(609e) A Theoretical Analysis of Mechanical Properties of Fiber Reinforced Nafion Membranes

PEM fuel cells (PEMFCs) typically consist of perfluorinated sulfonic acid (PFSA) polymers or its derivatives, of which a good example is Nafion®. However, native PFSA membranes do not offer sufficient mechanical strength to meet the durability requirements for PEMFCs and new hybrid membranes with improved mechanical strength are being developed. One approach, as adopted by the Asahi Glass Co, has been to dope the native PFSA membrane with polytetrafluoroetylene (PTFE) fibrils with diameters of order a few microns. A clear understanding of the extent to which fibrils reinforce fuel cell membranes and how they affect water absorption, a critical factor that determines performance, is not available. This work is an attempt at developing a theoretical tool for bridging that gap.

We propose a model based on slender body theory (SBT), an approach that has been very successful in predicting the flow properties of fiber suspensions and mechanical properties of fiber reinforced elastic media. The basic principle behind the slender body approximation is to obtain asymptotic solutions in two different regimes ? an inner regime valid for points lying close to the fiber axis, and an outer regime valid for points that are far away from the fiber. The unknown coefficients in these solutions are determined by matching the inner and outer solutions in the intermediate regime, where both are valid. We present results for spatial variations in the water volume fraction in the presence of the fibrils. We also study how the presence of fibrils affects the amount of water absorbed for varying membrane elastic properties. The theory can be extended towards understanding the mechanical properties of other commercially available hybrid membranes.