(635a) Stretched Perfluorosulfonic Acid Membranes for Hydrogen/Air and Direct Methanol Fuel Cells

To develop a durable and high performance proton exchange membrane for hydrogen/air and direct liquid methanol fuel cell applications, we have systemically examined uniaxial stretching effects on perfluorosulfonic acid (PFSA) polymers. PFSAs represent a family of commercially available polymers with a perfluorinated backbone and side chains that terminate in hydrophilic sulfonic acid groups. Three PFSA materials were studied: DuPont’s Nafion^® (with a long and branched side chain of −O–CF₂CFCF₃–O–(CF₂)₂–SO₃H), 3M Company PFSA (a short-side-chain version of Nafion with –O–(CF₂)₄–SO₃H pendant groups), and Aquivion^® from Solvay Solexis (with a very short side chain of –O–(CF₂)₂–SO₃H). The effect of uniaxial stretching on the crystallinity and fuel cell properties of these polymers was probed, where solution-cast membranes were stretched at a temperature above the polymer’s α-transition temperature, followed by a polymer annealing step.

Nafion 1100EW, 3M 1000EW and 825EW, Aquivion 830EW PFSA show an increase in crystallinity upon uniaxial stretching, with a decrease in methanol permeability and no change in proton conductivity. A direct methanol fuel cell employing a stretched 3M 825 EW PFSA membrane with a draw ratio (DR) of 4, for example, exhibits a 31% improvement in power density at 0.4V, as compared to a fuel cell with a Nafion 117 (1100 EW) membrane. The higher power output is associated with lower methanol crossover.

Interestingly, 3M’s 733 EW PFSA polymer has no crystallinity before or after stretching. The nanostructure of ionic domains, however, varied with draw ratio. From wide-angle x-ray diffraction studies, the ionomer peaks of a stretched membrane shift to higher values of 2θ and a new ionomer peak appears when the draw ratio is greater than 4. For a stretched 733 EW membrane (DR=4), there is a 120% increase in the storage modulus with a proton conductivity equal to that of an unstretched film. This combination of properties makes stretched 3M 733 EW PFSA a promising PEM material for a hydrogen/air fuel cell. From an open circuit voltage humidity cycling fuel cell test at 80°C, for example, it was found that the lifetime of a stretched 3M 733 EW PFSA membrane (DR=4) was extended by a factor of 3.5, as compared to that of an unstretched membrane.