Nanophase-Segregation and Transport in Nafion 117 from Molecular Dynamics Simulations: Effect of Monomeric Sequence
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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 10-5vs 0.59 10-5cm2/s at 300 K). The experimental value (0.50 10-5cm2/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.
