Molecular Dynamics Simulation of Swollen Membrane of Perfluorinated Ionomer

Abstract

Molecular dynamics simulations of the swollen membrane of perfluorinated ionomer, which is composed of poly(tetrafluoroethylene) backbones and perfluosulfonic pendant side chains, have been undertaken to analyze the static and dynamic properties of the water and the side chain in the membrane. The calculations were carried out for four different water contents, 5, 10, 20 and 40 wt %, at 358.15 K and 0.1 MPa. The results are summarized as follows: (1) The sulfonic acid is the unique site to which water molecules can bind, and the other sites in the pendant side chain have no bound water even at high water concentration. (2) Sulfonic acids aggregate in the short range within 4.6-7.7 A despite the electrostatic repulsion between them. In such aggregates, a water molecule bridges two sulfonic acids. (3) Pendant side chains prefer to orient perpendicular to the hydrophilic/hydrophobic interface, and long-range correlation of side chain orientations is observed at 20 and 40 wt % water uptake membranes. (4) In a low water uptake membrane, the dynamics of water is substantially restricted due to strong attractive interactions with acidic sites. In contrast, at high water content, even the water locating near the sulfonic acid is relatively mobile. The short residence time of the bound water reveals that such water can frequently exchange position with relatively free water, which locates in the center of water cluster, in highly swollen membranes.

Modelling of morphology and proton transport in PFSA membranes

“…In a more recent study, Urata et al 73 used an explicit allatom description of a fully ionized sulfonic acid side chain combined with a UA model of the Nafion backbone, with torsional potentials and partial atomic charges calculated using hybrid DFT and MO theory. NPT dynamics simulations were run on systems at 358.15 K and 0.1 MPa with four different water contents: 5, 10, 20 and 40 wt%, using hydronium as the neutralising counterion.…”

“…We will therefore briefly review some of the recent mesoscale modelling work on PFSA polymer membranes. Between the first ''polymeric'' MD simulations by Vishnyakov and Neimark, 35,67,68 and those of Urata et al 73,76 Khalatur, Khoklov and co-workers published two mesoscale modelling studies of PFSA polymers based on Nafion but using rather different approaches to study polymer morphology. The first was based on a hybrid Monte Carlo/reference interaction site model (MC/RISM), 77 which used a combination of an MC method based on rotational isomeric state (RIS) theory, developed originally by Flory 78 to predict conformations of isolated polymer chains, and semi-emperical quantum mechanical calculations of short-range interactions, using the AM1 Hamiltonian, to produce single chain conformations that were then used to calculate intramolecular distribution functions.…”

Molecular Dynamics Simulations of Proton Transport in Proton Exchange Membranes Based on Acid-Base Complexes

“…The rigid body models are applied to all the solvent molecules including water, hydronium, pyrrole, pyrazole, and imidazole. The TIP3P model is applied to water molecule (Jorgensen, Chandrasekhar et al, 1983), and the all-atom rigid body force field model is applied to hydronium cation (Urata, Irisawa et al 2005). The molecular structure of the heterocyclic compounds are optimized by density functional theory at the B3LYP/6-31G(d) level of theory.…”

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

“…Computer simulations, especially molecular level techniques, have consequently become a powerful tool complementing experiments to probe the fundamentals of CL formation. 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].…”