Low Temperature PEM Degradation: Failure Modes & Mitigation Strategies

Abstract

There are three critical membrane degradation mechanisms that can lead to failure of polymer electrolyte fuel cell systems: chemical degradation, mechanical degradation, and 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. The effect of simultaneous chemical and mechanical degradation will also be discussed.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. 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. Durability of PFSA membranes will be discussed and an overview of the degradation of alternate PEM chemistries, such as hydrocarbon membranes, will also be provided. Insight into the degradation of these materials is limited because there is hardly any field data which can be used to determine the root causes of failure and validate that the ASTs are accelerating the relevant stressors and, also, because appropriate diagnostics, such as those used for PFSA membrane systems, have not been developed.This talk will also address shorting of PEMs in fuel cell systems. Shorting is a two step process during which soft shorts are created by penetration of conductive materials though the thickness of the membrane. Soft shorts become hard shorts when there is a voltage excursion that leads to high local temperatures and subsequent thermal decay of the PEM. These hard shorts are the severe events that lead to fuel cell stack failure. Results and analysis suggest that the PEM type is not the limiting factor in preventing membrane shorting, and that mitigation is best achieved by a combination of design and operating strategies.