Development and characterization of new proton-conducting composite membranes for fuel cells
Élaboration et caractérisation de nouvelles membranes composites à conduction protonique pour les piles à combustible
Résumé
The perfluoro-sulfonated ionomer membranes are employed as separators in many industrialapplications such as fuel cells, chloro-alkali industry, electrodialysis and gaining inclininginterest in aqueous rechargeable or redox-flow batteries where the knowledge of their ionictransport and transfer properties is fundamental.Particularly, Nafion is adopted as a referencemembrane for polymer electrolyte membrane (PEM) fuel cells due to its thermal stability andgood proton conductivity. However, Nafion membranes have several disadvantages such as a decrease in the proton conductivity at low relative humidity (<50%) and high temperatures(>80°C), and excessive dimensional changes due to the swelling/deswelling, leading tomechanical instabilities.To circumvent these problems, novel proton conducting membraneshave been developed, either by completely replacing or by using organic and/or inorganiccomponents to Nafion.3 In this regard, a large spectrum of membranes have been elaboratedconsidering many attributes such as high proton conductivity, physical separation between theanode and the cathode and fuel barrier characteristics, good chemical and physical stability andlow elaboration cost of the membrane. Two types of additives were examined to improve the performances, particularly : Membranes based on Nafion with Chitosan biopolymer. This naturel polymer is consideredas the second most abundant polysaccharide after cellulose.6 Chitosan improves the physical andchemical stability of the membrane in the presence of water, and it is considered as a less costlyadditive to Nafion7.The improvement of the proton conductivity with pristine chitosan isessentially challenging. Previous studies demonstrated that vehicularandGrotthuss mechanismjointly govern the proton transfer in chitosan membranes.In the vehicular mechanism, the protons diffuse together with solvent molecules in the form of hydronium ions byforming acomplex such as H5O2+ and H9O4+. In the Grotthuss mechanism, however, the protons jump fromone solvent molecule or functional group to the next by the continuous formation and breakingof hydrogen bonds. Membranes based on Nafion with Halloysite nanotubes (HNT). These clays confer to themembrane high proton conductivity by constructing large and continuous conductionpathways.These inorganic additives also improve the thermal and mechanical properties of PEM. Composite membranes of Nafion/Chitosan- SO3H and Nafion/HNT-SO3H are prepared. Theresulting composite membranes were studied by various conventional structural characterizationtechniques. H+ conductivity measurements were performed and the values obtained are higherthan those of pristine Nafion at various relative humidity (RH%) levels and temperatures (30°C-80°C). Our results highlight the beneficial character of functionalized chitosan biopolymer andHalloysite nanotube clays as additives to improve PEM performances.
Le Nafion a été considéré comme électrolyte modèle pour les piles à combustible (PAC), grâce à sa stabilité thermique et chimique ainsi que sa bonne conductivité protonique. Cependant, la conductivité protonique du Nafion se détériore à des taux d’humidité < 50% et à des températures >80°C. Pour cette raison de nouvelles membranes hybrides ont été élaborées afin d’en améliorer les performances. Plusieurs pistes ont été envisagées comme par exemple i) Membranes à base de chitosane, considéré comme le second polysaccharide le plus abondant après la cellulose. Ce polymère naturel permet d’assurer la stabilité physique et chimique de la membrane en présence d’eau, sans oublier son coût de revient qui reste moins cher en comparaison avec celui du Nafion et ii) Membranes à base de Nafion et d'argiles fibreuses (HNT), ces dernières confèrent à la membrane une conductivité protonique élevée en construisant des voies de transfert larges et continues. Cela permet aussi d'améliorer les propriétés thermiques et mécaniques des PEM. Notre étude est basée sur l'élaboration de membranes composites, nafion, chitosane et HNT. Des mesures de conductivité ont été entreprises et les valeurs obtenues comparées à celles du nafion. Des mesures d'ac-électrogravimétrie ont aussi été entreprises afin de mieux aborder les mécanismes de conduction.
Domaines
Polymères
Origine : Version validée par le jury (STAR)
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