Damage to fuel cell membranes. Reaction of HO˙ with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O2

Dockheer, Sindy M.; Gubler, Lorenz; Bounds, Patricia L.; Domazou, Anastasia S.; Scherer, Günther G.; Wokaun, Alexander; Koppenol, Willem H.

doi:10.1039/c0cp00082e

Cited by 38 publications

(51 citation statements)

References 56 publications

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“…15 The hydroxyl radical is a very powerful oxidant, E (HO /H 2 O) ¼ 2.72 V (Table I). Because the H-O bond in water is very strong, abstraction of a hydrogen atom from another compound by HO is highly favorable.…”

Section: Reactive Intermediatesmentioning

confidence: 99%

“…In addition, at low pH, the OH-adduct can undergo an acid catalyzed water elimination (reaction 19), forming benzyl radicals. 15,80 The balance of reactions 18 and 19 depends on the local oxygen concentration, low [O 2 ] favoring reaction 19 and benzyl radical formation, which can lead to chain fragmentation. The hydroxylated product (compound E) is stable and, being a phenolic compound, could in fact enhance the stability of the polymer against radical induced degradation.…”

Section: Attack On Pssa Based Ionomersmentioning

confidence: 99%

“…In PSSA, the attack by HO is the starting point for a complex framework of follow-up reactions, intermediates and potential chain degradation pathways. 15 The kinetic scheme adopted in this study represents a zerodimensional description of the chemical environment in the fuel cell membrane in the presence of H 2 and O 2 under conditions typical of an open circuit voltage (OCV) hold test, which is often used as an accelerated stress test to probe chemical stability of the MEA. 2 The approach is based on the assumption that H 2 O 2 is created at the electrodes and diffuses into the membrane, where it engages in reactions that form radicals.…”

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confidence: 99%

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Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

Gubler¹,

Dockheer²,

Koppenol³

2011

J. Electrochem. Soc.

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257

258

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Formation of radicals, such as HO , H and HOO , in the membrane of the polymer electrolyte fuel cell and their attack on perfluoroalkylsulfonic acid (PFSA) and poly(styrenesulfonic acid) (PSSA) ionomers was simulated based on a kinetic framework with H 2 O 2 as "parent" molecule and with contaminating Fe as parameter. Analysis under quasi-steady state conditions yielded radical concentrations of around 10 À19 M for H , 10 À16 M for HO and 10 À10 M for HOO . H is formed via the reaction of HO with H 2 dissolved in the membrane. The attack of the PFSA ionomer was assumed to proceed via weak carboxylic end-groups. The corresponding calculated fluoride emission rate (FER) showed good agreement with experimental data under ex situ Fenton test conditions. The predicted FER under fuel cell operating conditions was underestimated by 2-3 orders of magnitude. It is likely that degradation via side-chain attack is prevalent during open circuit voltage hold tests. The oxidative degradation of PSSA ionomer follows an entirely different pathway, because, in addition to a-hydrogen abstraction by HO , the aromatic ring effectively scavenges HO to form an OH-adduct. Follow-up reactions lead to chain scission and formation of a stable hydroxylated degradation product.The proton conducting membrane in the polymer electrolyte fuel cell (PEFC) is exposed to considerable oxidative stress due to the presence of reactive intermediates formed in the membrane electrode assembly (MEA), which attack the electrolyte membrane and lead to chain scission, loss of polymer constituents, membrane thinning and, eventually, failure of the cell. 1,2 The chemical breakdown of the polymer additionally causes loss of mechanical integrity with ensuing mechanical failure of the membrane due to pinhole or crack formation. 3 The nature of the reactive intermediates formed during electrochemical H 2 and O 2 conversion in the PEFC has been discussed for many years in many articles. The insight that H 2 O 2 is involved in membrane degradation was already gained in the 1960s. 4,5 The observation that iron or other redox-active metal ions appeared to accelerate the degradation led to the suggestion that oxygen radicals are produced through the metal-ion-catalyzed decomposition of hydrogen peroxide (Fenton reaction). The working hypothesis that hydroxyl radicals (HO ) and hydroperoxyl radicals (HOO ) are the responsible species for membrane degradation has been accepted for a long time. Direct detection of radical intermediates in fuel cells was accomplished only recently by introducing spin-trapping agents into the MEA and placing a miniature fuel cell into an electron spin resonance (ESR) spectrometer. Using this approach, Roduner et al. proposed that HO and polymer derived carbon centered radicals are formed. 6 More recently, Schlick et al., using a similar approach, proved the presence of carbon centered radicals, HO , HOO as well as H in an operating fuel cell. 7 The reaction mechanisms leading to the formation of radical intermediates have been the subject ...

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Section: Reactive Intermediatesmentioning

confidence: 99%

Section: Attack On Pssa Based Ionomersmentioning

confidence: 99%

mentioning

confidence: 99%

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Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

Gubler¹,

Dockheer²,

Koppenol³

2011

J. Electrochem. Soc.

Self Cite

257

258

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“…However, the formation rate of benzyl radicals may be affected by RH. A variety of reaction pathways of PSSA degradation have been identified in our previous research (18,19). Around 90% of HO • radicals add to the aromatic rings, which results in the formation of an OH-adduct; while the remaining 10% of the HO • attack the C-H bond at the α position and abstract a hydrogen to yield benzyl radicals, which subsequently can undergo chain scission.…”

Section: Resultsmentioning

confidence: 99%

The Effect of Relative Humidity on Chemical Degradation of Styrene Based Radiation Grafted Membranes in the PEFC

et al. 2013

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The chemical degradation of styrene based radiation grafted membranes has been investigated by testing the MEAs at 50% RH for 110 h and different current densities. Post-test analysis using FTIR spectroscopy was performed to qualitatively and quantitatively analyze membrane degradation. The results show that higher water content in the MEA leads to a higher degradation rate of styrene based radiation grafted membranes. A range of working hypotheses is discussed from the point of view of both formation rate of reactive intermediates and chain degradation mechanism of ionomers. The strong relative humidity dependency of membrane conductivity causes the poor performance in the fuel cell when the membrane is dry. Imbalanced humidification of reactant gases (e.g. 50% at cathode and 100% at anode) can decrease chemical degradation of styrene based radiation grafted membranes, meanwhile maintaining a fuel cell performance comparable to that under fully humidified conditions.

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“…68,69 The interaction of hydrogen peroxide with the membrane is frequently used in order to probe the long term stability of PEFC membranes. [70][71][72][73] The presence of radicals formed from hydrogen peroxide was mainly detected at the cathode of PEFCs and is promoted by small impurities like iron ions. [74][75][76] In real fuel cell applications the membrane also interacts with other components.…”

Section: Degradation Of Polymer Membranesmentioning

confidence: 99%

Accelerated Degradation of High-Temperature Polymer Electrolyte Fuel Cells: Discussion and Empirical Modeling

Reimer

Schumacher

Lehnert

2014

J. Electrochem. Soc.

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Accelerated stress tests are commonly applied in order to obtain a prediction of fuel cell life time within a short testing period. The stress test in this work considers a fuel cell which is constantly under load with frequent periods of very high current. Four different cells were operated each with a specific load profile. As a result severe performance degradation was observed in the region of high current densities. A short overview over the phenomena which may contribute to this specific fuel cell degradation is compiled from literature. Based on this overview major degradation modes are identified and combined with a simple polarization curve model. The modeling results allow for two different interpretations. Carbon corrosion reactions can explain the observed effects if cell voltage is assumed as the driving force for degradation. A better fit was obtained by using the overall heat flux as degradation criterion. In this case an increase in local temperature could lead to redistribution and loss of phosphoric acid from the MEA. The expected lifetime of a polymer electrolyte fuel cell (PEFC) ranges from 5 000 h for mobile application to 40 000 h for stationary application.1,2 In order to gain information about time dependent degradation in advance accelerated test protocols are used.3,4 Depending on the design of these tests valuable information about the nature of the main degradation mode under the applied operation conditions can be obtained. Despite the difference in operation conditions for PEFC, HT-PEFC and PAFC the structure and material of the electrodes are almost the same. Consequently, observed degradation phenomena show strong similarities.5 A good description of these phenomena can be found in several review papers 6-13 as well as in at least four books 1,2,14,15 where recent experimental facts and interpretations are summarized. A compact description can also be found in a recent review paper on modeling transport phenomena in PEFCs. 16 The purpose of this paper is to present the results of a specific accelerated degradation test for high temperature polymer electrolyte fuel cells (HT-PEFC) with phosphoric acid doped polybenzimidazole (PBI/H 3 PO 4 ) membranes. The fuel cell is constantly under load with frequent periods of very high current. In order to accelerate degradation effects the fuel cell is operated beyond it's point of maximum power. A simple polarization curve model is used as starting point for the analysis of data. Based on this first analysis a degradation model is proposed which allows for a straightforward physical interpretation. The complexity of degradation effects which occur simultaneousy usually leads to models which either resolve a specific mechanism in depth or describe more general effects. This paper aims at general effects which requires a broader understanding of degradation. In the following it is attempted to provide an overview over the phenomena which may contribute to fuel cell degradation. Based on this overview major degradation modes are identified and ...

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Damage to fuel cell membranes. Reaction of HO˙ with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O2

Cited by 38 publications

References 56 publications

Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

The Effect of Relative Humidity on Chemical Degradation of Styrene Based Radiation Grafted Membranes in the PEFC

Accelerated Degradation of High-Temperature Polymer Electrolyte Fuel Cells: Discussion and Empirical Modeling

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Damage to fuel cell membranes. Reaction of HO˙ with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O2

Cited by 38 publications

References 56 publications

Radical (HO•, H• and HOO•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

Radical (HO•, H• and HOO•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

The Effect of Relative Humidity on Chemical Degradation of Styrene Based Radiation Grafted Membranes in the PEFC

Accelerated Degradation of High-Temperature Polymer Electrolyte Fuel Cells: Discussion and Empirical Modeling

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Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells