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WIREs Energy Environ.
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A review of PEM fuel cell durability: materials degradation, local heterogeneities of aging and possible mitigation strategies

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Through a tight collaboration between chemical engineers, polymer scientists, and electrochemists, we address the degradation mechanisms of membrane electrode assemblies (MEAs) during proton exchange membrane fuel cell (PEMFC) operation in real life (industrial stacks). A special attention is paid to the heterogeneous nature of the aging and performances degradation in view of the hardware geometry of the stack and MEA. Macroscopically, the MEA is not fuelled evenly by the bipolar plates and severe degradations occur during start‐up and shut‐down events in the region that remains/becomes transiently starved in hydrogen. Such transients are dramatic to the cathode catalyst layer, especially for the carbon substrate supporting the Pt‐based nanoparticles. Another level of heterogeneity is observed between the channel and land areas of the cathode catalyst layer. The degradation of Pt3Co/C nanocrystallites employed at the cathode cannot be avoided in stationary operation either. In addition to the electrochemical Ostwald ripening and to crystallite migration, these nanomaterials undergo severe corrosion of their high surface area carbon support. The mother Pt3Co/C nanocrystallites are continuously depleted in Co, generating Co2+ cations that pollute the ionomer and depreciate the performance of the cathode. Such cationic pollution has also a negative effect on the physicochemical properties of the proton‐exchange membrane (proton conductivity and resistance to fracture), eventually leading to hole formation. These defects were localized with the help of an infrared camera. The mechanical fracture‐resistance of various perfluorosulfonated membranes further demonstrated that polytetrafluoroethylene‐reinforced membranes better resist hole formation, due to their high resistance to crack initiation and propagation. WIREs Energy Environ 2014, 3:540–560. doi: 10.1002/wene.113 This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Fuel Cells and Hydrogen > Systems and Infrastructure Energy Research & Innovation > Science and Materials
PEMFC voltage (blue curve) and current densities through each of the 20 segments of the segmented cell presented in Figure , as a function of time. The fuel cell was started‐up by injecting 10 standard liters per hour (slph) of hydrogen in the anode compartment initially filled with air.
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Segmented cell with five straight parallel channels used in Ref . The flow field plate is divided into 20 segments and it can be used either on the cathode side or on the anode side; the other plate is not segmented. The water circuit used to control the cell temperature flows through the aluminium end‐plate located under the gold‐plated flow field. (Reproduced with permission from Ref . Copyright 2013, Elsevier)
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(a) Exploded view of a PEMFC system (without the H2 tank). Courtesy of AXANE fuel cell systems (http://www.axane.fr/). (b) SEM image of a commercial membrane‐electrode assembly, Reproduced with permission from Ref . Copyright 2010, The Electrochemical Society. (c) Transmission electron microscopy (TEM) image of the Pt‐Co/C cathode electrocatalyst. (Reproduced with permission from Ref . Copyright 2013, Elsevier)
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(a) Variation of the degradation rate per unit cell versus the operation time on the field for Axane systems. The number of start/stop procedures undergone by the stacks is also given for information. The nature of the MEA operated in these stacks is not identical and proprietary. In most cases, the customer stopped exploitation of the system before any failure. (b) Example of cell voltage drop monitored for a segmented cell during repetitive start‐up/shut‐down procedures (the cell was operated 10 min at j = 0.50 A cm−2 between each start‐up and shut‐down events); (c) the overall voltage drop of 190 mV, measured between the beginning of life (BoL) and the end of life (EoL) after 272 start‐up with 2 slph of H2 in an air‐filled anode, results from very local degradations.
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Methodology developed to define both 1‐the position of the holes (a) in the stack, (b) in the MEA and (c) in the membrane, and 2‐the causes for failure, which might be related to mechanical issues, such as chain orientation or chemical contamination of the sulfonic groups with cationic foreign moieties.
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Required energy to initiate (left) and propagate (right) the formation of crack in PFSA as a function of the IEC of various membranes. The chemical structure does not alter much this behavior measured with the essential work fracture (EWF) method. Layered structures do present significantly higher toughness according to this method (not shown on this graph) up to 23 kJ m−2 and 10 MJ m−3 for we and βwp, respectively. The substitution of protons by any cations from a salt, induces a dramatic plasticity loss, as revealed for instance by a βwp value close to zero (blue dot). Lines were added to guide the eye.
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Dependence of the proton conductivity of membranes with the IEC, for various chemical structures. The conductivity is governed by the IEC, following percolation behavior with a threshold close to 0.60 meq g−. The controlled addition of cations (R+/− is the number of cations divided by the number of SO3 in the polymer) induces the gradual decrease of the proton conductivity, as if the IEC was reduced regardless of the nature of the cations. R+/− is also denoted as γM+ in the literature and in Section Degradation of the Catalytic Layers, with M+ the metal cation contaminant.
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Absolute position of the 970 cm−1 and 1070 cm−1 bands from FTIR experiments performed on dried membranes. Significantly different behaviors are observed depending on the nature and amount of the cations that was added on purpose to replace the protons in the PFSA.
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(a) Linear sweep ORR voltammograms measured for Pt/C in contact with fresh or Co2+‐contaminated Nafions 117 PEM, (b) corresponding Tafel plots of the mass‐transport corrected kinetic current, and (c) normalized diffusion limited current for fresh and Co2+‐contaminated Nafion 117 PEM. Temperature = 22°C, sweep rate = 0.001 V s−1. (d) Linear sweep ORR voltammograms for Pt/C in contact with a noncontaminated (γCO2+ = 0, black curves) and a Nafion 117 PEM contaminated with Co2+ ions (γCO2+ = 0.7, red curves) and linear sweep voltammograms in argon‐saturated solution into the hydrogen evolution region. (Reproduced with permission from Ref . Copyright 2012, Royal Society of Chemistry)
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(a) Density of isolated (full symbols) and aggregated (open symbols) Pt‐Co/C nanoparticles (NPs) and (b) surface‐averaged diameter () statically estimated by measuring the size of isolated Pt‐Co/C particles versus PEMFC operation time. The PEMFC short stacks were operated in various experimental conditions (j = 0.24 or 0.60 A cm−2 in stationary conditions or j = 0.60 A cm−2 in 1 h start/1 h stop, T = 70°C, 65% RH), as detailed in Refs and .
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Particle size distributions and representative TEM images of the Pt‐Co/C cathode electrocatalyst before/after operation at j = 0.60 A cm−2, T = 70°C, 65% RH for various durations. (Reproduced with permission from Ref . Copyright 2013, Elsevier)
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(a) Raman spectra of the fresh/aged Pt‐Co/C cathode electrocatalysts before/after operation at j = 0.60 A cm−2, T = 70°C, 65% RH for various durations. Reproduced with permission from Ref . Copyright 2013, Elsevier. (b) Variation of the I(D)/I(G) ratio and the mean crystallite size (La) versus the PEMFC operation time in various experimental conditions (j = 0.24 or 0.60 A cm−2 in stationary conditions or j = 0.60 A cm−2 in 1 h start/1 h stop, T = 70°C, 65% RH), as detailed in Refs and .
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Schematics of the four main processes of Pt/C nanoparticles degradation during PEMFC operation. (Reproduced with permission from Ref . Copyright 2010, The Electrochemical Society)
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FEG‐SEM images obtained in back‐scattered electrons for MEAs operated at j = 0.60 A cm−2, T = 70°C, 65% RH after various durations of operation in PEMFC 16‐cell stack. (Reproduced with permission from Ref . Copyright 2013, Elsevier)
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Potential distribution in a cell during a start‐up in air with reference to the RHE. (Reproduced with permission from Ref . Copyright 2005, The Electrochemical Society)
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Schematics of the active and passive part of a cell during (a) start‐up and (b) shut‐down, and possible electrochemical reactions occurring during the transients. In that case the heterogeneities generated are at the level of the centimeters to tens of centimeter scale. (Reproduced with permission from Ref . Copyright 2013, Elsevier)
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