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Novel Silicate-based Membranes for Direct Methanol Fuel Cells.

Felice, Valeria (2011). Novel Silicate-based Membranes for Direct Methanol Fuel Cells. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 208 p.

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The symbols and special characters used in the original abstract could not be transcribed due to technical problems. Please use the PDF version to read the abstract.

The symbols and special characters used in the original abstract could not be transcribed due to technical problems. Please use the PDF version to read the abstract. Direct methanol fuel cells (DMFCs) attract a lot of interest, due to the high energy density of methanol and the simple and quick refill of the device. However, the DMFC performance is limited by the methanol crossover through Nafion, the state of the art electrolyte. In this work zeolite-based materials are proposed as electrolyte for DMFCs. Zeolites are alluminosilicates with a well-ordered pore system which could act as a barrier to the passage of methanol. However, an improvement of their proton conductivity (~1O-4 S cm-1) is required to be competitive with Nafion (~0.1 S cm-1 at 25°C). Three commercial Faujasites (CBV600, CBV720 and CBV780) with different Si/Al ratios (3.4 to 48.7) and surface areas (530 to 823 m2 g-1) were used as starting materials. In order to increase the proton conductivity and to make them suitable to be used in a fuel cell assembly, Faujasites were functionalized with sulfonic acid groups and blended with polymers. The stability of Faujasite in acidic conditions was first tested by exposure to 6 mol dm-3 HCI for up to 7000 hours. Dealumination due to the acid exposure occurred within the first 24h but the zeolite structure remained intact. The dealumination induced secondary porosity and an increase of the water uptake at high relative humidity, which increased the proton conductivity. The functionalization was carried out by grafting of mercapto-silane and subsequent conversion of the mercapto- groups into sulfonic acid groups by H2O2 treatment. The effect of the silane precursor concentration, and of the chemical and textural characteristics of the parent zeolites on the extent of functionalization were evaluated. In spite of a significant decrease of specific surface area and water uptake, the proton conductivity increased from 2.2xlO-6 to 1 X 10-5 S cm-1 for the CBV600 and by a factor of 3 for the CBV720 and CBV780 series (up to 3.4xl0-4 S cm-1 ). The difference in the extent of the increase and in the conductivity values are attributed to a H+ dilution effect and to different specific surface areas and water uptakes. Finally, water and methanol sorption measurements on parent and functionalized samples showed that the surface functionalization improves significantly the zeolite selectivity in terms of water over methanol. Selected zeolites (as-received and functionalized with sulfonic acid groups) were used in the fabrication of two types of composite membranes: Nafion/CBV780 (0-40 wt%) and CBV780 (up to 60 wt.%) blended with non-ionomer binders (Teflon, PVDF, HDPE, SEBS). The Nafion/CBV780 composites with a zeolite loading below 2 wt%, show higher thermal stability, higher water mobility and proton conductivity compared to unfilled Nafion. An improvement of the DMFC perfonnance was also observed, although at high methanol concentration (10 mol dm-3) the effect of the methanol crossover through the composite membranes was still significant. Among the non-ionomer composites, SEBS/60% functionalized CBV780 composite showed higher proton conductivity (~10-3 S cm-1). Although an improvement is still required, important criteria for the achievement of this goal, related to the zeolite surface modification, selectivity, composite fabrication and the effect of zeolite on the water mobility, are established and presented in this work.

Type de document: Thèse
Directeur de mémoire/thèse: Tavares, Ana C.
Informations complémentaires: Résumé avec symboles
Mots-clés libres: piles; combustible; zéolithes
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 22 janv. 2013 16:38
Dernière modification: 18 nov. 2015 15:41
URI: http://espace.inrs.ca/id/eprint/717

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