Nafion((R))/poly(vinyl alcohol) blends: Effect of composition and annealing temperature on transport properties
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In this study, the transport properties (proton conductivity and methanol permeability) of Nafion 117, solution-cast Nafion, poly(vinyl alcohol) (PVA) and Nafion/PVA blend membranes were measured as a function of annealing temperature (60-250 C) and blend composition for application to the direct methanol fuel cell (DMFC). A Nafion/PVA blend membrane at 5 wt% PVA (annealed at 230 C) resulted in similar proton conductivity, but three times lower methanol permeability compared to Nafion 117. In addition, an unusual trend was observed in Nafion/PVA (50 wt% PVA) blend membranes, where proton conductivity remained relatively constant, but methanol permeability decreased by approximately one order of magnitude with increasing annealing temperature. Infrared spectroscopy reveals a band shift in the hydroxyl peak to higher wavenumbers in Nafion/PVA blends (25-90 wt% PVA) with increasing annealing temperature suggesting an increase in the interaction between the hydroxyl groups in PVA and the sulfonic acid groups in Nafion. For Nafion alone, proton and methanol transport rates increased and then decreased with increasing annealing temperature with a maximum at 210 C for both solution-cast and as-received (extruded) Nafion. This trend coincides with two transition temperatures observed by other investigators using differential scanning calorimetry, suggesting that transport properties are affected by morphological changes in Nafion. 2006 Elsevier B.V. All rights reserved.
