density between the diblock and random copolymers, Jang et al 21 also observed a qualitative difference in the form of the nanophase-separated morphologies; namely that the sulfonate groups were more aggregated in the former case, and more uniformly distributed in the latter case. However, the pair correlation function of sulfur atoms in each case was similar, suggesting that it is the sulfonate groups on the same chain in the diblock polymer that aggregate, whereas in the random copolymer there are significant associations between sulfonate groups on different chains.…”

Molecular Dynamics Study of a Surfactant-Mediated Decane−Water Interface: Effect of Molecular Architecture of Alkyl Benzene Sulfonate

“…62 These force fields were extensively tested and also successfully used in our previous studies. 63,64 For the surfactant, the benzene sulfonate part was described by the explicit all-atom model using the Dreiding force field, 65 and the alkyl tail part was described by the same united atom model used for decane. The total potential energy is given as follows:…”

Effect of Pt nano-particle size on the microstructure of PEM fuel cell catalyst layers: Insights from molecular dynamics simulations

“…Classical molecular dynamics (MD) simulations have for instance been used to study the hydronium ions and water transport in the bulk membrane of PEMFCs [10][11][12][13][14][15][16]. These studies have provided important insight on the conduction mechanisms of proton through sulfonated acid groups via water clusters [10,11];o n the influence of membrane water content [10] and of morphology of nanophase in Nafion [13,14]; and on the effect of temperature [15,16]. Although atomistic models are widely used to investigate ion transport in bulk membrane, there have been relatively few studies of composite catalyst layer using this approach.…”