Regular durability testing of heavy duty fuel cell systems for transit bus application requires several thousand hours of operation, which is costly and time consuming. Alternatively, accelerated durability tests are able to generate failure modes observed in field operation in a compressed time period, by applying enhanced levels of stress. The objective of the present work is to design and validate an accelerated membrane durability test (AMDT) for heavy duty fuel cells under bus related conditions. The proposed AMDT generates bus relevant membrane failure modes in a few hundred hours, which is more than an order of magnitude faster than for regular duty cycle testing. Elevated voltage, temperature, and oxidant levels are used to accelerate membrane chemical stress, while relative humidity (RH) cycling is used to induce mechanical stress. RH cycling is found to significantly reduce membrane life-time compared to constant RH conditions. The role of a platinum band in the membrane is investigated and membranes with Pt bands demonstrate a considerable life-time extension under AMDT conditions, with minimal membrane degradation. Overall, this research serves to establish a benchmark AMDT that can rapidly and reliably evaluate membrane stability under simulated heavy duty fuel cell conditions.

The mechanical stability of catalyst coated membranes (CCMs) is an important factor for the overall durability and lifetime of polymer electrolyte fuel cells. In this article, the evolution of the mechanical properties of degraded CCMs is comprehensively assessed. A combined chemical and mechanical accelerated stress test (AST) was applied to simulate field operation and rapidly generate partially degraded CCM samples for tensile and expansion experiments under both room and fuel cell conditions. The tensile results indicated significant reductions in ultimate tensile strength, toughness, and fracture strain as a function of AST cycles, accompanied by a mild increase in elastic modulus. The increased brittleness and reduced fracture toughness of the CCM, caused primarily by chemical membrane degradation, is expected to play an important role in the ultimate failure of the fuel cell. The expansion tests revealed a linear decay in hygrothermal expansion, similar in magnitude to the loss of mechanical strength. The decline in CCM sensitivity to environmental changes leads to non‐uniform swelling and contraction that may exacerbate local degradation. Interestingly, the hygrothermal expansion in the late stages of degradation coincided with the fracture strain, which correlates to in situ development of fractures in chemically weakened membranes.

A CeO 2 supported membrane electrode assembly (MEA) was fabricated by hot-pressing CeO 2 -coated electrodes and a PFSA ionomer membrane. Upon application of a combined chemical and mechanical accelerated stress test (AST), the CeO 2 supported MEA showed six times longer lifetime and 40 times lower fluoride emission rate than a baseline MEA without cerium. The membrane in the CeO 2 supported MEA effectively retained its original thickness and ductility despite the highly aggressive AST conditions. Most of the cerium applied on the anode migrated into the membrane and provided excellent mitigation of joint chemical and mechanical membrane degradation. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0081504eel] All rights reserved.Manuscript submitted January 13, 2015; revised manuscript received February 11, 2015. Published February 20, 2015 In a polymer electrolyte fuel cell (PEFC), it is imminent to achieve extension of membrane lifetime for enhancing durability and hence cost-competitiveness of the PEFC system. Hydroxyl radicals, generated from hydrogen peroxide through the Fenton reaction, 1 are known to be responsible for chemical degradation of perfluorosulfonic acid (PFSA) ionomer membranes used in PEFCs.2 One approach of mitigating the attack of hydroxyl radicals is to incorporate the Ce 3+ /Ce 4+ redox system as a regenerative radical scavenger into the membrane [3][4][5][6] or catalyst layers 7,8 which has been shown to reduce the fluoride emission rate during low humidity and open circuit voltage (OCV)-hold condition. Although uniform incorporation of Ce 3+ by ion-exchanging of protons represented the most powerful scavenging effect on the attack of hydroxyl radicals, 9,10 it can also introduce tradeoffs such as loss in high power performance due to the associated reduction in membrane conductivity.10 Moreover, cerium initially present inside a membrane was observed to migrate toward the catalyst layers during an accelerated stress test, where its mitigation function may not be preserved. 8 The objective of the present work is to demonstrate the effectiveness of cerium under combined chemical and mechanical membrane degradation, representative of the actual membrane degradation mechanism during field operation of PEFCs.Catalyzed gas diffusion electrodes (GDEs) were fabricated by coating a micro-porous layer onto a non-woven carbon paper gas diffusion layer (GDL) substrate, followed by coating a catalyst layer (CL) consisting of carbon-supported platinum catalyst and PFSA ionomer. A baseline MEA was prepared by hot-pressing a standard PFSA membrane with anode and cathode G...

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The application of highly charged anion exchange membranes is often limited by strong water sorption leading to excessive swelling and eventual dissolution, especially at elevated temperatures. The cross-linking of polymers has been shown to be an excellent mitigation strategy but this often restricts membrane and ionomer processing methods. Here, we explore the reaction, stability, and utility of a cross-linking agent for the recently discovered class of cationic polymers: methylated, C2protected poly(benzimidazolium)s. In situ reaction and formation of p-xylyl cross-linking groups is found to provide novel, highly functionalized, hydroxide-stable membranes and films with enhanced anion conductivities and superior mechanical properties compared to un-cross-linked polymers. The versatility of this strategy will be important in the design of polymers for the preparation of stable, hydroxide-conducting films, membranes, and ionomers.

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