Substantially Enhanced\nPerformance of Proton Exchange\nMembranes by Metal–Imidazole Coordination within Metal–Organic\nFramework

Metal–organic frameworks (MOFs)\nare often used\nas proton\nconductor fillers to enhance the proton conductivity of hybrid membranes.\nIt is interesting but still challenging to discuss the influence of\nthe molecular morphology and distribution of proton carriers within\nMOFs on their proton exchange membranes (PEMs) and their overall performance.\nIn this work, two types of imidazole-loaded proton conductors were\nsynthesized based on MIL-101(Cr), with one containing free imidazole\nmolecules and the other containing imidazole molecules coordinating\nwith Cr-metal sites. Furthermore, series composite membranes (CPM/IM-<i>x</i> and CPM@IM-<i>x</i>, <i>x</i> = 1,\n2, 3) were fabricated using imidazole-loaded proton conductor as filler\nand a cross-linked poly(vinyl alcohol) (CL-PVA) that contains both\nhydrophilic and hydrophobic structures as matrix. The CPM/IM membranes\nwith coordinated imidazole molecules exhibit significantly enhanced\nwater affinity, stability, and ion exchange capacity. This is mainly\ndue to the coordinated imidazole improving the two-phase dispersion,\npromoting imidazole deprotonation, and facilitating low-energy-barrier\nproton transport, while the CPM@IM membranes with the free imidazole\nmolecules exhibit relatively lower proton conductivity but higher\nactivation energy (<i>E</i><sub>a</sub>). Surprisingly,\nthe proton conductivity of the CPM/IM-2 membrane can reach up to 2.7\n× 10<sup>–2</sup> S cm<sup>–1</sup> at 85 °C\nand 90% RH, but with the lowest <i>E</i><sub>a</sub> of\n20.9 kJ/mol. This remarkable performance indicates more efficient\nH<sup>+</sup> transportation. Moreover, the single-cell power density\nof CPM/IM-2 is up to 89.7 mW cm<sup>–2</sup>, proving it exhibits\nhigh potential application prospects as a proton exchange membrane\nfor hydrogen-based fuel cells.