(Invited) Durability of Polymer Electrolyte Membranes for Automotive Applications
Authors: Gittleman Craig S , Lai Yeh-Hung , Coms Frank D. 
Published: February 2020

There are three critical membrane degradation mechanisms that can lead to failure of polymer electrolyte fuel cell systems: chemical, mechanical and thermal degradation. Chemical and mechanical degradation can occur during normal fuel cell operation whereas thermal degradation should only occur at extreme temperatures such as those that can be induced by membrane shorting. This talk will focus on the individual failure modes; with attention dedicated to describing the fundamental model-based mechanistic understandings, appropriate accelerated stress tests that enable rapid screening and relevant in-situ diagnostics to track membrane health during fuel cell operation, as well as discussions of effective mitigation strategies to prevent or minimize the risk of failure caused by the specific modes of membrane degradation with focus on the effect of simultaneous chemical and mechanical degradation.

Both chemical and mechanical degradation of perfluorosulfonic acid (PFSA) membranes have been extensively studied, and effective mitigation strategies have been developed that can significantly extend PFSA proton exchange membrane (PEM) lifetimes. For example, porous polymer supports such as (expanded polytetrafluoroethlene) ePTFE can be used to make PFSA membrane which exceed US Department of Energy mechanical durability requirements and hydroxyl radical scavenging additives such as Ce3+ and Mn2+ can enable PFSA membranes to meet chemical stability requirements. Accelerated stress tests (ASTs) have been developed that can be used to screen various membrane types and mitigation strategies, and effective in-situ diagnostics have been developed to track degradation of PFSA membranes during fuel cell operation. The failures observed in these accelerated tests as seen by postmortem analysis mimic the failures of PFSA membranes in field tests. A new highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. The HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the gas outlets. HASTs were conducted using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a state-of-the-art robust membrane containing both an ePTFE mechanical support and a Ce-based chemical stabilizing additive. Result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. Further results will be presented that decouple the cerium migration effect from the mechanical/chemical synergistic degradation effect.