Poly(2,4-phenylene oxide)s (PPOs)-based anion exchange membranes (AEMs) with four of the most widely investigated head groups were prepared. Through a combination of experimental and simulation approaches, the effects of the different types of head groups on the properties of the AEMs, including hydroxide conductivity, water content, physicochemical stability, and fuel cell device performance were fully explored. Unlike other studies, in which the conductivity was mostly investigated in liquid water, the conductivity of the PPObased AEMs in 95% relative humidity (RH) conditions as well as in liquid water was investigated. The conductivity trend in 95% RH condition was different from that in liquid water but corresponded well with the actual cell performance trend observed, suggesting that the AEM fuel cell performance under in situ cell conditions (95% RH, 60 °C, H 2 /O 2 ) is more closely related to the conductivity measured ex situ under 95% RH conditions (60 °C) than in liquid water. On the basis of the conductivity data and molecular simulation results, it was concluded that the predominant hydroxide ion-conducting mechanism in liquid water differs from that in the operating fuel cell environment, where the ionomers become hydrated only as a result of water vapor transported into the cells.

We synthesized various imidazolium-based cations (IM + -1 to IM + -5) such as OH − -conducting groups, and the substitution effect of imidazoliums with substituents at their C2, C4, and C5 positions on their alkaline stability is investigated through 1 H NMR spectroscopy and molecular simulation. The effect of the methoxy group at the C2 position on the imidazolium shows increased lowest unoccupied molecular orbital (LUMO) values, suggesting enhanced alkaline stability for the methoxy-substituted ones. However, the LUMO isosurface analysis, together with Mulliken charge and Fukui nucleophilic function values, indicate that the methoxy group makes the imidazolium ring more unstable. In addition, the alkyl chain length effect at the C4/C5 position reveals similar behaviors by showing that the longer alkyl group enhances the electrophilicity of the imidazolium ring, making it unstable against OH − ions. This investigation reveals an important information that density functional theory calculation-based comparisons of the stability of the conductors should be performed by considering not only the LUMO energy but also other parameters such as the LUMO isosurface of the developed conductors. Overall, the IM + -1 and IM + -2 with methyl and ethyl groups at C4/C5 and the 2,6-dimethylphenyl group at the C2 position reveal much higher alkaline stability than the typically used conductors in anion exchange membranes.

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