In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers’ complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradat...

New Insights into Perfluorinated Sulfonic-Acid Ionomers

The recent release of hydrogen economy roadmaps for several major countries emphasizes the need for accelerated worldwide investment in research and development activities for hydrogen production, storage, infrastructure and utilization in transportation, industry and the electrical grid. Due to the high gravimetric energy density of hydrogen, the focus of technologies that utilize this fuel has recently shifted from light-duty automotive to heavy-duty vehicle applications. Decades of development of cost-effective and durable polymer electrolyte membrane fuel cells must now be leveraged to meet the increased efficiency and durability requirements of the heavy-duty vehicle market. This Review summarizes the latest market outlooks and targets for truck, bus, locomotive and marine applications. Required changes to the fuel-cell system and operating conditions for meeting Class 8 long-haul truck targets are presented. The necessary improvements in fuel-cell materials and integration are also discussed against the benchmark of current passenger fuel-cell electric vehicles. Fuel cells are increasingly being considered for powertrains of heavy-duty transportation. Cullen et al. survey the technical challenges of fuel cells at both the system and materials level for transportation application and outline the roadmap for future development.

New roads and challenges for fuel cells in heavy-duty transportation

A review of the stability and durability of non-precious metal catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells

https://iopscience.iop.org/article/10.1149/2.0211514jes/pdf

Review-Electromobility: Batteries or Fuel Cells?

A review of proton exchange membrane water electrolysis on degradation mechanisms and mitigation strategies

A detailed thermochemical and kinetic analysis of PFSA ionomers and the reactive oxygen species formed in an operating fuel cell is reported. The analysis reveals that hydroxyl radical is the only oxygen species capable of abstracting a hydrogen atom from carboxylic acid intermediates, thereby propagating the PFSA main chain unzipping process. Two chemically specific, main chain scission mechanisms are proposed. The first involves formation of sulfonyl radicals under dry conditions and the second invokes the action of hydrogen atoms formed from hydrogen gas and hydroxyl radical. The thermochemical analysis provides a strong basis from which to rationalize the results from a broad range of in-situ and ex-situ fuel cell degradation studies.

The Chemistry of Fuel Cell Membrane Chemical Degradation

Cerium and manganese ions are very effective reversible scavengers of •OH in an operating PFSA-based PEM fuel cell. The use of these ions in very small quantities can reduce the fluoride release rate by up to three orders of magnitude relative to an unmitigated sample and thereby afford extremely durable membranes. A chemically rational mechanism that accounts for the remarkable effectiveness of these chemical mitigants is presented.

https://iopscience.iop.org/article/10.1149/1.2982015/pdf

Mitigation of Perfluorosulfonic Acid Membrane Chemical Degradation Using Cerium and Manganese Ions

Publisher Summary
Proton exchange membranes (PEMs) are the most promising membranes for automotive applications because of their relatively high proton conductivity at low temperatures. This chapter addresses the three primary root causes of membrane failure in automotive fuel cell systems. PEM fuel cells for high power density operation utilize perfluorosulfonic acid (PFSA) membranes. Ultimately, fuel cells fail because of pinholes that develop and propagate within the polymer membranes and can also fail if electronic current passes through the membranes causing the system to short. Both chemical and mechanical degradation of PFSA membranes are extensively studied and effective mitigation strategies are developed that can significantly extend PFSA-PEM lifetimes. The approaches described in this chapter can be applied to such alternative PEMs, but new fundamental models for both chemical and mechanical degradation are likely required. Even for the case of PFSA membranes, further work is required to develop the predictive models required to enable accurate estimation of membrane life in a real system. This chapter includes the first in-depth fundamental study of membrane shorting of PEMs in subsequent thermal decay of the PEM. Results and analysis suggest that the PEM type is not the limiting factor in preventing membrane shorting, and that mitigation is best achieved by a combination of design and operating strategies.

Chapter 2 – Membrane Durability: Physical and Chemical Degradation

We present experiments in an in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that allowed separate monitoring of radical formation at anode and cathode sides. The in situ FC was operated at 300 K under closed and open circuit voltage conditions, CCV and OCV, respectively, with a membrane-electrode assembly (MEA) based on Nafion 117 10% neutralized by Ce(III) and Pt as catalyst, notation MEA/Ce. The presence of the unstable intermediates was determined by addition of 5,5-dimethylpyrroline-N-oxide (DMPO) as a spin trap, simulation of the spectra from the DMPO adducts, and analysis of the corresponding magnetic parameters and relative intensities. The main objective of these experiments was to identify early events in the mitigation mechanism of Ce(III) on Nafion degradation. The present results were therefore compared to an in situ FC based on Nafion 117 in the acid form, notation MEA/H (Danilczuk et al. J. Phys. Chem. B 2009, 113, 8031−8042) that served as baseline. The differences...

Cerium(III) as a Stabilizer of Perfluorinated Membranes Used in Fuel Cells: In Situ Detection of Early Events in the ESR Resonator

Low-molecular weight model compounds (MCs) for Nafion membranes used in fuel cells were exposed at 300 K to ·OH radicals produced by UV irradiation of aqueous H2O2 solutions. The MCs contained fluorinated and partially fluorinated groups terminated by sulphonic or carboxylic acid groups. The fragmentation process in the MCs was studied by spin trapping electron spin resonance (ESR) methods, using 5,5-dimethylpyrroline-N-oxide (DMPO), N-tert-butyl-α-phenylnitrone (PBN) and 2-methyl-2-nitrosopropane (MNP) as the spin traps. The objective of these experiments was to assess the effect of the type of ionic groups (sulphonic or carboxylic) and of fluorine substitution on the spin adducts detected. DMPO experiments led to the detection of spin adducts of ·OH and of carbon-centred radicals (CCRs), and allowed the determination of the ·OH attack site on the ionic and/or on the protiated or fluorinated groups. CCR adducts were also detected when using PBN as a spin trap; a key point in the interpretation of the PBN results was, however, the realisation that MNP is formed during PBN exposure to UV irradiation and oxygen or other oxidants such as H2O2. Experiments with MNP as the spin trap were the most informative in terms of structural details for adducts obtained from each MC. The results allowed the identification of CCRs present as adducts, based on large hyperfine splittings (hfs) from, and the number of, interacting 19F nuclei; in addition, oxygen-centred radicals (OCRs) as MNP adducts were also identified, with much lower hfs from 19F nuclei. Taken together, the results deduced by spin trapping suggest that both sulphonic acid and acetic acid groups can be attacked by ·OH radicals and confirm two possible degradation mechanisms in Nafion membranes: initiated at the backbone and at the side chain.

Fragmentation of Fluorinated Model Compounds Exposed to Oxygen Radicals: Spin Trapping ESR Experiments and Implications for the Behaviour of Proton Exchange Membranes Used in Fuel Cells

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