Atomistic simulation and molecular dynamics of model systems for perfluorinated ionomer membranes

Elliott, James A.; Hanna, Simon; Elliott, Alice M. S.; Cooley, Graham E.

doi:10.1039/a905267d

Cited by 118 publications

(120 citation statements)

References 21 publications

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“…To describe inter-and intramolecular interactions, we used the DREIDING force field 67 (previously used by other investigators to study hydrated Nafion system 25,47,48,68 ) but with the fluorocarbon parts described with a recently developed force field 69 and the water described using the F3C force field. 70 We used the standard combination rules of DREIDING in mixing these FF.…”

Section: Dr )mentioning

confidence: 99%

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

et al. 2004

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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.

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Section: Dr )mentioning

confidence: 99%

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

et al. 2004

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“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Cheng

Malek

Sui

2010

Electrochimica Acta

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The effect of Pt nano-particles size on the microstructure of catalyst layers in a Polymer Electrolyte Fuel Cell is investigated by means of molecular dynamics simulations. The catalyst layer model includes carbon-supported platinum, perfluorosulfonate ionomer (PFSI), hydronium ions and water molecules. Three different Pt nano-particle sizes, i.e. 1, 2 and 3 nm, are studied, and simulations provide visualization of the distinct micro-morphologies of the CL corresponding to each nano-particle size. The results are analyzed using pair correlation functions, showing that different microstructures are obtained for different Pt nano-particle sizes, and also that inclusion of PFSI in the simulations impacts significantly the final configuration of Pt nano-particles. Water molecules are found to distribute near the side chains of PFSI and surface of Pt nano-particles, but far from the graphite surface. Side chains form clusters and exhibit different dispersion toward the Pt surface. The orientations of the side chains in the vicinity of the Pt surface are analyzed in detail. The dispersion of perfluorosulfonate ionomer is found to strongly influence the merging of Pt nano-particles and, consequently, the CL microstructure formation.

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“…The first fully atomistic MD studies of ionomers were based on polyelectrolyte analogues. [46][47][48] A large body of work was devoted to MD simulation of hydrated PFSA. [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] Atomic-level MD simulations of PFSA-based polymer materials, Hyflon/Dow, Nafion and Aciplex, were reported in Refs.…”

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confidence: 99%

Molecular Dynamic Study of Water-Cluster Structure in PFSA and PFIA Ionomers

Atrazhev

Astakhova

Sultanov³

et al. 2017

J. Electrochem. Soc.

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Molecular-dynamics simulations were used to study the structure of water clusters in different perfluorinated ionomers. The ionomers included in this work were Perfluorosulfonic Acid (PFSA) and Perfluoroimide Acid (PFIA). Different shapes and sizes of water domains in PFSA and PFIA were observed. In PFSA, ionomer water domains have spherical-like shape while water domains have complicated branch structure in PFIA ionomer. The mean water-cluster size for PFSA and PFIA ionomers as a function of water content lie on one universal curve, which indicates that the size of water clusters depends primarily on water content and weakly on ionomer chemistry. In PFSA, almost all hydronium ions are located predominantly close to the water cluster/backbone interface for both large and small values of water content; while in PFIA a relatively large fraction of hydronium ions are distributed within the bulk of the water clusters. A similar distribution of acidic groups was observed in both ionomers. In PFSA ionomer, all sulfonic groups are located in the surface water layer at the distance from 3 Å to 6 Å from the backbone. In PFIA ionomer, approximately 60% of sulfonic groups are located in the surface water layer and 40% of sulfonic groups are immersed into the bulk of water cluster. Perfluorinated ion-exchange membranes are widely used in both fuel cells and flow-battery cells. Flow batteries, such as vanadium redox batteries (VRBs), are promising devices for the large-scale Electrical Energy Storage (EES) applications 1 and are receiving increasing interest due to the demand for grid-scale EES solutions and recent improvements in VRB cell performance.2 The ion-exchange membrane is a critical component in these different types of electrochemical flow cells, which conducts the charge-carrier ions (e.g., protons) while preventing the flow of electrons through it. Additionally, in a flow-battery cell, the membrane separates the positive and negative liquid electrolytes containing the active redox-species ions. For example, in a VRB cell, it is desirable to prevent the crossover of vanadium ions through the membrane since the result is a decrease in both the energy efficiency and energy capacity of the VRB system. 2,3 In VRBs, this energy-capacity loss is recoverable, since the vanadium can be simply rebalanced by pumping some of the electrolyte with the excess V to the other electrolyte tank (albeit with an additional energy loss due to self-discharging that results from mixing the two reactants). In flow-battery chemistries with dissimilar active-species ions (e.g., Fe-Cr systems), crossover can also cause contamination of the electrolytes and/or electrodes, which can result in additional adverse effects on the flow-battery system. 1 In any case, the activespecies ions are transported through the membrane in the hydrophilic phase (i.e., the water clusters) of the membrane. Therefore, the study of the water-cluster structure is required to understand and model the transport of different ions in the membrane. One of the ultimate...

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Atomistic simulation and molecular dynamics of model systems for perfluorinated ionomer membranes

Cited by 118 publications

References 21 publications

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

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

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

Molecular Dynamic Study of Water-Cluster Structure in PFSA and PFIA Ionomers

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