Characterization of Nafion®XL Properties after Ex-Situ and In-Situ Degradations

Abstract

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