Mechanical properties of catalyst coated membranes: A powerful indicator of membrane degradation in fuel cells

摘要

Mechanical durability of perfluorosulfonic acid (PFSA) ionomer membranes in polymer electrolyte fuel cells (PEFCs) is investigated in this thesis. This work contributes to a systematic characterization of the decay in mechanical properties of membranes and catalyst coated membranes (CCMs) that are subjected to controlled chemical and/or mechanical degradation mechanisms. During field operation of PEFCs, the membrane is subjected to a combination of chemical and mechanical degradation, resulting in the loss of mechanical integrity and ultimately leading to lifetime-limiting mechanical membrane failure. Accelerated stress tests (ASTs) were performed in this study in order to investigate the decay rate caused by each individual degradation mechanism, and to simulate the failure modes of field operated fuel cells. Mechanical degradation was studied using humidity cycling (in-situ) or mechanical fatigue stress (ex-situ). Chemical degradation was evaluated via open circuit voltage (OCV) or elevated voltage (in-situ) or Fenton’s reagents (ex-situ). Moreover, the combined chemical and mechanical degradations were also taken into account following recently developed protocols. In order to investigate the evolutions in mechanical properties during the degradations, different mechanical experiments were utilized including tensile, fatigue, thermal and hygral expansion, and creep tests in a wide range of hygrothermal conditions from the defined room conditions (23°C – 50% RH) to the fuel cell operating conditions (70°C – 90% RH) covering the expected range of operating conditions in PEFCs. Once the mechanical properties of the baseline membrane and CCM were characterized, the effect of each individual degradation mechanism was carefully investigated. Microstructural characterization techniques were also utilized in order to obtain supplementary evidences to the changes in mechanical properties. As a result, chemical degradation was revealed to be the dominant mechanism that controls the decay in mechanical properties of the PFSA membranes and can result in early stage mechanical failure in the presence of mechanical or hygrothermal stress. However, pure mechanical degradation was also recognized to be capable of creating membrane physical damage but at lower rates compared to chemical degradation mechanisms. Slight decay in mechanical properties of the 8,200 hours field operated CCMs was observed, indicating their relatively milder operating conditions when compared to the accelerated stress tests, and further suggesting that the membranes were still in rather good health after this amount of field operation. According to the outputs of this work, critical degradation routes on membrane mechanical stability were diagnosed and mitigation strategies were introduced in order to enhance the membrane mechanical durability and overall fuel cell lifetime.