Tracking the Water-Uptake Behavior of Fuel Cell Membranes during Accelerated Stress Testing

Proton Exchange Membrane (PEM) failure is one of the key criteria in deciding the durability of the PEM fuel cell system. Physical properties such as ionic conductivity and mechanical properties depend on the microstructure. The microstructure is itself strongly dependent on the water content of the membrane (1, 2). It is therefore important to fully understand the effect of degradation stressors on the morphological changes and water-uptake characteristics in order to improve membrane durability.

In this work, standard perfluorosulfonic acid (PFSA) ionomer membranes are subjected to a combined chemical and mechanical accelerated stress test (AST) used for rapid benchmarking of in-situ membrane stability. The degradation stressors in the combined chemical/mechanical AST was previously shown by our group to change the molecular structure and lead to membrane material loss (3). 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. The observed structural changes are anticipated to alter the water-uptake properties of the membrane, as shown previously for ex-situ testing (4). The degradation also alters the morphology, ion exchange capacity, mechanical properties, and proton conductivity of the membrane (5).

 Water-uptake measurements of partially AST degraded catalyst coated membrane (CCM) samples are carried out at room temperature using a dynamic vapor sorption system. In spite of severe fluoride release and membrane thinning, the water-uptake per unit mass of the AST degraded CCMs is found to be relatively constant. Since the test specimens are CCMs which contain a significant amount of catalyst layer material, an attempt is made to measure and separate the mass of the ionomer by thermogravimetric analysis (TGA). The ionomer material loss throughout the AST process is significant due to fluoride release and  ionomer degradation, up to roughly 50% at end-of-life (3). The water-uptake per unit mass of ionomer in the CCM is determined by extracting the ionomer weight obtained by heating the CCM in the TGA chamber. In contrast to the previous results normalized by specimen dry weight, the water-uptake increases significantly per unit mass of the ionomer in the CCM, as shown in Fig. 1. Although the increased water-uptake may indicate enlarged solvated hydrophilic domains in the membrane, which is good for enhanced proton mobility, the proton conductivity is found to decrease (Fig. 1). This may be attributed to a decay in proton concentration due to sulfonic acid functional group loss during side chain degradation. The overall effects of the combined chemical/mechanical AST on the membrane morphology and properties are presented and compared to provide additional insight into the complex degradation mechanism.

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. BPS is also acknowledged for providing access to experimental facilities, material samples and technical support.

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