Nafion®/poly(vinyl alcohol) blends:: Effect of composition and annealing temperature on transport properties

In this study, the transport properties (proton conductivity and methanol permeability) of Nafion (R) 117, solution-cast Nafion (R), poly(vinyl alcohol) (PVA) and Nafion (R)/PVA blend membranes were measured as a function of annealing temperature (60-250 degrees C) and blend composition for application to the direct methanol fuel cell (DMFC). A Nafion (R)/PVA blend membrane at 5 wt% PVA (annealed at 230 degrees C) resulted in similar proton conductivity, but three times lower methanol permeability compared to Nafion (R) 117. In addition, an unusual trend was observed in Nafion (R)/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 (R)/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 (R). For Nafion (R) alone, proton and methanol transport rates increased and then decreased with increasing annealing temperature with a maximum at 210 degrees C for both solution-cast and as-received (extruded) Nafion (R). 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 (R). (c) 2006 Elsevier B.V. All rights reserved.