Electrochemical/Mechanical Coupling in Ion-Conducting Soft Matter

Kusoglu, Ahmet; Weber, Adam Z.

doi:10.1021/acs.jpclett.5b01639

Cited by 36 publications

(33 citation statements)

References 53 publications

(122 reference statements)

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“…19 The synergistic pathways for membrane degradation by the combined mechanical and chemical stressors have been summarized in Kusoglu and Weber. 22 In general, the most often observed mechanical/chemical synergistic pathway in a device level AST is via a cycle of four steps as explained by Kusoglu and Weber: "(i) the crossover of reactant gases, (ii) initiation of defects due to chemical decomposition, (iii) resulting failure initiation sites that serve as stress-concentration points under mechanical loads, and (iv) growth of defects under mechanical cyclical stresses leading to higher rates of gas crossover further accelerating all the subsequent processes (i → iv)". In this pathway, the mechanical stressor enhances the chemical stressor through the increased gas crossover by the macroscopic formation and growth of defects.…”

Section: F3218mentioning

confidence: 99%

“…Kusoglu and Weber suggested that there is a yet to be determined nanoscale pathway by which chemical degradation is accelerated by mechanical stress. 22 However, the cerium distribution maps show that the location of the severe chemical degradation also coincides with that of the most cerium depletion. Thus, further study is recommended to decouple cerium migration effects from possible mechanical/chemical synergistic degradation effects.…”

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confidence: 96%

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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%

mentioning

confidence: 96%

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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“…For the former, hydroxide radicals that are generated as byproducts of the electrochemical reactions during PEFC operation attack the ionomer's main chain or side-chain, thereby leading to its decomposition [4]. These two phenomena can also interact, [14] where chemical degradation changes the mechanical properties of membrane, making it fragile and vulnerable to deformation [13][14][15][16][17], and mechanical stresses could accelerate the rate of chemical degradation [18,19]. These two phenomena can also interact, [14] where chemical degradation changes the mechanical properties of membrane, making it fragile and vulnerable to deformation [13][14][15][16][17], and mechanical stresses could accelerate the rate of chemical degradation [18,19].…”

Section: Introductionmentioning

confidence: 99%

“…These two phenomena can also interact, [14] where chemical degradation changes the mechanical properties of membrane, making it fragile and vulnerable to deformation [13][14][15][16][17], and mechanical stresses could accelerate the rate of chemical degradation [18,19]. Thus, it is the coupled chemical-mechanical degradation that dominates the membrane's lifetime, [14] and therefore, current mitigation strategies focus on ionomers that are both chemically stable and mechanically durable. Thus, it is the coupled chemical-mechanical degradation that dominates the membrane's lifetime, [14] and therefore, current mitigation strategies focus on ionomers that are both chemically stable and mechanically durable.…”

Section: Introductionmentioning

confidence: 99%

Structure/property relationship of Nafion XL composite membranes

Shi

Weber

Kusoglu

2016

Journal of Membrane Science

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138

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Perfluorosulfonic-acid (PFSA) ionomer membranes (most commonly Nafion ® ) are currently the prototypical proton-exchange-membrane in polymer-electrolyte fuel cells (PEFCs), for which durability still represents a technical barrier to their commercialization. In an effort to address the durability demands, PFSA membranes with reinforcement and/or stabilizers have become of great interest as they have demonstrated superior durability in PEFCs compared to their unreinforced analogues. One such particular membrane that is tailored for enhanced durability and commonly employed in PEFCs is Nafion XL, a Nafion-based ionomer membrane with mechanical reinforcement and chemical stabilizers. Despite an increasing number of recent studies demonstrating its improved lifetime in accelerated stress testing (AST), its structure and transport properties have not been investigated in a systematic fashion. In this paper, we report water uptake, dimensional change, conductivity, and mechanical properties of Nafion XL membrane, as well as its strong anisotropy, in comparison to (unreinforced) Nafion 212 membrane.Moreover, water-domain spacing and crystallinity of Nafion XL membrane, determined from small-and wide-angle X-ray scattering (SAXS/WAXS) experiments, are correlated with the measured properties to establish a structure/property relationship, and discussed within the context of composite materials. It is also found that (pre)conditioning of the membrane by heating in water at different temperatures could have significant impacts on its structure/property relationship, in particular, the mechanical stability and conductivity, which were related to morphological changes observed from microscopy 2 studies. The findings reported here not only provide a new dataset that can be used for PEFC performance and durability modeling but also benefit the efforts on developing composite ionconductive membranes. 1.Highlights  The composite Nafion XL membrane's structure/property relationship is investigated  Impact of pretreatment on membrane's transport and mechanical properties are shown  Membrane's composite morphology are revealed through TEM, SEM and SAXS/WAXS

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“…15 Although the chemical degradation effect is beyond the scope of this paper, the model developed here to predict the mechanical behaviors of a membrane under in-situ conditions is a significant step forward. In the future, the modeling capability may be extended to capture the chemical stressors within the current mechanical modeling framework.…”

mentioning

confidence: 99%

Model-Based Analysis of PFSA Membrane Mechanical Response to Relative Humidity and Load Cycling in PEM Fuel Cells

Hasan

Goshtasbi

Chen

et al. 2018

J. Electrochem. Soc.

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The hydration/dehydration cycling of the membrane during the fuel cell operation and the resulting mechanical stress are in part responsible for the mechanical failures of a polymer electrolyte membrane. To thoroughly investigate the mechanical behaviors of the membrane under in-situ cyclic conditions, in this paper, we have interfaced a comprehensive two-dimensional transient fuel cell transport model with a viscoelastic-plastic membrane mechanical model. The transport model is used to produce the spatiotemporal profiles of membrane water content and temperature in an operating cell, which are then sent to the membrane mechanical model to calculate the mechanical parameters of interest. This provides the extended capability of studying the membrane mechanical response under in-situ conditions, which was not possible with the original mechanical model. The effects of cycling relative humidity and voltage and current at different temperatures on the membrane stresses are studied using the coupled model. It is found that the location of maximum in-plane tensile stress can vary significantly with the operating temperature during voltage cycling, whereas the highest in-plane compressive stress occurs under the land for all cases, particularly near the cathode. The simulation results confirm the need for coupling the two models to capture comprehensive transport phenomena in studying the membrane mechanical behaviors, and represent an important step toward improved understanding of various synergistic mechanical failure mechanisms that affect the membrane in an operating fuel cell. Mechanical failure of the polymer electrolyte membrane in the form of cracks, pin-holes and delamination has been identified as a limiting factor in the durability of the fuel cell. The hydration/dehydration cycling of the membrane during the fuel cell operation and the resulting mechanical stress are in part responsible for the mechanical failures.1,2 For example, the cyclic mechanical stresses have been shown to play a significant role in propagating a crack through the thickness of a membrane.3 Recent in-situ experiments have shown that membrane damage can be dominated by these cracks. 4 Since operation of a fuel cell causes hydration/dehydration cycling along with spatiotemporal variations of temperature that can influence the membrane response as well, 5 it is critical to thoroughly understand the mechanical behavior of the membrane under in-situ conditions, so that alleviating strategies focusing on the fuel cell operating conditions can be developed.Previously, a 2D viscoelastic-plastic membrane model of a representative fuel cell unit volume has been developed in ABAQUS and preliminarily validated. [6][7][8][9][10] In this prior work, the hydration states (water volume fraction) and temperatures at the interface of gas diffusion media and the gas channels are the model inputs. The spatiotemporal water transport in the electrodes and membrane is governed by diffusion only, without the in-situ features including electro-osmotic drag ...

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Electrochemical/Mechanical Coupling in Ion-Conducting Soft Matter

Cited by 36 publications

References 53 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

Structure/property relationship of Nafion XL composite membranes

Model-Based Analysis of PFSA Membrane Mechanical Response to Relative Humidity and Load Cycling in PEM Fuel Cells

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