Characterization of the Fracture Energy of a PFCB/PVDF Polymer Electrolyte Fuel Cell Membrane Using a Knife Slit Test

Polymer electrolyte membranes (PEM) undergo hygrothermal stress cycling in an operating fuel cell which may lead to pinhole or crack formation and propagation resulting in membrane failure. The fracture energy of a material, measured by fracture tests, is the energy needed for a crack to propagate throughout the material. In this study, the fracture energy of a promising novel fuel cell membrane comprised of a blend of a sulfonated perfluorocyclobutane (PFCB) block copolymer and polyvinylidene fluoride (PVDF) is investigated in various environmental conditions using a knife slit test. Fracture energies determined using the knife slit test have been shown to be several orders of magnitude lower, and therefore closer to the intrinsic fracture energy of a material, than those found by other fracture tests of related membranes. It is believed that the intrinsic fracture energy can give insight into the fracture resistance and durability of the polymer blend membrane. A polymer blend of 70% PFCB and 30% PVDF was tested at dry and nominally 10% relative humidity conditions at 40, 70, and 90°C, as well as at 70°C and nominally 50% relative humidity, to assess the effect of environmental conditions on fracture energy. Results show that the PFCB/PVDF blend had comparable fracture energy to a baseline fuel cell material, Nafion® NRE 211. In addition, the fracture energy of the blend was found to lie between that of the PFCB and PVDF components.