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Etude des interactions moléculaires polymère-eau lors de l'hydratation de la membrane Nafion, électrolyte de référence de la pile à combustible

Abstract : Nafion is a polymer. Thanks to its high conductivity (up to 10^-2 S.cm^-1) at high relative humidity (RH), it is a reference electrolyt for a fuel cell. However its conductivity falls during low hydration conditions. To solve this problem, we can add a hygroscopic compound, like ziconium phosphate (ZrP), into the membrane.

The conductivity is linked to three factors : the structure of the membrane, the proton diffusion mechanisms and the interactions between water molecules and the polymer; we are interested by this last field of research. Infrared spectroscopy are used to establish the hydration mechanisms at a molecular scale for a Nafion and a Nafion-ZrP membrane. This technique can be coupled with a molecular dynamic study, which we have begun for the Nafion. The infrared spectra of Nafion and Nafion-ZrP have been measured on the whole range of RH.

We found 5 hydration mechanisms for the Nafion membrane. The ionisation of sulfonic groups SO_3H is very fast at the beginning of hydration. Then the protons H^+ move away from the sulfonate groups SO_3^- and the net of hydrogen bonds around these ionic groups changes. For a RH of 40%, bulk water appears inside the membrane. We have thus a "photograph" of the inner membrane at each stage of RH. The adding of an inoganic compound ZrP has no influence on the hydration mechanisms.

According to the comparison between our mechanisms and the curve of conductivity, all the sulfonic groups have to be dissociated to reach optimal diffusion of the proton, probably assured by the Grotthuss mechanism.
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Submitted on : Friday, June 6, 2008 - 5:33:59 PM
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Jérémy Chabe. Etude des interactions moléculaires polymère-eau lors de l'hydratation de la membrane Nafion, électrolyte de référence de la pile à combustible. Physique [physics]. Université Joseph-Fourier - Grenoble I, 2008. Français. ⟨tel-00285999⟩

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