Structure/property relationship of Nafion XL composite membranes

Shi, Shouwen; Weber, Adam Z.; Kusoglu, Ahmet

doi:10.1016/j.memsci.2016.06.004

Cited by 135 publications

(148 citation statements)

References 59 publications

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“…Empirical relationships between Ce ion transport properties and PEM water content.-Using water sorption data for Nafion XL PEMs, 19 empirical relationships between Ce ion diffusivity and mobility as a function of PEM water content (defined as λ = nH 2 O/nSO 3 H) may be estimated. Assuming that ion diffusivity and mobility are zero for λ = 0, linear fitting of the data yields:…”

Section: Discussionmentioning

confidence: 99%

Cerium Ion Mobility and Diffusivity Rates in Perfluorosulfonic Acid Membranes Measured via Hydrogen Pump Operation

Baker

Babu

Mukundan

et al. 2017

J. Electrochem. Soc.

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Ion mobility and diffusivity coefficients were determined for cerium ions in Nafion XL perfluorosulfonic acid ionomer membranes at 100% and 50% relative humidity in a conductivity cell using a hydrogen pump. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). In order to decouple simultaneous effects of Ce ion mobility and back-diffusion which occur due to potential and concentration gradients, respectively, a one-dimensional model was developed and fit to these intermittent XRF profiles. The resulting mobility and diffusivity coefficients demonstrate the dramatic effects of potential and concentration gradients on Ce ion migration during PEM fuel cell operation. Cerium ions enhance the durability of polymer electrolyte membrane (PEM) fuel cell components by rapidly and reversibly scavenging radical species which are generated during operation.1 Such species react with and degrade the perfluorosulfonic acid (PFSA) ionomer which comprises the PEM and is present in the catalyst layers (CLs). Recent reviews by Rodgers 2 and Zatoń 3 extensively describe the origins of these radical species, their detrimental roles in PEM fuel cell degradation, and strategies to prevent degradation by mitigating their reactions with PFSA.While Ce is an excellent radical scavenger, Ce ions are known to migrate between the PEM and the CLs due to potential and concentration gradients, 1,4-6 water flux, 7 and degradation of Ce-exchanged sulfonic acid groups. 7,8 Ce species have also been observed to irreversibly stabilize in the CLs, 4,7-9 which is attributed to an interaction with the carbon catalyst supports. 7It is believed that Ce migration into the cathode CL will generate performance losses due to diminished CL ionomer conductivity. 10,11 Ex situ microelectrode studies of ionomer thin films exchanged with Ce ions at concentrations previously measured in the cathode CL 7 also demonstrate significant performance losses.12 These losses are attributed to reduced ionomer water content which results from cation contamination, 13 which, in turn, reduces oxygen diffusion to the Pt electrode by collapsing the ionomer structure.14 Further, because the diffusion length of the most reactive hydroxyl radical is only 40 nm, 15 migration of Ce ions away from an ionomer region may leave it more susceptible to degradation. Therefore, in order to stabilize it in the MEA and localize it to areas of highest radical generation, it is critical to understand different transport properties of Ce ions, such as diffusivity and mobility due to concentration and potential gradients, respectively.In this work, Ce ion migration was induced in PEMs using a hydrogen pump. Migration profiles were subsequently mapped using X-ray fluorescence (XRF). In order to decouple simultaneous Ce ionic diffusivity and mobility which occurs during these experiments, a onedimensional (1-D) model was developed and subsequently fit to the experimental data to extract these transport properties. ExperimentalCon...

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Section: Discussionmentioning

confidence: 99%

Cerium Ion Mobility and Diffusivity Rates in Perfluorosulfonic Acid Membranes Measured via Hydrogen Pump Operation

Baker

Babu

Mukundan

et al. 2017

J. Electrochem. Soc.

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“…The nominal thicknesses are ∼12 μm for the PTFE layer and ∼9 μm for each external PFSA layer. 10,17 A reinforced membrane which operated in Axane Evopac fuel cell system was also investigated. This system, composed of two stacks of 55 cells, was studied in References 18 and 12.…”

Section: Methodsmentioning

confidence: 99%

Towards a NMR-Based Method for Characterizing the Degradation of Nafion XL Membranes for PEMFC

Robert¹,

Kaddouri²,

Perrin³

et al. 2018

J. Electrochem. Soc.

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PFSA-based reinforced membranes are used today as the benchmark material for the electrolyte in PEMFCs. Although greatly improved relatively to their unreinforced version, they still suffer from aging and degradation during fuel cell (FC) operation. In this study we first performed proton NMR to characterize the different water populations in the pristine Nafion XL reinforced membrane. Then we used proton and fluorine NMR, FTIR and sorption measurements in order to qualitatively observe the differences induced in the membrane's chemical structure and properties by long term FC operation. Proton NMR is seen to be an adapted tool to quickly measure a signature that is correlated to the degradation state while FTIR can serve as a local probe of the chemical structure. The degradation of the proton exchange membrane (PEM) is one of the main factors limiting the proton exchange membrane fuel cell (PEMFC) stability and performance.1,2 The development of PEM with increased durability remains thus today a critical obstacle that restricts the large scale spreading of PEMFC systems.The decomposition of the membrane is induced by several factors among which mechanical stress and chemical degradation prevail.The mechanical degradation is initiated by humidity cycling that creates alternating shrinkage/swelling events and a non-uniform stress distribution in the membrane plane. The resulting reduction of the polymer mechanical strength can lead to the formation of cracks and to the final failure of the membrane electrode assembly (MEA). 3-5The chemical decomposition of the PEM in the MEA is caused by gas crossovers. The electrochemical reactions of these gases cause the formation of free radicals and the attack of the polymer chemical structure, which results in scissions in the main chain and in the side chain and finally to the thinning of the membrane. 6,7 Many studies have focused on PEM chemical degradation and a comprehensive review of the current understanding of the mechanisms in perfluorosulfonic acid (PFSA) membranes was recently published. 8 PFSA membranes, such as Nafion, Flemion or Aciplex are today the most widely used fuel cell electrolytes thanks to their good chemical robustness and their high proton conductivity.9 Through the years, efforts have been made to reduce the membrane's ionic resistivity without compromising the mechanical properties. This was made possible by the introduction of chemical stabilizers and mechanical reinforcement. For instance, the Nafion XL is a reinforced membrane composed of two external PFSA layers with stabilizers to reduce chemical attack and a central microporous polytetrafluoroethylene (PTFE)-rich support layer to provide additional mechanical strength and enable the use of thinner ionomer. The central PTFE layer is impregnated with ionomer to provide a continuous conductive pathway across the membrane's thickness.The structure and basic properties of the Nafion XL membrane were investigated by Shi and co-workers. 10 The authors examined the effect of reinforcement and pre-...

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“…This was due to possible structural changes, leading to internal stress relaxation in the expanded PTFE membranes. The cross‐sections of Nafion™ XL membrane prior to and after pretreatment are shown in Figures 7d to 7g . Prior to pretreatment, the fibers are dispersed along the thickness direction, as shown in Figures d and e.…”

Section: Resultsmentioning

confidence: 99%

“…It should be noted that the improvement in mechanical stability is not at the expense of reduced performance. It was found that Nafion™ XL membrane showed comparable conductivity with the benchmark material Nafion™ 212 membrane . In addition, The Nafion™ XL composite membrane exhibited longer lifetime during accelerated stress test (AST) and field test , .…”

Section: Introductionmentioning

confidence: 99%

Effect of Pretreatment on Microstructure and Mechanical Properties of Nafion™ XL Composite Membrane

et al. 2019

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As an ion‐conductive polymer electrolyte, Nafion™ XL composite membrane is used to meet the requirement for cost reduction, performance and durability improvement in polymer electrolyte fuel cells. While significant durability is observed for Nafion™ XL membrane, the mechanisms of improved mechanical durability, especially the impact induced by pretreatment, is still left unexplored. In this paper, ex situ mechanical tests of pretreated membranes are carried out, including tensile, fracture and fatigue crack propagation tests, to unravel the complex interplay between strength, fracture toughness and cyclic hygrothermal stress that controls the mechanical durability. Pretreatment increases strain‐hardening modulus at the expense of reduced ductility, while the break stress is not affected. Results from in situ SEM tensile tests reveal that pretreatment increases the orientation resistance during tension, leading to increased stress responses. In addition, the fracture resistance and fatigue crack propagation resistance increase after pretreatment, which is ascribed to increased membrane stiffness and reinforcement fiber reorientation, respectively. From a microscopic point of view, fibers along the cross‐sectional reinforcement layer collapse, corresponding to fiber reorientation at a smaller size scale and accounting for the above increased resistance. The findings reported here will provide more insights into the mechanical durability of composite ion‐conductive membranes.

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Structure/property relationship of Nafion XL composite membranes

Cited by 135 publications

References 59 publications

Cerium Ion Mobility and Diffusivity Rates in Perfluorosulfonic Acid Membranes Measured via Hydrogen Pump Operation

Cerium Ion Mobility and Diffusivity Rates in Perfluorosulfonic Acid Membranes Measured via Hydrogen Pump Operation

Towards a NMR-Based Method for Characterizing the Degradation of Nafion XL Membranes for PEMFC

Effect of Pretreatment on Microstructure and Mechanical Properties of Nafion™ XL Composite Membrane

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