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

Abstract

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-...

Time-resolved monitoring of composite Nafion™ XL membrane degradation induced by Fenton's reaction

“…In recent years, the Ce 3+ /Ce 4+ couple has become a promising candidate to efficiently neutralize radicals and protect ionic groups thanks to its self-regenerative property. However, despite the presence of mechanical reinforcements and radical scavengers, current membranes are still insufficiently durable during fuel cell operation [16,41].…”

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

“…While remarkable changes in microstructure are observed in this study, pretreating membranes by heating them in 70 °C water does not result in much changes in microstructure . As the reinforced membranes are normally tested at 80 °C, no significant decay in durability is reported , . With increasing demand to work at elevated temperature or low relative humidity, composite membranes with short‐side‐chain (SSC) PFSA ionomer were found to have great potential to meet such requirement .…”

“…To address this issue, mechanical reinforced composite membranes, e.g., with porous expanded polytetrafluoroethylene (ePTFE), are used incorporating radical scavengers, such as CeO 2 . Among them, GORE-SELECT membranes from W. L. Gore & Associates, Inc. [7][8][9] and Nafion XL membranes [10,11] manufactured by Dupont are good examples. The relatively stiff porous reinforcement layer not only reduces in-plane swelling which induces mechanical stress during humidity cycling, but also increases the mechanical strength [12].…”

Magnetic Resonance Imaging Measurements in Engineering Science

“…Actuellement, les membranes à base de polymè re PFSA (Perfluoro Sulfonic Acid Membranes, dont le Nafion®) sont les plus utilisé es car elles pré sentent à la fois de bonnes proprié té s de transport visà -vis des ions H + et une bonne stabilité chimique et mé canique. Depuis plusieurs anné es, nous utilisons des mé thodes RMN et IRM pour é tudier le transport d'eau dans ces maté riaux et comprendre en particulier les relations structure/transport [23,24] ainsi que l'influence de vieillissements sur les proprié té s [25]. Le polymè re Nafion é tant perfluoré , il repré sente un bon systè me d'étude pour la RMN car le signal dé tecté provient uniquement de l'eau dans la membrane, non des chaî nes polymè res.…”