A Mechanistic Study of Perfluorosulfonic Acid Membrane Water Permeance Degradation in Air

Chemical degradation of perfluorosulfonic acid (PFSA) membranes of proton exchange membrane (PEM) fuel cells is a well-known process which can limit PEM fuel cell lifetimes (1, 2). Chemical degradation can weaken membranes to the point where they no longer provide an effective barrier to reactant gas permeation or electrical shorting. PEM chemical degradation is the result of a complex interplay between mechanical and chemical degradation processes, both of which are strong functions of operating conditions. In particular, hot and dry operating conditions induce high rates of chemical degradation whereas relative humidity cycling between wet and dry states challenges the mechanical integrity of the membranes.

Because chemical degradation of PEMs is accelerated under low humidity operating conditions, membrane lifetimes are significantly enhanced by providing humidified inlet gas streams to the fuel cell stack.  In addition, fuel cell performance is improved with the use of humidified input streams. An efficient method of humidifying the cathode inlet stream employs a passive water vapor transfer (WVT) device that delivers water vapor from the wet cathode exhaust to the cathode inlet stream (Figure 1). The key component of a WVT device is a highly water permeable, but gas impermeable membrane. Not surprisingly, ionomeric membranes such as PFSAs function as excellent WVT materials.

Ideally, WVT membrane materials will maintain adequate durability and performance characteristics to provide necessary stack humidification throughout the target operational life of an automotive fuel cell system (>10years />8000 operational hours). Several general pathways known to reduce water permeance values include cation contamination, mechanical degradation and chemical degradation. We have observed during in-situ and ex-situ testing of WVT materials in air that the water permeance values of PFSA-based WVT membranes can degrade by 75% over time. This degradation of water permeance is associated with a physical and chemical change of the membrane.

The extent of ionomer transformation is readily monitored using transmission FTIR spectroscopy by the appearance of new absorption bands centered at 3200 and 1434 cm^-1 and also by the decreases in water stretching and bending bands centered near 2800 and 1700 cm^-1, respectively (Figure 2). In addition to water permeance, other membrane properties including proton conductivity and ion exchange capacity decrease as a function of this degradation reaction. The fully reversible, deuterium exchanged FTIR spectrum proves that the degradation product is not the cross-linked sulfonic acid anhydride as previously suggested (3). Through an extensive series of kinetic studies and analytical characterization, we have determined that the membrane degradation product results from a reaction between the ionomer and a trace atmospheric gas. We will demonstrate that the definitive structural assignment is fully consistent with all known properties and behaviors of the degraded membrane. The revelation of the PFSA degradation product and its formation mechanism will aid in development of highly durable fuel cell systems.

References

 

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