Reduced in-plane swelling of Nafion by a biaxial modification process

A biaxial modification process to reduce the in-plane swelling of perfluorosulfonic acid polymers is demonstrated for Nafion 212 membranes. The process is based on the one-way shape memory effect of the polymer, which can be installed by drying under constrained conditions and observed by water swelling. The through-plane/in-plane swelling ratio at 30 °C for treated Nafion 212 increases from 1.4 to 5.8 (ex situ) and ≈25 (after humidity cycling in a fuel cell). Young modulus and tensile strength increase. X-ray diffraction analysis shows an increase of crystallinity. The influence on through- and in-plane proton conductivity is studied. Finally, fuel cell tests are presented. The results suggest that the process enhances the durability of fuel cell systems under changing humidity conditions. A one-way shape memory effect is used to change the direction of water-induced membrane swelling from in-plane to through-plane. Because membranes are clamped between bipolar plates in a fuel cell, the shape memory effect is re-established when membranes dry. This prevents the membrane expanding and forming wrinkles, and therefore could significantly increase the lifetime of fuel cells.