Collette, Floraine (2008) Vieillissement hygrothermique du Nafion. PhD thesis Mécanique et matériaux, Laboratoire d'ingénierie des matériaux, ENSAM 2008ENAM0029 p.171.

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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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Table of content

SOMMAIRE - 5

INTRODUCTION GENERALE - 11

I. ETAT DE L’ART - 15

1. La membrane électrolyte Nafion® dans la pile à combustible - 17

1.1. Fonctionnement de la pile - 17

1.2. Caractéristiques physiques de la membrane - 20

2. Le matériau polymère Nafion® - 22

2.1. Structure chimique du Nafion® - 22

2.2. Outils d’analyse - 24

2.3. Propriétés physiques et mécaniques - 35

3. Durabilité du Nafion® - 42

3.1. Durabilité de la membrane Nafion® : Tests in-situ - 42

3.2. Durabilité du polymère Nafion® : Test ex-situ - 46

4. Conclusion Générale - 52

II. METHODES EXPERIMENTALES - 53

1. Matériaux - 55

2. Protocoles de vieillissement - 55

3. Méthodes de caractérisation - 56

3.1 Caractérisation mécanique : Essais de traction - 56

3.2. Essais de sorption d’eau par DVS - 57

3.3. Dosage des acides sulfoniques/Capacité d’échange ionique - 59

3.4. Conductivité protonique - 60

3.5. Caractérisation de la structure chimique - 60

III. CARACTERISATION INITIALE : Nafion® 112 / Nafion® 212-CS - 63

1. Propriétés mécaniques - 65

2.Hydrophilie - 66

2.1. Forme de l’isotherme de sorption - 68

2.2. Diffusion - 68

3. Capacité d’échange ionique - 70

4. Conductivité ionique - 70

5. Structure chimique - 70

5.1. Spectroscopie IR - 70

5.2. Résonance Magnétique Nucléaire (RMN) - 71

8

IV. RESULTATS EXPERIMENTAUX : Comparaison du vieillissement du Nafion® 112 et

du Nafion® 212-CS à 80°C, en enceinte climatique (80%RH) - 75

1. Propriétés mécaniques - 77

1.1. Nafion® 112 vieilli à 80°C, en enceinte climatique (80%RH) - 78

1.2. Nafion® 212-CS, en enceinte climatique (80%RH) - 79

1.3. Comparaison Nafion® 112 et Nafion® 212-CS vieillis à 80°C, 80%RH - 80

1.4. Conclusion sur les propriétés mécaniques - 81

2. Hydrophilie - 82

2.1. Evolution de la sorption du Nafion® 112 vieilli à 80°C en enceinte climatique

(80%RH) - 82

2.2. Evolution de l’hydrophilie du Nafion® 212-CS vieilli à 80°C en enceinte climatique

(80%RH) - 84

2.3. Comparaison du Nafion® 112 et du Nafion® 212-CS vieillis à 80°C et 80%RH - 86

3. Capacité d’échange ionique - 87

4. Conductivité ionique - 88

5. Structure chimique du Nafion® - 89

5.1. Spectroscopie IR - 89

5.2. Etude de la structure chimique du Nafion® par spectroscopie RMN - 93

6. Discussion : mécanisme de vieillissement - 96

7. Stabilité du Nafion® 212-CS et catalyse de la réaction de condensation - 100

V. RESULTATS EXPERIMENTAUX : Influence de l’humidité relative sur le vieillissement

du Nafion® 112 et 212-CS - 103

1. Propriétés mécaniques - 105

2. Hydrophilie - 108

2.1. Evolution de la sorption du Nafion® 212-CS à 80°C en étuve (0%RH) et en enceinte

climatique (80%RH et 95%RH) - 108

2.2. Influence du facteur d’exposition humidité - 110

3. Diffusion - 111

4. Capacité d’échange ionique - 113

5. Structure chimique du Nafion® - 114

5.1. Spectroscopie IR - 114

5.2. Etude de la structure chimique du Nafion® par spectroscopie RMN - 116

6. Discussion : Influence de l’humidité relative sur le mécanisme de vieillissement - 119

6.1. Mécanisme de vieillissement ionique - 120

6.2. Plastification du Nafion® - 124

9

VI. REACTION INVERSE DE LA CONDENSATION : HYDROLYSE DE

L’ANHYDRIDE - 125

1. Hydrolyse du Nafion® vieilli en présence d’eau liquide à 80°C - 127

2. Hydrolyse du Nafion® vieilli en milieu acide - 128

2.1. Structure chimique caractérisée par spectroscopie IR - 129

2.2. Influence de la concentration d’acide chlorhydrique - 131

2.3. Structure chimique caractérisée par RMN - 131

2.4. Hydrolyse en présence de catalyseur (acide) - 133

2.5. Propriétés : Rajeunissement du polymère - 135

3. Hydrolyse du Nafion® vieilli en milieu basique - 137

4. Conclusion - 138

CONCLUSION GENERALE - 141

ANNEXES - 147

Références bibliographiques - 165

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