Tear Resistance of Proton Exchange Membranes

Through the thickness flaws or “pinholes” in proton exchange membranes (PEM) allow gas crossover that can lead to fuel cell failure. The formation of these flaws is not fully understood, but one possible mechanism is that small flaws could grow through crack propagation in the fracture mechanics sense. Although relatively brittle features are sometimes observed in failures resulting under simulated fuel cell conditions, the stress strain plots of the membranes themselves exhibit considerable ductility. In an effort to use fracture mechanics principles to characterize PEMs, fracture parameters associated with the essential work of fracture from double edge notch tensile (DENT) specimens; the tear energy obtained from the trouser tear test; and cutting energies associate with knife slitting were measured and compared. Presumably through reducing crack tip blunting, the knife slitting test is able to measure fracture energies as low as 200J/m², two orders of magnitude smaller than measured in the other tests. The results are sensitive to rate, temperature, and moisture level. Although the implications of these properties to fuel cell durability are not yet understood, they may have applicability in the more brittle features that are sometimes observed.