Molecular dynamics simulations were performed to investigate the relationship between the molecular structure of perfluorosulfonic acid (PFSA) ionomers and the nanoscale morphology of the hydrated membranes. Three structural features are examined including (i) the length of the side chain to which the sulfonic acid group is attached, (ii) the equivalent weight (EW) of the electrolyte ionomer, and (iii) the molecular weight (MW) of the polymer electrolyte. Membrane morphologies are studied from the water content λ = 3 (λ represents number of water molecules per sulfonate group) to saturation (λ = 22). We find that with the longer side chain, there is more clustering of the sulfonate groups and more local water−water clustering, but a more poorly connected aqueous domain. When one decreases the equivalent weight (EW) in either the short side chain (SSC) PFSA or Nafion, there is more clustering of the sulfonate groups and more local water−water clustering and a better connected aqueous domain. Because connectivity enhances and confinement reduces water mobility, a decrease in EW, which enhances connectivity and reduces confinement, results in an increase in diffusivity. An increase in side chain length diminishes connectivity but reduces confinement, which together result in little change in the observed water diffusivity. For the short chains studied, we find these results to be independent of MW.

The nanoscale structure of the ionomer film located in the catalyst layer of polymer exchange membrane fuel cells (PEMFCs) is of vital importance to proton transport and catalyst utilization. Classical molecular dynamic simulations are conducted to explore the molecular-level structure as well as the structure−property relationships in the ionomer film. Twenty-four systems are simulated to investigate the effect of (i) hydration, (ii) ionomer film thickness, (iii) oxidation of the carbon support surface, and (iv) the presence of catalyst nanoparticles on film adhesion and morphology. The ionomer does not form a continuous film on the carbon surface; rather, the ionomer forms irregular patches through which proton transport from the catalyst to the membrane must occur. These ionomer films are not able to retain water to the same extent as bulk ionomer membranes. However, thicker films retain proportionally more water than thinner films, allowing for a larger and better connected aqueous domain required for proton transport. Oxidation of the carbon support surface through either epoxidation or hydroxylation strongly impacts the water distribution throughout the film and thus the film adhesion. Hydroxylation enhances adhesion of the film relative to a pristine surface. Epoxidation can result in partial delamination of the film, an effect that is more pronounced for thinner films. The presence of Pt or PtO nanoparticles impacts the distribution of water and the ionomer. An aqueous layer forms around the nanoparticles and provides pathways for protons into the film. These insights provide a molecular-level basis for the experimental observations such as the inhomogeneous distribution of the Nafion film on the carbon support, the existence of an optimal content of recast ionomer in the catalyst layer, and the impact of surface oxidation on the restructuring of polymer chains and thus on PEMFC performance. This work also implies that oxidation during operation can result in ionomer film delamination, which reduces the binding energy of the catalysts, a possible precursor to catalyst detachment.

Atomistic and coarse-grained (CG) molecular dynamics (MD) simulations were conducted for a cross-linked and sulfonated poly­(1,3-cyclohexadiene) (xsPCHD) hydrated membrane with λ­(H2O/HSO3) = 10 and 20. From the atomistic level simulation results, nonbonded pair correlation functions (PCFs) of water–water, water–H3O+ ion, H3O+–H3O+, polymer–water, and polymer–H3O+ ion pairs were obtained and studied. The water self-diffusivity and H3O+ vehicular self-diffusivity were also obtained. Membrane structure was further studied at CG level using a multiscale modeling procedure. Nonbonded PCFs of polymer–polymer pairs were obtained from atomistic simulation of hydrated membrane with λ = 10 and 20. Two sets of CG nonbonded potentials were then parametrized to the PCFs using the iterative Boltzmann inversion (IBI) method. The CGMD simulations of xsPCHD chains using potentials from above method satisfactorily reproduced the polymer–polymer PCFs from atomistic MD simulation of hydrated membrane system at each hydration level. The transferability of above two set of CG potentials was further tested through CGMD simulation of hydrated membrane at an intermediate hydration level (λ = 15). Limited transferability was observed, presumably due to the use of an implicit solvent. Using an analytical theory, proton conductivity was calculated and compared with that from experimental measurement under similar conditions. Good agreement was obtained using inputs from both atomistic and CG simulation. This study provides a molecular level understanding of relationship between membrane structure and water and H3O+ ion transport in the xsPCHD membrane.

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