Current understanding of chemical degradation mechanisms of perfluorosulfonic acid membranes and their mitigation strategies: a review

Zatoń, Marta; Rozière, Jacqués; Jones, Deborah J.

doi:10.1039/c7se00038c

Cited by 251 publications

(260 citation statements)

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“…The chemical degradation by side chains loss has already been observed for monolayer PFSA in previous ex-situ and in-situ studies. 7,8,[34][35][36] The decrease of the IEC g from 0.87 to 0.83 indicates here that the global IEC is only slightly impacted by FC operation.…”

Section: Chemical Structure Analyses-mentioning

confidence: 99%

“…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.…”

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confidence: 99%

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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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Section: Chemical Structure Analyses-mentioning

confidence: 99%

mentioning

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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“…All XRF analysis was based on the relative intensity of fluoresced Ce L-lines, which are the most prominent fluorescent Xrays emitted by Ce and arise at energy levels of 4.8 keV. 18 Ce area density measurements from XRF were normalized to the ionomer thickness in units of mg Ce /cm 3 ionomer.…”

Section: Methodsmentioning

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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“…The hydrogen peroxide yield is enhanced in the presence of acetonitrile, acetylene, methyl methacrylate, naphthalene, and propene [60][61][62][63]. The elevated level of hydrogen peroxide in turn promotes ionomer degradation [64] and structural modifications to the catalyst layer that are relatively more impactful for the lower catalyst loading. Therefore, in view of the lower cell voltage and cathode potential for a lower catalyst loading (Figure 1, Table 1), a higher hydrogen peroxide yield [60][61][62][63] and ionomer degradation are expected.…”

Section: Impedance Spectramentioning

confidence: 99%

Impact of the Cathode Pt Loading on PEMFC Contamination by Several Airborne Contaminants

St‐Pierre

Zhai

2020

Molecules

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Proton exchange membrane fuel cells (PEMFCs) with 0.1 and 0.4 mg Pt cm −2 cathode catalyst loadings were separately contaminated with seven organic species: Acetonitrile, acetylene, bromomethane, iso-propanol, methyl methacrylate, naphthalene, and propene. The lower catalyst loading led to larger cell voltage losses at the steady state. Three closely related electrical equivalent circuits were used to fit impedance spectra obtained before, during, and after contamination, which revealed that the cell voltage loss was due to higher kinetic and mass transfer resistances. A significant correlation was not found between the steady-state cell voltage loss and the sum of the kinetic and mass transfer resistance changes. Major increases in research program costs and efforts would be required to find a predictive correlation, which suggests a focus on contamination prevention and recovery measures rather than contamination mechanisms.Molecules 2020, 25, 1060 2 of 15 focusing on the cathode catalyst loading effect were found [19,20]. However, contamination data in [19] are not directly comparable because both the catalyst layer design and catalyst loading were concurrently altered. The authors also refer to 10 ppb SO 2 data obtained by another group that showed more severe fuel cell damage with a catalyst loading decrease from 0.4 to 0.3 mg Pt cm −2 . In contrast, the effect of 2,6-diaminotoluene, a species that leaches out of the fuel cell system balance of plant materials, was more impactful after the Pt catalyst loading was lowered from 0.4 to 0.1 mg Pt cm −2 [20]. In comparison to the anode, the higher cathode potential is expected to affect the contamination mechanism with, for example, a different Pt surface charge, altered contaminant adsorbates and reaction intermediates, catalyst coverage, and cell voltage loss. This situation is exacerbated with a catalyst loading change, which affects the overpotential of the irreversible oxygen reduction reaction and the cathode potential. Information about chemical and electrochemical reactions for specific contaminants may be available in the literature. However, the presence of relevant cathode reactants, oxygen and water, may not be considered. For instance, novel intermediates or products were not detected with chlorobenzene in air [21]. However, the presence of acetylene in air led to small amounts of methane [22] that were not expected based on acetylene chemistry and electrochemistry. Therefore, tests completed under these significantly different operating conditions are needed in part because contaminant reactions are not currently predictable in assessing catalyst coverage and cell voltage loss.This report documents the impact of the cathode Pt catalyst loading effect for PEMFCs contaminated with seven model organic airborne species, which were previously evaluated and selected from a larger pool of 21 contaminants [23]: Acetonitrile (nitrile), acetylene (alkyne), bromomethane (halocarbon), iso-propanol (alcohol), methyl methacrylate (ester), naphthalene (polycyclic ...

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Current understanding of chemical degradation mechanisms of perfluorosulfonic acid membranes and their mitigation strategies: a review

Abstract: This article provides a comprehensive perspective of perfluorosulfonic acid fuel cell membrane degradation phenomena, reviews and appraises the effectiveness of key concepts for the mitigation strategies and identifies future research priorities.

Cited by 251 publications

References 300 publications

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

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

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

Impact of the Cathode Pt Loading on PEMFC Contamination by Several Airborne Contaminants

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