Modified Nafion as the Membrane Material for Direct Methanol Fuel Cells
Jun Lin1, Ryszard J. Wycisk1, J. Lee1, Peter N. Pintauro2, and Michael Kellner3. (1) Case Western Reserve University, Department of Chemical Engineering, Cleveland, OH 44106-7217, (2) Department of Chemical Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7217, (3) Hawken School, Gates Mills, OH 44040
Nafion® is widely studied in PEM fuel cells, but its high cost and high methanol crossover have spurred research on alternative membrane materials (e.g., sulfonated polysulfones, polyketones, and polyphosphazenes). Another approach is to improve the performance/properties of Nafion by blending it with another polymer or by modifying its microstructure. In the present talk, two modified Nafion membrane systems for direct methanol fuel cells are described: (1) blends of Nafion with an inert fluoropolymer (FEP-Teflon®) and (2) stretched recast Nafion. Nafion/FEP blends were prepared by melt-mixing (using Nafion precursor in the sulfonyl fluoride form), followed by hot pressing to form a defect-free film, and then hydrolyzing the precursor groups. Stretched recast Nafion membranes were prepared from DMAc solvent, where uniaxial film stretching was carried out prior to annealing. The resulting membranes, 50-100 μm in thickness, were characterized in terms of proton conductivity and methanol permeability. Methanol fuel cell tests were performed with 1.0-10.0 M methanol and ambient pressure air. The Nafion-FEP blended membranes contained 3.5-9 times less perfluorosulfonic acid polymer as compared to Nafion 117 and exhibited good mechanical and electrochemical properties. Their fuel cell performance (maximum power density) was superior to that of Nafion 117 with 1.5-times lower methanol crossover. Uniaxial stretching of recast Nafion had no effect on proton conductivity but it did lower methanol permeability. After annealing, morphological changes in recast/stretched Nafion were permanent. Stretched Nafion films worked very well in a direct methanol fuel cell at 1.0 M methanol, with lower crossover as compared to Nafion 117 and high power output.