Probing the Morphological Changes of in-Situ Degraded Membranes in Polymer Electrolyte Fuel Cells

Membrane durability is one of the key criteria to be met to successfully commercialize polymer electrolyte fuel cell (PEFC) technology. Commonly used perfluorosulfonic acid (PFSA) ionomer membranes are prone to early degradation under the influence of common degradation stressors encountered in PEFCs. The important functional properties of PFSA membranes such as ionic conductivity and mechanical properties depend on its microstructure due to its unique hydrophobic backbone and side chains with hydrophilic end groups (1). Severe molecular changes to the PFSA membrane structure have been reported due to chemical and mechanical membrane degradation. Such changes in the molecular structure were found to affect thermal stability, water uptake, proton conductivity, and mechanical properties of the membrane; as well as the performance and durability of the fuel cell as previously published by our group (2-5). Although the effects of molecular changes on the degraded membrane characteristics have been explored, the impact on its internal morphology is generally not known. The present objective is therefore to understand the effect of degradation stressors on the morphological changes in order to improve membrane durability.

In this work, standard PFSA membranes were subjected to a combined chemical and mechanical accelerated stress test (AST) used for rapid benchmarking of in-situ membrane stability. The chemical phase of the AST generates hydroxyl radicals that attack both the side chain and main chain of the polymer, while the mechanically generated stresses due to humidity cycling accelerate mechanical failures. Beginning-of-life (BOL) and end-of-life (EOL) membrane samples extracted after in-situ degradation were imaged using transmission electron microscopy (TEM). In order to enhance image contrast, the sulfonic acid end group sites in the membrane were selectively exchanged with Pb ions by soaking it in saturated lead acetate solution. The samples embedded in epoxy resin were sliced to thin films (~70-90 nm) using ultra microtome and collected on a Cu grid for imaging. The hydrophilic regions in the Pb exchanged membrane appear as dark spots in the TEM images due to enhanced electron scattering, while the hydrophobic backbone dominated regions appear bright. The BOL membrane micrograph is presented in figure 1(A), exhibiting a regular phase separated morphological layout typical for PFSA ionomer membranes. Since the chemical degradation operates at random locations through out the membrane and commences with ionomer side chain cleavage (2-3), the degraded regions at the nanoscopic level exhibited low concentration of dark spots and the image brightness becomes non-uniform. (figure 1(B)). The lower concentration and absence of dark spots in the degraded region is expected to resemble the absence of ion attractive sulfonic acid groups at that site due to side chain degradation. Also, the dark spots tend to align in a long range order as a result of combined chemical/mechanical degradation. Apart from the severely degraded regions, mildly degraded regions which exhibit BOL morphology were also seen while scanning the membrane at EOL. The images of cracks (not shown here) recorded at EOL reveal that cracks preferentially propagate along the region of lowest concentration of dark spots which represent depletion of SO₃^-in the ion-rich hydrophilic domains.

Elemental mapping of the degraded zone was carried out by selectively capturing the generated x-rays with characteristic energy due to electron interacting with the sample.

The mapping of fluorine in the degraded zone revealed its absence at the most degraded sites (figure 1(C)). This can be correlated to the severe fluoride release from the membrane as a result of combined chemical/mechanical degradation.  Overall, the observed effects of combined chemical/mechanical degradation on the membrane morphology contributes to the fundamental understanding of the complex membrane degradation mechanism occurring during PEFC operation.

ACKNOWLEDGMENTS

Research funding provided by Automotive Partnership Canada (APC), Natural Sciences and Engineering Research Council of Canada (NSERC) and Ballard Power Systems (BPS) is gratefully acknowledged. We also thank BPS for providing material samples and Chan Lim for carrying out AST experiments.

REFERENCES

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