Hydrated Arrays of Acidic Surface Groups as Model Systems for Interfacial Structure and Mechanisms in PEMs

Roudgar, Ata; Sp, Narasimachary; Eikerling, Michael

doi:10.1021/jp063189v

Cited by 51 publications

(78 citation statements)

References 63 publications

(164 reference statements)

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“…Side chain separation has a vital impact on hydrogen binding between side chains; it determines water binding and mechanisms of proton transport at the hydrated ionomer surface. A separation of 0.7 nm and a hexagonal arrangement optimize H-bond interactions between side chains [57]. The high side chain density is indicative of a surface dominated mechanism of proton transport, as discussed in [58].…”

Section: Structure Of Ionomer In CLmentioning

confidence: 91%

“…The square lattice arrangement of side chains is an artifact, caused by the neglect of hydrogen bonding between side chain and hydronium beads in our CGMD simulations. Strong hydrogen bonds with hydronium will favor the formation of a hexagonal arrangement of side chains [57]. Side chain separation has a vital impact on hydrogen binding between side chains; it determines water binding and mechanisms of proton transport at the hydrated ionomer surface.…”

Section: Structure Of Ionomer In CLmentioning

confidence: 99%

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Microstructure of Catalyst Layers in PEM Fuel Cells Redefined: A Computational Approach

2011

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This work comprises an extensive coarse-grained molecular dynamics study of self-organization processes that define the mesoscopic structure of catalyst layers used in polymer electrolyte fuel cells. The detailed structural analysis focuses on agglomeration of Pt-decorated primary particles of graphitized carbon black, formation of ionomer domains, emergence of the porous network, and formation of interfaces between the distinct phases. Insights obtained enable us to decisively redraw the existing structural picture of the catalyst layer. As a key result, we found that ionomer forms a thin adhesive film, which partially covers agglomerates of Pt/carbon. Densely arranged charged side chains of ionomer form a highly ordered array on the ionomer film surface. The preferential orientation of these charged side chains depends on the surface wetting properties of the agglomerates. As a major consequence, results on ionomer structure and distribution, presented in this work, seem to invalidate the classical electrolyte-flooded agglomerate model that has been widely applied to catalyst layers in polymer electrolyte fuel cells. Instead, the structural analysis provided defines a need for novel models of proton transport, water distribution, and Pt effectiveness that account for the thin-film morphology of ionomer and the specific arrangement of surface groups.

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Section: Structure Of Ionomer In CLmentioning

confidence: 91%

Section: Structure Of Ionomer In CLmentioning

confidence: 99%

Microstructure of Catalyst Layers in PEM Fuel Cells Redefined: A Computational Approach

2011

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“…A more detailed classification of non-frozen water into non-freezable, freezable and free water was given in references [17][18][19] , it may exhibit freezing point depressions, and the free water will appear when the water content is relatively high. According to Eikerling et al [20] and Roudgar et al [21], water can also be sorted into surface water and bulk water. Surface water is tightly bound to H + SO 3 − and bulk water is regarded as liquid which is loosely bound to H + SO 3 − .…”

Section: States and Phase Changes Of Water In Pemfcsmentioning

confidence: 99%

A Review on Cold Start of Proton Exchange Membrane Fuel Cells

et al. 2014

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Successful and rapid startup of proton exchange membrane fuel cells (PEMFCs) at subfreezing temperatures (also called cold start) is of great importance for their commercialization in automotive and portable devices. In order to maintain good proton conductivity, the water content in the membrane must be kept at a certain level to ensure that the membrane remains fully hydrated. However, the water in the pores of the catalyst layer (CL), gas diffusion layer (GDL) and the membrane may freeze once the cell temperature decreases below the freezing point (T f ). Thus, methods which could enable the fuel cell startup without or with slight performance degradation at subfreezing temperature need to be studied. This paper presents an extensive review on cold start of PEMFCs, including the state and phase changes of water in PEMFCs, impacts of water freezing on PEMFCs, numerical and experimental studies on PEMFCs, and cold start strategies. The impacts on each component of the fuel cell are discussed in detail. Related numerical and experimental work is also discussed. It should be mentioned that the cold start strategies, especially the enumerated patents, are of great reference value on the practical cold start process. OPEN ACCESSEnergies 2014, 7 3180

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“…Distinct to the MD simulations implementing an EVB methodology to describe the breaking and forming of covalent bonds critical in proton transport, two recent investigations 119,129 have employed ab initio molecular dynamics (AIMD) simulations implemented using the VASP code [130][131][132][133] to investigate Fig. 11 Fully optimized B3LYP/6-311G** molecular fragments of the SSC polymer with 5 H 2 Os.…”

Section: This Journal Is C the Owner Societies 2007mentioning

confidence: 99%

Modelling of morphology and proton transport in PFSA membranes

Elliott

Paddison

2007

Phys. Chem. Chem. Phys.

225

255

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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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Hydrated Arrays of Acidic Surface Groups as Model Systems for Interfacial Structure and Mechanisms in PEMs

Cited by 51 publications

References 63 publications

Microstructure of Catalyst Layers in PEM Fuel Cells Redefined: A Computational Approach

Microstructure of Catalyst Layers in PEM Fuel Cells Redefined: A Computational Approach

A Review on Cold Start of Proton Exchange Membrane Fuel Cells

Modelling of morphology and proton transport in PFSA membranes

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