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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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A proton-exchange membrane fuel cell (PEMFC) constitutes today one of the preferred technologies to promote hydrogen-based alternative energies. However, the large-scale deployment of PEMFCs is still hampered by insufficient durability and reliability. In particular, the degradation of the polyelectrolyte membrane, caused by harsh mechanical and chemical stresses experienced during fuel cell operation, has been identified as one of the main factors restricting the PEMFC lifetime. An innovative chemical-mechanical ex situ aging device was developed to simultaneously expose the membrane to mechanical fatigue and an oxidizing environment (i.e., free radicals) in order to reproduce conditions close to those encountered in fuel cell systems. A cyclic compressive stress of 5 or 10 MPa was applied during several hours while a degrading solution (H2O2 or a Fenton solution) was circulated in contact with the membrane. The results demonstrated that both composite Nafion™ XL and non-reinforced Nafion™ NR211 membranes are significantly degraded by the conjoint mechanical and chemical stress exposure. The fluoride emission rate (FER) was generally slightly lower with XL than with NR211, which could be attributed to the degradation mitigation strategies developed for composite XL, except when the pressure level or the aging duration were increased, suggesting a limitation of the improved durability of XL.

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During fuel cell operation, proton-exchange membranes (PEMs) are subjected to aggressive chemical environment and mechanical fatigue that alter their properties and may lead to performance loss or, in the worst cases, fuel cell shutdown due to membrane failure. Membrane degradation due to chemical and mechanical stresses remains one of the main factors limiting PEM fuel cell lifetime [1]. Chemical damages are mostly due to radical attacks leading to the scission of the polymer chains (backbone or/and side chains), while mechanical stress comes from repeated swelling/shrinkage cycles caused by the membrane water-uptake. Despite quantities of works focusing independently on chemical (most often) or mechanical (more rarely) degradations, only a few studies were published on the effect of both combined [2]–[4]. Among them, Kusoglu et al. demonstrated that a static compression affects its microstructure and accelerates the chemical decomposition of the polymer chains in Nafion® membranes [2]. In this study, we investigate the impact of both chemical and mechanical degradations on the morphology and physicochemical properties of Nafion® membranes: chemical structure, as well as water transport and sorption. The membrane degradation process was induced by a custom-made device that mimics the operating conditions of the fuel cell by exposing the membrane simultaneously to a free radical environment and cyclic compression. The generation of free radical environment is based on the circulation of a continuous flow of hydrogen peroxide or Fenton’s solution (i.e. hydrogen peroxide and ferrous ions), while mechanical fatigue is induced by a cyclic compression to reproduce the swelling/shrinkage sequences entailed by the membrane water-uptake during transient fuel cell operation. Preliminary studies were necessary to determine the appropriate experimental conditions, both from a chemical and a mechanical point of view. Firstly, an investigation similar to the one of Frensch et al. was performed to determine the effect of Fenton’s reagent concentrations on membrane degradation [5]: fluoride emission measurements showed that a higher degradation rate was obtained at a low iron ions concentration. Moreover, at low iron concentration, the degradation rate depended also significantly on the hydrogen peroxide concentration. On the other hand, high hydrogen peroxide concentration (≈ 30 %vol) entailed strong membrane morphology modification with the appearance of many bubbles - with a diameter varying from some micrometers to several millimeters- at the surface of both membranes [6]. Such morphology evolutions cannot be considered as representative of the degradations occurring during fuel cell operation. The first mechanical and chemical degradation tests were carried out using a 0.0503 cm3.s-1 flow of either pure water or 3%vol diluted H2O2 through two thermostated (80°C) half-cells. The half-cells and the membrane were inserted between the clamps of an electromechanical testing machine (MTS load frame model 312.21) and the mechanical stress consisted in a sinusoidal constraint between 0 and 5 MPa at 0.1 Hz. As expected, the application of this sinusoidal constraint increased significantly the degradation rate (Figure): the fluoride emissions were 5 to 7 times higher than under chemical stress only. Indeed, Nafion® XL’s degradation rate increases from 11 µg/h/gNafion to 68 µg/h/gNafion when mechanical stress was applied in addition to chemical stress. Likewise, the values increased from 11 to 54 µg/h/gNafion with NR-211. Moreover, with the combination of cyclic compression and H2O2 solution exposure, the degradation rates approached the values obtained with a Fenton’s reagents exposure. [1] R. Borup et al., Chem. Rev., 107 (10), 3904–3951, 2007. [2] A. Kusoglu et al., ECS Electrochem. Lett., 3 (5), F33–F36, 2014. [3] S. velan Venkatesan et al., J. Electrochem. Soc., 163 (7), F637–F643, 2016. [4] V. M. Ehlinger et al., 166 (7), F3255–F3267, 2019. [5] S. H. Frensch et al., J. Power Sources, 420, 54–62, 2019. [6] S. Mu et al., J. Appl. Polym. Sci., 129 (3), 1586–1592, 2013. Figure 1

Among the different PEMFC components the electrolyte membrane is still the object of many studies as its degradation remains a limiting factor of its durability. Quantities of work have focused on the impact of chemical damages due radical attacks leading to the scission of the polymer chains [1, 2]. Recent studies have pointed out the combined effect of mechanical and chemical stressors on the PEM properties [3] and it was demonstrated that the compression level imposed on the membrane changes the microstructure and accelerates the chemical decomposition of the polymer. A set of experimental techniques are available to characterize the membranes properties after degradation, including EIS (proton conductivity), water sorption, scattering methods (microstructure), IR, Raman and 19F-NMR spectroscopy (chemical structure) or DMA (mechanical behavior). These methods are complementary, and it is usually necessary to combined several of them to obtain an accurate description of the membrane properties. Ultimately, the experimental data are analyzed and correlated to understand the link between the chemical structure, the microstructure and the transport properties. In the present study we investigated the link between the changes observed in the chemical structure and the evolution of the water sorption and transport properties in the composite Nafion XL membrane after degradation in controlled conditions (ex-situ Fenton tests) and after long term fuel cell operation in the field. We used a multi-characterization approach, combining 19F-NMR and IRTF spectroscopy, water sorption and 1H-pulsed field NMR to quantify the changes observed in the chemical structure and the evolution of the water sorption and diffusion. The IR measurements demonstrate a heterogeneous chemical degradation between the anode and the cathode side (Figure 1) while the 19F data show a decrease of the IEC after both ex-situ and in-situ degradation. This loss of IEC, mainly due to side chain scissions, is correlated to an important decrease of the membrane water sorption capacity. Interestingly, the 1H-NMR spectra reveal two proton resonances (Figure 2) attributed to two non-exchangeable water populations, respectively in the two external layers of ionomer and in the ionomer dispersed in the central PTFE layer of the composite membrane. The ratio between the two populations is seen to decrease after degradation because the two external layers are decomposed first. However, the diffusion measurements show that, at a similar water content, the water diffusion coefficient is lower after degradation in the external layers as well as in the central layer, showing that the degradation also impacts water dynamics in the ionomer dispersed in the PTFE central reinforcement. This multi-characterization approach was also developed with the aim to propose an efficient and simple protocol able to quickly characterize and correlate the Nafion XL chemical and transport properties. [1] Ghassemzadeh, L.; Holdcroft, S. Quantifying the Structural Changes of Perfluorosulfonated Acid Ionomer Upon Reaction with Hydroxyl Radicals. J. Am. Chem. Soc. 2013, 135 (22), 8181−8184. [2] Zaton, M.; Rozière, J.; Jones, D.J. Current understanding of chemical degradation mechanisms of perfluorosulfonic acid membranes and their mitigation strategies: a review. Sustainable Energy & Fuels. 2017, 1, 409-438. [3] Kusoglu, A.; Calabrese, M.; Weber, A. Z. Effect of Mechanical Compression on Chemical Degradation of Nafion Membranes. ECS Electrochem. Lett. 2014, 3, F33−F36. Figure 1