Cerium Migration through Hydrogen Fuel Cells during Accelerated Stress Testing

Stewart, S. Michael; Spernjak, Dusan; Borup, Rodney L.; Datye, Abhaya K.; Garzon, Fernando

doi:10.1149/2.008404eel

Cited by 72 publications

(83 citation statements)

References 9 publications

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“…Further complicating the effective AST design is the need to consider the cation mobility 9,20,25,26 in chemically robust membranes using cerium or manganese as radical scavengers. Since cation mobility is strongly dependent on the humidification conditions during the test, 20 it is important that the humidification conditions of the AST also properly mimic the conditions in the field.…”

Section: F3218mentioning

confidence: 99%

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

Lai

Rahmoeller

Hurst

et al. 2018

J. Electrochem. Soc.

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A highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. Using a 50-cm 2 cell cycled between 0.05 and 1.2 A/cm 2 with a low inlet RH in the co-flow configuration, 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 outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a robust membrane containing both a mechanical support and a chemical stabilizing additive, the 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. One of the key challenges facing the commercialization of automotive fuel cells is the development of membrane electrode assemblies (MEAs) that can meet durability targets. In proton exchange membrane (PEM) fuel cells, the PEM serves to conduct protons from the anode to the cathode of the fuel cell while simultaneously insulating electronic current from passing across the membrane as well as preventing crossover of the reactant gases, H 2 and O 2 . State-of-the-art PEM fuel cells for high power density operation utilize perfluorosulfonic acid (PFSA) membranes that are typically no more than 25 μm thick. To be viable for automotive applications, these membranes must survive 10 years in a vehicle and 8000 hours of operation including transient operation with start-stop and freeze-thaw cycles. Fuel cells cannot operate effectively when gas can permeate the membrane through microscopic pinholes or excessively reduced thickness. Ultimately, fuel cells can fail because such excessive crossover leaks develop and propagate within the polymer membranes. Fuel cells can also fail if electronic current passes through the membranes and causes the system to short. It is thus critical that these membranes are sufficiently robust to thinning, cracking and thermal decomposition over the range of conditions experienced during fuel cell operation. In automotive fuel cell systems, there are three primary root causes of membrane failure: (1) Chemical degradation: polymer decomposition caused by the direct attack on the polymer from radical species generated as byproducts or side reactions of the fuel cell electrochemical reactions; (2) Mechanical degradation: membrane fracture caused by cyclic fatigue stresses imposed on the membrane via hygrothermal fluctuations in a constrained cell; and (3) Thermal degradation: membrane degradation caused by ohmic heating thr...

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Section: F3218mentioning

confidence: 99%

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

Lai

Rahmoeller

Hurst

et al. 2018

J. Electrochem. Soc.

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“…CeO x , MnO x ) into the membrane electrode assembly (MEA). [3][4][5][6]10 Specifically, it has been shown that by incorporating CeO x into the MEA, membrane degradation (as measured by fluoride ion release) can be reduced by ∼1000 x (vs. ∼ 100 x for MnO x ). 5 While these results are highly promising, recent work by our group has indicated that during accelerated stress testing (AST), these metal oxides will dissolve, and the resulting cations will transport to the cathode catalyst layer (CCL) where they bind strongly with the -HSO 3 groups in the ionomer.…”

mentioning

confidence: 99%

Effect of CeOx Crystallite Size on the Chemical Stability of CeOx Nanoparticles

Banham

Cheng

et al. 2014

J. Electrochem. Soc.

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CeOx is an excellent free radical scavenger to improve polymer electrolyte membrane durability. However, this metal oxide will dissolve during accelerated stress testing (AST), with the resulting cations transporting to the cathode catalyst layer (CCL) leading to performance reduction/degradation of the PEMFC. Controlling the rate of CeOx dissolution is therefore of great importance, as it may be possible to maintain sufficient Ce cations for free radical scavenging while minimizing the impact of these cations on the CCL. Here the effect of CeOx crystallite size on CeOx dissolution was investigated. Three CeOx additives were prepared having crystallite sizes of 6, 13, or 25 nm. An ex-situ method was used to evaluate the chemical stability of these three CeOx samples, as well as one commercially available CeOx. It was determined that surface area, rather than crystallite size, is the best predictor of chemical stability. In-situ membrane electrode assembly AST cycling was then performed, demonstrating that when low loadings of CeOx (0.006 mg/cm 2 ) are used, the ex-situ method correctly predicts trends in end of life (EOL) performance. Finally, it is shown that increasing the anode RH during AST cycling leads to significantly higher EOL performance losses. Fuel cells are electrochemical devices capable of efficiently converting chemical energy into electrical energy while producing very little pollutants (e.g. NO x , SO x , particulates).1 Due to their higher efficiency and cleaner operation vs. conventional internal combustion engines, fuel cells have become highly promising candidates for clean power generation. Much like a battery, PEMFCs contain an anode, a cathode and an electrolyte. The electrolyte dictates the required operating conditions of the fuel cell (temperature, pH, etc.), and thus fuel cells are classified according to the electrolyte that they employ. Currently, one of the most promising candidates for stationary, mobile, and automotive applications is the proton exchange membrane fuel cell (PEMFC), which utilizes a perfluorinated sulfonic acid (PFSA) polymer membrane (typically Nafion). These membranes must be humidified with liquid water in order for facile proton conduction to occur, and thus PEMFCs typically operate at relatively low temperatures (60-85• C). While PFSA membranes are capable of achieving high proton conductivities (∼10 S/m), they suffer from several modes of degradation during normal operating conditions. 2 Of particular concern are the highly reactive free radicals (OH . , OOH . , H . ) that are formed from the decomposition of H 2 O 2 (an intermediate in the oxygen reduction reaction (ORR)) within the membrane.3-5 While H 2 O 2 alone is not particularly damaging, the presence of even trace amounts of Fenton's catalyst (e.g. Fe 2+ /Fe 3+ ) greatly accelerates the decomposition of H 2 O 2 into aggressive free radicals.6-8 These free radicals are known to attack PFSA membranes through an "unzipping" mechanism in which decomposition is initiated at one end of a PFSA unit and con...

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“…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.…”

mentioning

confidence: 99%

“…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.…”

mentioning

confidence: 99%

“…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.…”

mentioning

confidence: 99%

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Fuel Cell Durability Enhancement with Cerium Oxide under Combined Chemical and Mechanical Membrane Degradation

Lim

Alavijeh

Lauritzen

et al. 2015

ECS Electrochemistry Letters

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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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Cerium Migration through Hydrogen Fuel Cells during Accelerated Stress Testing

Cited by 72 publications

References 9 publications

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

Effect of CeOx Crystallite Size on the Chemical Stability of CeOx Nanoparticles

Fuel Cell Durability Enhancement with Cerium Oxide under Combined Chemical and Mechanical Membrane Degradation

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