No abstract

Relations between morphology and transport sensitively govern proton conductivity in perfluorsulfonate ionomers (PFSIs) and thus determine useful properties of these technologically important materials. In order to understand such relations, we have conducted a broad systematic study of H + -form PFSI membranes over a range of uniaxial extensions and water uptakes. On the basis of small-angle X-ray scattering (SAXS) and 2 H NMR spectroscopy, uniaxial deformation induces a strong alignment of ionic domains along the stretching direction. We correlate ionic domain orientation to transport using pulsed-field-gradient 1 H NMR measurements of water diffusion coefficients along the three orthogonal membrane directions. Intriguingly, we observe that uniaxial deformation enhances water transport in one direction (parallel-to-draw direction) while reducing it in the other two directions (two orthogonal directions relative to the stretching direction). We evaluate another important transport parameter, proton conductivity, along two orthogonal in-plane directions. In agreement with water diffusion experiments, orientation of ionic channels increases proton conduction along the stretching direction while decreasing it in the perpendicular direction. These findings provide valuable fodder for optimal application of PFSI membranes as well as for the design of next generation polymer electrolyte membranes.

No abstract

Perfluorosulfonic acid ionomers (PFSAs), such as Nafion®, have become the benchmark membrane material for proton exchange membrane fuel cells (PEMFCs). Despite their commercial success, little is known about the complex morphology-property relationships governing the chemical and physical properties. For example, the detailed structure of PFSA crystallites within the amorphous phase and the spatial arrangement of the crystallites around the ionic aggregates of PFSA materials are virtually unknown. In addition, the effect of processing on the membrane performance and durability in PEMFC applications is not yet fully understood. The first part of this work focuses on the significance of the crystalline component with respect to polymer-water interactions. The second part of this work addresses the impact on changes in morphology that could occur during long-term operation in a fuel cell including the effects of heat and humidity.

Poly(ethylene oxide) (PEO) containing pentaerythritol triacrylate (PETA) was electrospun to yield fiber mats that were subsequently crosslinked by exposure to UV irradiation. These mats were then supplemented with lithium perchlorate and swollen with ethylene/dimethyl carbonates to generate a structured polymer-gel electrolyte for Li-polymer battery applications. The fibrous mats were found to be composed of a nonwoven collection of ca. 1μm diameter PEO fibers with up to 85% porosity. Following UV exposure, the mats were rendered insoluble by chemical crosslinking, yielding gel fractions in excess of 80%. Upon incorporation of a liquid lithium ion electrolyte, the swollen mats were shown to maintain the fiberous framework and exhibit conductivities as high as 1.2 × 10-2 S/cm at room temperature. In contrast, a control mat of electrospun poly(vinylidene fluoride) containing fibers of comparable dimensions and identical electrolyte loading was found to yield a conductivity of only 5 × 10-3 S/cm.

No abstract

Elastomeric gaskets are commonly used between cells within a fuel cell stack to ensure that the reactant gases are isolated. Failure of a fuel cell gasket can cause the reactant gases to mix and can lead to failure of the fuel cell. An investigation of the durability of a hydrocarbon elastomeric seal material developed for proton exchange membrane fuel cells was performed by comparing the tearing energy required for crack propagation of as‐received and environmentally aged samples. Tear force was recorded as a function of crosshead displacement rate and the critical strain energy release rate was calculated and plotted against crack growth rate. Data obtained at different temperatures were then used to generate a fracture energy master curve. Additional samples were aged in selected relevant environments and compared to the as‐received material to study the effect of environmental aging on tear energy master curves. Comparison of tearing energy master curves for different test conditions showed an increase in the tearing energy for all aging environments. The tear energy master curve for testing carried out in water suggested that the crack propagation rates as low as of 10–8 ms–1 can be seen as the fracture energy approaches 80 Jm–2.