Effect of Mechanical Compression on Chemical Degradation of Nafion Membranes

Kusoglu, Ahmet; Calabrese, Michelle A.; Weber, Adam Z.

doi:10.1149/2.008405eel

Cited by 46 publications

(71 citation statements)

References 33 publications

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“…Due to the synergy of the chemical and mechanical degradation modes, humidity cycling results in larger FERs and PEM thinning than constant RH tests, alone. 4,7 In all ASTs, anode and cathode FERs were observed to be uniform.…”

Section: Resultsmentioning

confidence: 90%

Cerium Migration during PEM Fuel Cell Accelerated Stress Testing

Baker¹,

Mukundan²,

Spernjak³

et al. 2016

J. Electrochem. Soc.

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Cerium is a radical scavenger which improves polymer electrolyte membrane (PEM) fuel cell durability. During operation, however, cerium rapidly migrates in the PEM and into the catalyst layers (CLs). In this work, membrane electrode assemblies (MEAs) were subjected to accelerated stress tests (ASTs) under different humidity conditions. Cerium migration was characterized in the MEAs after ASTs using X-ray fluorescence. During fully humidified operation, water flux from cell inlet to outlet generated in-plane cerium gradients. Conversely, cerium profiles were flat during low humidity operation, where in-plane water flux was negligible, however, migration from the PEM into the CLs was enhanced. Humidity cycling resulted in both in-plane cerium gradients due to water flux during the hydration component of the cycle, and significant migration into the CLs. Fluoride and cerium emissions into effluent cell waters were measured during ASTs and correlated, which signifies that ionomer degradation products serve as possible counter-ions for cerium emissions. Fluoride emission rates were also correlated to final PEM cerium contents, which indicates that PEM degradation and cerium migration are coupled. It is proposed that cerium migrates from the PEM due to humidification conditions and degradation, and is subsequently stabilized in the CLs by carbon catalyst supports. Widespread adoption of polymer electrolyte membrane (PEM) fuel cell technology is currently hindered by insufficient component durability and high cost.1 During operation, reactive radical species generated by electrochemical fuel cell processes attack vulnerable functional groups in the ion-conducting, or ionomer, molecules which constitute the PEM and are present in the catalyst layers (CLs).2 These attacks reduce PEM thickness and generate local pinholes, which release hydrofluoric acid (HF), sulfuric acid (H 2 SO 4 ), and fluorinated polymer fragments into effluent cell waters; increase crossover of reactant gases through the PEM; and lead to cell failure.1,2 Since the PEM is constrained by cell hardware, hygrothermal cycling generates mechanical stresses which cause physical damage to the PEM in the form of cracks, tears, and pinholes.1,2 Furthermore, during typical operation, cells experience both chemical and mechanical stresses simultaneously, which results in synergy between the degradation modes. Localized mechanical stresses increase PEM susceptibility to radical attack by reducing the activation energy necessary for such attacks to proceed 3,4 and chemical degradation of the ionomer diminishes the bulk mechanical properties of the PEM, such as ultimate tensile strength, strain-to-failure, and fracture toughness, which further increases its susceptibility to physical failure. 5-11Owing to its rapid and regenerative redox with radical species and stability in acidic media, 7 cerium dramatically improves PEM durability by neutralizing radicals before they attack the ionomer. Cerium ions may be directly exchanged with protons in the ionomer 12,13 or ...

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Section: Resultsmentioning

confidence: 90%

Cerium Migration during PEM Fuel Cell Accelerated Stress Testing

Baker¹,

Mukundan²,

Spernjak³

et al. 2016

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%

Structure/property relationship of Nafion XL composite membranes

Shi

Weber

Kusoglu

2016

Journal of Membrane Science

145

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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“…Nafion ® membrane's high proton conductivity and mechanical stability are critical factors for achieving cell performance and durability targets, respectively. In PEFCs, the chemical and mechanical stressors are coupled, are strongly affected by temperature and humidity, and their impact dependent on the membrane's hygrothermal history, therefore making it difficult to characterize the role of individual stressors on overall properties . It is of particular interest to uncouple the effects of hygrothermal aging, that is prolonged exposure to hot and humid environments, from those of chemical degradation to understand better the mechanistic origins controlling the membrane's response and operational lifetime.…”

Section: Introductionmentioning

confidence: 99%

Impact of hygrothermal aging on structure/function relationship of perfluorosulfonic-acid membrane

Shi

Dursch

Blake

et al. 2015

J. Polym. Sci. Part B: Polym. Phys.

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Perfluorosulfonic‐acid (PFSA) membranes are widely used as the solid electrolyte in electrochemical devices where their main functionalities are ion (proton) conduction and gas separation in a thermomechanically stable matrix. Due to prolonged operational requirements in these devices, PFSA membranes’ properties change with time due to hygrothermal aging. This paper studies the evolution of PFSA structure/property relationship changes during hygrothermal aging, including chemical changes leading to changes in ion‐exchange capacity (IEC), nanostructure, water‐uptake behavior, conductivity, and mechanical properties. Our findings demonstrate that with hygrothermal aging, the storage modulus increases, while IEC and water content decrease, consistent with the changes in nanostructure, that is, water‐ and crystalline‐domain spacings inferred from small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) experiments. In addition, the impact of aging is found to depend on the membrane's thermal prehistory and post‐treatments, although universal correlations exist between nanostructural changes and water uptake. The findings have impact on understanding lifetime, durability, and use of these and related polymers in various technologies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 570–581

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Effect of Mechanical Compression on Chemical Degradation of Nafion Membranes

Cited by 46 publications

References 33 publications

Cerium Migration during PEM Fuel Cell Accelerated Stress Testing

Cerium Migration during PEM Fuel Cell Accelerated Stress Testing

Structure/property relationship of Nafion XL composite membranes

Impact of hygrothermal aging on structure/function relationship of perfluorosulfonic-acid membrane

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