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Anion exchange membranes prepared from imidazolium cations still need to overcome the drawbacks of water solubility and chemical stability in strong base. Here, we use density functional theory (DFT) to focus on alkaline stability of imidazolium-based monomers with different C4/C5-position substituents and analyze their degradation mechanism when treated with 10 M NaOH. On the basis, we select methyl-substituted imidazolium (MeIM) cationic compound to construct ether-free main-chain type polymers via superacid-catalytic reaction, together with the introduction of branched monomer to inhibit water swelling of membrane. The optimized branched structure maintains stable dimension performance under 80 °C compared to linear polymer dissolved in water. Meanwhile, the alkaline-resisted membrane with branched structure expands the operating temperature range of fuel cell to 90 °C and maintains long-term durability. [Display omitted] • The main-chain type imidazolium-based AEM is prepared by superacid-catalytic polymerization. • Excellent alkaline stability of methyl-substituted imidazolium on C4 and C5 position when treated 10 M KOH. • The branched polymer structure is designed to suppress water swelling of imidazolium-based AEM. • The operating temperature of fuel cell is expanded to 90 °C for AEM's long-term stability. [ABSTRACT FROM AUTHOR]

Abstract: The permeation of helium, oxygen and nitrogen in perfluorosulphonic acid ionomeric (PFSI) membranes with short and long side chains, namely Aquivion® and Nafion® 117, was studied in the relative humidity range between 0 and 90%, and at temperatures between 35 °C and 65 °C. The presence of humidity enhances, up to a factor of 100, the gas permeability in the membranes, due to the permeation of gas molecules in the hydrophilic domains: the enhancement is rather pronounced for O2 and N2, and less marked for He permeability. The relative permeability increase, Pgas/Pgas,0, shows a complex dependence on the relative humidity, as the water content in the membrane is itself a non linear function of this parameter. The water volume fraction in the membrane at each activity was accurately estimated from measurements of vapor-induced swelling, which indicate that the partial molar volume of water is smaller than its pure liquid value, in both membranes at 35 °C. When plotted against water volume fraction, the gas permeability increases exponentially in the range between 2% and 20%; the slope of the curve is higher for Nafion® than for Aquivion®, as it is reasonable due to their different microstructures. The ideal selectivity of the two membranes for He over N2, O2 over N2 and He over O2, decreases markedly with increasing water content in the membrane. [Copyright &y& Elsevier]

To ameliorate the trade-off effect between ionic conductivity and water swelling of anion exchange membranes (AEMs), a crosslinked, hyperbranched membrane (C-HBM) combining the advantages of densely functionalization architecture and crosslinking structure was fabricated by the quaternization of the hyperbranched poly(4-vinylbenzyl chloride) (HB-PVBC) with a multiamine oligomer poly(N,N-Dimethylbenzylamine). The membrane displayed well-developed microphase separation morphology, as confirmed by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Moreover, the corresponding high ionic conductivity, strongly depressed water swelling, high thermal stability, and acceptable alkaline stability were achieved. Of special note is the much higher ratio of hydroxide conductivity to water swelling (33.0) than that of most published side-chain type, block, and densely functionalized AEMs, implying its higher potential for application in fuel cells. [ABSTRACT FROM AUTHOR]