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Vieillissement hygrothermique du Nafion

Abstract : For the development of Proton Exchange Membrane Fuel Cells (PEMFC), membrane durability is a crucial issue. Nafion® in-situ aging in fuel cell did not lead to determine a degradation mechanism. That is why ex-situ aging tests were performed, in drying oven and in climatic chambers, over 500 days on Nafion® 112 and on Nafion® 212-CS in practical fuel cell usage conditions (e.g. 80°C, with a hygrometric rate between 0%RH and 95%RH).
The evolution of the mechanical properties shows an increase of the Young modulus and a decrease of the breaking strain while the hydrophilicity (directly linked to the protonic conductivity) measured by DVS (Dynamic Vapour Sorption) decreases. The evolution of the polymer chemical structure, monitored by infrared spectroscopy, underlines sulfonic anhydride formation. An indirect proof of the anhydride formation is given by Nuclear Magnetic Resonance.
The suggested mechanism is the sulfonic acid condensation to form an anhydride. The properties evolution is explained via this mechanism.
The comparison of Nafion® 112 and Nafion® 212-CS shows that Nafion® 212-CS gets aged three times faster than Nafion® 112. The condensation reaction catalysis by metallic cations which concentrations is higher in Nafion® 112 explains the aging kinetics difference.
Moreover, the tests done at different relative humidities show that when the hygrometric rate increases, the condensation reaction is accelerated. We suggest an SN1 type ionic degradation mechanism which implies the formation of RSO2+ as an intermediate.
Lastly, we point out that, in presence of a catalyser, the condensation reaction is reversible: The anhydride is hydrolysed which leads to recovered properties. The membrane is rejuvenated.
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Submitted on : Wednesday, January 21, 2009 - 11:38:08 AM
Last modification on : Tuesday, October 20, 2020 - 11:00:41 AM
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  • HAL Id : tel-00354847, version 1

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Floraine Collette. Vieillissement hygrothermique du Nafion. Matériaux. Arts et Métiers ParisTech, 2008. Français. ⟨NNT : 2008ENAM0029⟩. ⟨tel-00354847⟩

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