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

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Molecular dynamics (MD) simulations are undertaken on a series of five copolyimides based on two different dianhydrides: the flexible 4,4′-oxydiphthalic dianhydride (ODPA) and the rigid bulky bicyclo(2.2.2)-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA). The diamines are respectively 4,4′oxydi(phenylamine) (ODA), 2-trifluoride-4,4′-oxydi(phenylamine) (CF 3ODA) and 2-methoxy-4,4′-oxydi-(phenylamine) (MeOODA). These are potential candidates for gas separation membranes, and the effects of increasing BCDA content in ODPA/BCDA copolyimides as well as adding trifluoromethyl or methoxy substituents on the ODA are studied at the molecular level. Amorphous long-chain models are built using a hybrid pivot Monte Carlo/MD sampling preparation procedure. The reproducibility of this approach is tested on a series of eight independently prepared systems. Densities, cohesive energies, Hildebrand parameters, conformational characteristics, intermolecular structures, and the available void spaces are analyzed for each system under study. Both the BCDA moiety and the trifluoromethyl substituent on the diamine are found to have similar consequences on the properties of the copolyimide by decreasing chain cohesion and increasing the available void space. This is related to the steric effect of the BCDA dianhydride, while the trifluoromethyl combines both steric and electronic repulsion. The steric effect of the methoxy substituent on the diamine is not strong enough to significantly differ from the unsubstituted system.

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Unwersite de Savoie -Batiment m-Campus scientiiue 73376 LA? Bourget-du-Lac Cedex, RaneeA series of polyimides has been synthesized based on different diarhydrides and diamines. The microstructure of polyimide films has been analyzed by performing density measurements, X-ray diffraction and W-visible spectrophotometry. The dynamic mechanical behavior was investigated according to both chemical and microstructural aspects. Thus, it was pointed out that the broadness of the p subglass transition is dependent not only on dianhydride flexibility but also on para-or meta-diamine links. Such behavior is discussed in terms of intra-charge transfer complex. On the contrary, the temperature location of the y-transition appears to be controlled by interchain packing. Based on a statistical approach, a correlation between microstructural data and gamma relaxation temperature is established.ies: Cheng et aL conclude that the molecular motion is mainly connected to the dianhydride moiety (10).

O2M3M5MPYR] as the cation and bis(trifluoromethanesulfonyl)imide [NTf 2 ], trifluoromethanesulfonate [TfO], dicyanamide [N(CN) 2 ], thiocyanate [SCN] or iodide as the anion have been synthesized, and their physicochemial properties have been studied. Thermal properties, solubilities in water and water contents, UV-vis spectra, densities, refractive indices, electrochemical windows, and toxicities on Vibrio fischeri have been measured. These ionic liquids, most of which are liquid at room temperature, exhibit high refractive indices and increased hydrophobicity. Despite high toxicity values toward V. fischeri, ionic liquids containing a dialkylpyridinium or trialkylpyridinium cation with an octyl chain represent good alternatives to extracting solvents. † Part of the "Josef M. G. Barthel Festschrift".

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Some properties of perfluorosulphonated ionomer membranes contaminated by a series of 10 counter ions were investigated by infrared spectroscopy (FTIR), thermogravimetric analysis coupled to mass spectroscopy (TG-MS), and dynamic mechanical spectrometry (DMA). Distinctive parameters were extracted and regarded as a function of the cations' properties. An optimum interaction between sulfonate group and cation was found for cations with Lewis Acid Strength (LAS) in the 0.2-0.3 range. This critical value is found to be the Lewis Basic Strength (LBS-SO À 3 ) of the sulfonate anion in Nafion membrane. Thermal stability analyses also point out the influence of this cation parameter on the polymer degradation process. Cations with LAS values lower than LBS-SO À 3 improve the thermal stability of the side chains while cations with LAS values higher than LBS-SO À 3 enhance the thermal degradation. Moreover, the temperature of the modulus drop increases with the LAS of the counter ion. For cations with values lower 0.5, the transition is attributed to the glass relaxation of the polymer while for cations showing LAS values higher than 0.5, the loss of stiffness originates from the polymer thermal degradation process. The overview of the experimental data allows the definition of calibration curves as a function of the cations' LAS.