Ion transport and clustering in nafion perfluorinated membranes

Hsu, Wei‐Chung; Gierke, T. D.

doi:10.1016/s0376-7388(00)81563-x

Cited by 903 publications

(737 citation statements)

References 12 publications

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“…Although no direct visualizations of the structure were published, the authors found the presence of isotropic hydrophilic clusters containing up to 100 water molecules and separated by regions of fluorocarbon material. Contrary to the phenomenological model of Hsu and Gierke, 65,66 there was no evidence of a percolating water-filled network; instead, the authors proposed that water and protonic transport occurred by the transient formation of connective pathways between clusters, with a characteristic timescale for their manifestation and disappearance of order 100 ps. However, due to the continuously evolving network structure, it was difficult to distinguish between motions of water molecules within a cluster, and from one cluster to another.…”

Section: Classical Molecular Mechanics Modelsmentioning

confidence: 68%

Modelling of morphology and proton transport in PFSA membranes

Elliott

Paddison

2007

Phys. Chem. Chem. Phys.

221

253

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Computational modelling studies of the structure of perfluorosulfonic acid (PFSA) ionomer membranes consistently exhibit a nanoscopic phase-separated morphology in which the ionic side chains and aqueous counterions segregate from the fluorocarbon backbone to form clusters or channels. Although these investigations do not unambiguously predict the size or shape of the clusters, and whether or not the channels percolate the matrix or if the connections between them are more transient, the sequence of co-monomers along the main chain appears strongly to influence the domain size of the ionic regions, with more blocky sequences giving rise to larger domain sizes. The fundamental insight that substantial rearrangement of the sulfonic acid terminated side chains and fluorocarbon backbone takes place during swelling or shrinkage is borne out by both molecular and mesoscale simulations of model PFSA polymers, along with ab initio electronic structure calculations of minimally hydrated oligomeric fragments. Molecularlevel modelling of proton transport in PFSA membranes attests to the complexity of the underlying mechanisms and the need to examine the chemical and physical processes at several distinct time and length scales. These investigations have revealed that the conformation of the fluorocarbon backbone, flexibility of the sidechains, and degree of aggregation and association of the sulfonic acid groups under minimally hydrated conditions collectively control the dissociation of the protons and the formation of Zundel and Eigen cations. The former appear to be the dominant charge carriers when the limiting water content allows only for the formation of a contact ion pair with the tethered sulfonate anion. As the water content increases, solventseparated Eigen ions begin to appear, indicating that the dominant mechanism for diffusion of protons occurs over a region approximately 4 Å away from the sulfonate groups. Finally, both the vehicular and Grotthuss shuttling mechanisms contribute to the mobility of the protons but, surprisingly, they are not always correlated, resulting in a lower overall diffusion coefficient. In summary, as the preceding observations indicate, the state of computational modelling of PFSA membranes has progressed sufficiently over the last decade to enable its use as a powerful predictive tool with which to guide the process of designing novel membrane materials for fuel cell applications.

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Section: Classical Molecular Mechanics Modelsmentioning

confidence: 68%

Modelling of morphology and proton transport in PFSA membranes

Elliott

Paddison

2007

Phys. Chem. Chem. Phys.

221

253

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“…One of the earliest models for the nanostructure of PFSA ion omers is the cluster-network model proposed by Gierke and Hsu [9,18,19]. They used a simplified geometric representation: a poly mer matrix containing a cubic array of spherical, inverted micellar clusters which grow upon water uptake [19].…”

Section: Spherical Cluster Modelmentioning

confidence: 99%

“…These water-filled hydrophilic domains are separated from the hydrophobic polymer matrix resulting in a two-phase nanostructure as observed through a vast number of scattering experiments [9,[20][21][22][23][24][25][34][35][36][37]. Upon water uptake, 3 water molecules initially ionize the SO 3 groups and remain bound to them.…”

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confidence: 99%

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Mechanics-based model for non-affine swelling in perfluorosulfonic acid (PFSA) membranes

Kusoglu

Santare

Karlsson

2009

Polymer

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“…One widely accepted empirical model for hydrated Nafion is the cluster-network model proposed by Hsu and Gierke 11,12 on the basis of small-angle X-ray scattering (SAXS) experiments. In this model, spherical hydrophilic clusters (∼4 nm diameter) of water are surrounded by sulfonate groups connected through cylindrical channels with ∼1 nm diameter.…”

Section: Introductionmentioning

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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Ion transport and clustering in nafion perfluorinated membranes

Cited by 903 publications

References 12 publications

Modelling of morphology and proton transport in PFSA membranes

Modelling of morphology and proton transport in PFSA membranes

Mechanics-based model for non-affine swelling in perfluorosulfonic acid (PFSA) membranes

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

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