Membranes based on basic polymers and perfluorinated acids for hotter and drier fuel cell operating conditions

Larson, J.M.; Hamrock, Steven J.; Haugen, Gregory M.; Pham, Phat; Lamanna, W. M.; Moss, A. Butenhoff

doi:10.1016/j.jpowsour.2007.04.010

Cited by 11 publications

(10 citation statements)

References 6 publications

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“…It is well-known that several factors affect the conductivity of ion-exchange membranes. The most important controls are the water content ,− and the ion-exchange capacity (IEC) or related degree of functionalization (DF) of the polymer electrolyte. ,,, The temperature of operation ,− and the chemical structure of the polymer ,,− also play important roles. The interplay of these variables is central in determining water and ion transport in fuel cell membranes. , However, there appears to be a major trade-off between high IEC and the equilibrium water uptake of the membrane, causing issues in preservation of the solid-like mechanical properties of the membrane required for cell operation. , …”

Section: Introductionmentioning

confidence: 99%

“…To preserve the conductivity of the membrane, commercial fuel cell membranes operate with a water content λ between 13 and 18 water molecules per ion in the polymer chain. , However, the electro-osmotic drag by the moving anions ,− under operando conditions creates a water gradient that results in water contents as low as λ = 5 close to one of the electrodes, limiting the overall conductivity of the membrane . The quest for maximizing conductivity under these conflicting constraints has spurred a great deal of research into understanding the roles of chemistry and architecture of the polymer electrolyte on the mobility of ions in membranes at low water contents. ,,,,,− …”

Section: Introductionmentioning

confidence: 99%

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Width and Clustering of Ion-Conducting Channels in Fuel Cell Membranes Are Insensitive to the Length of Ion Tethers

Barnett

Molinero

2021

J. Phys. Chem. C

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Ion transport is the main function of fuel cell membranes. Maintaining high ion conductivity, however, is challenging under the low water contents established by electro-osmotic drag under operando conditions. Separating the bound ion from the polymer backbone via tethers up to 7 carbons long enhances the conductivity of both anion- and cation-exchange membranes. This effect has been attributed to increased phase separation of hydrophilic and hydrophobic domains in the membranes. However, to date, there have not been systematic studies of the effect of tether length on the structure of ion-exchange membranes. Here, we use large-scale molecular simulations to investigate the structure of polyphenylene oxide trimethylalkylammonium anion-exchange membranes with the cations tethered by alkyl chains from 1 to 12 carbons long, as a model for fuel cell membranes with variable tether length. The diffusion of the anions in the simulations increases with tether length and saturates for chains with seven carbons or more. Nevertheless, the simulation reveals that the width and clustering of hydrophilic channels, as well as the local environment of the anions, are insensitive to tether length. Further comparison of the width of the hydrophilic channels along a wide range of zwitterion-, anion-, and cation-exchange membranes indicates that the width of hydrophilic channels is universal across all low-water-content ionic membranes. We conclude that the improved conductivity of fuel cell membranes with ion tethers does not have a structural origin.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Width and Clustering of Ion-Conducting Channels in Fuel Cell Membranes Are Insensitive to the Length of Ion Tethers

Barnett

Molinero

2021

J. Phys. Chem. C

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“…While high ionic conductivity of the membrane is achieved at high water contents, ,− the rigidity of the polymer suffers as water content increases, making for mechanically less stable membranes. − Further, water gradients created by electro-osmotic drag and reactions at the electrode in fuel cell membranes under operation conditions result in regions with water contents as low as λ = 5 water molecules per ion pair in the membrane (11.11 molal ionic concentration). − The increased resistance that can ensue in lower content regions has resulted in a strong push to optimize the polymer to increase ion mobility at low water contents. ,− …”

Section: Introductionmentioning

confidence: 99%

Mechanism of Facilitation of Ion Mobility in Low-Water-Content Fuel Cell Membranes

Barnett

Molinero

2021

J. Phys. Chem. C

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Ion-exchange membranes for fuel cells have nanophase-segregated structures with percolated hydrophilic channels. Adding tethers up to seven carbons long between polymer ions and backbone has been shown to increase the conductivity of anion-and cation-exchange membranes at low water contents that are established under fuel cell operando conditions. This effect had been attributed to the development of better-clustered, wider ion-conducting channels. However, recent studies indicate that the width and clustering of the channels are insensitive to tether length. This poses the question of how do tethers increase ion mobility in membranes. Here, we use large-scale molecular simulations to elucidate the mechanisms of ion diffusion in polyphenylene oxide trimethylalkylammonium (PPO-TMA) anion-exchange membranes with cations tethered by alkyl chains from 1 to 12 carbons long, as a model for fuel cell membranes with variable tether length. We find that tethers increase the conductivity through a purely dynamical facilitation mechanism that originates in the synergism of strong cation−anion pairing and the restricted mobility of the polymer-bound cation. In the absence of a tether, the anion diffusion is dominated by ∼0.3 nm jumps into positions of their solvation water. Local mobility of tethered cations removes the jamming for anions, increasing the contribution of the faster continuous diffusion mechanism. The facilitation mechanism saturates for approximately seven carbon long tethers, revealing why that tether length is used in the best-performing commercial membranes.

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“…It seems that addition of simple metal oxides provided optimal conductivity for the PBI membranes (582). Membranes made from PBIO (trade name for a phenylene oxide benzimidazole from Fumatech and bis-fluorinated acids and silica) exhibit better fuel cell performance at 100-110 • C than PA-doped PBI membranes (583).…”

Section: Support Matrixmentioning

confidence: 99%

Rigid‐Rod Polymers

Wang¹,

Jiang²,

Adams³

2011

Encyclopedia of Polymer Science and Technology

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Pursuing high strength and high modulus polymer fibers and novel electrooptical properties have motivated much research activity in rigid‐rod polymers in the past 40 years. In this review, we examine the synthesis, structures, and properties of rigid‐rod polymers with emphasis on polybenzobisoxazoles (PBO), polybenzobisthiazoles (PBZT) and polybenzimidazoles (PBI), beginning with their solution properties, especially lyotropic liquid crystallinity. High performance fibers spun from lyotropic liquid crystalline polyphosphoric acid (PPA) solution are described. The characterization of fiber properties including mechanical, thermal, electrical, and optical properties is summarized. PBO (Zylon) and PIPD (M5) fibers were successfully commercialized in high strength applications such as body armor, ropes, cables, and recreational equipment in the last 10 years, and we discuss the possible causes of rapid degradation of some PBO fibers. The concepts of molecular composites and nanocomposites provide new applications in antiballistic materials, fire protection, athletic equipment, cables, strings, ropes, cordage, sails, multilayer circuits, and catalyst supports. Recently, rigid‐rod polymer films have drawn attention in the electrooptical fields, especially for phosphoric acid (PA)‐doped PBI membranes (Celtec MEAs) in the applications of high temperature fuel cells. Computer simulation has been employed as a powerful tool to predict the properties of the rigid‐rod polymers.

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Membranes based on basic polymers and perfluorinated acids for hotter and drier fuel cell operating conditions

Cited by 11 publications

References 6 publications

Width and Clustering of Ion-Conducting Channels in Fuel Cell Membranes Are Insensitive to the Length of Ion Tethers

Width and Clustering of Ion-Conducting Channels in Fuel Cell Membranes Are Insensitive to the Length of Ion Tethers

Mechanism of Facilitation of Ion Mobility in Low-Water-Content Fuel Cell Membranes

Rigid‐Rod Polymers

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