Control of Nafion/Poly(vinylidene fluoride-co-hexafluoropropylene) Composite Membrane Microstructure to Improve Performance in Direct Methanol Fuel Cells

Abstract

The structure of Nafion/poly(vinylidene fluoride--hexafluoropropylene) composite membranes was controlled by solvent casting under an applied electric field. The field-induced structural changes caused the membranes to display an enhanced water uptake, proton conductivity, and methanol permeability in comparison to solvent cast membranes prepared without an applied field. Although both methanol permeability and proton conductivity are enhanced by the applied field, the permeability increase is relatively lower than the proton conductivity improvement, which results in enhanced proton/methanol selectivity and improved direct methanol fuel cell (DMFC) performance. The experimental evidence suggests that the ordered arrangement of Nafion domains at the micrometer scale and the limiting swelling of Nafion at the nanometer scale are two main reasons for the improved selectivity. The electric-field-processed membranes show comparable or better performance than commercial Nafion 117 in DMFCs, depending on the operating conditions.