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Chapter 4 Nafion®/H-ZSM-5 composite membranes with superior performance for DMFCs 1. Introduction Direct methanol fuel cells (DMFCs) gain recently attention as candidates for mobile power sources to portable electronic devices [1-5] due to their high energy density and easy and significantly fast charging times. In order to increase the proton conductivity and decrease the methanol cross-over of the electrolyte membrane, zeolites can be used. Zeolites have two important properties: 1. they have very high water retention ability, which can only be eliminated at around 200°C and 2. they have molecular sieving properties, which can help to selective separations based on molecular size and shape [6-8]. Unfortunately, membranes made of pure zeolites suffer from the brittleness, fragility and defects [9]. Therefore composite membranes can be made using zeolites as fillers and a polymeric matrix as a host. If the zeolites are well dispersed in the matrix, they can serve as extra route for proton transport in the membrane in addition to the already existing water channels. This leads to increase of the membrane conductivity and at the same time, the tortuous pathway, created by the zeolites, can decrease the methanol cross-over. DuPont’s Nafion® is a good choice as a host material for the zeolites due to its excellent proton conductivity, mechanical strength and thermal and chemical stability [10-14]. In fact, researchers made already several attempts to prepare and characterize Nafion/zeolite composite membranes for DMFC applications [15-17]. In most cases the proton conductivity and methanol cross-over is reported without DMFC results. The ratio of proton conductivity and methanol permeability (often indicated as “β”) is then used to predict DMFC performance. As we showed in earlier study [18], very often β value has very low predictive power. In this study, H-ZSM-5 zeolites with various Si/Al ratios are prepared in house using microwaves-assisted heating. Then, they are incorporated into Nafion membranes at various loadings (1, 3 and 5 wt %). All composite membranes are tested with respect to swelling degree, proton conductivity and methanol permeability. Selected membranes with the highest proton conductivity and lowest methanol cross-over are tested fro several days in DMFC. For comparison, the performance of pure DE2020 (membrane prepared using DE2020 Nafion® dispersion) membranes are also investigated. 54PDF Image | Development of membrane materials for direct methanol fuel cells
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