Detecting the Conduction\nProperty of Proton Carriers\nat Different Relative Humidity Based on Postsynthetic MOF Nanosheets

Exploring the transfer ability of proton carriers at\ndifferent\nrelative humidity (RH) is vital for the rational design and development\nof high-performance proton exchange membranes (PEMs). However, the\nhighly humidity-dependent transfer channel and random carrier distribution\ndisqualify most membrane materials. Herein, a series of MIL-53 metal–organic\nframework (MOF) nanosheets with stable, quantifiable pore structures\nand different conducting groups are prepared through postsynthetic\nligand exchange, followed by spin coating to assemble lamellar membranes.\nWe demonstrated that proper binding energy between the carrier group\nand water molecule is favorable for proton transfer based on the vehicle\nmechanism at low RH. Particularly, strong binding energy traps water\nmolecules, hindering proton transfer even though the carrier possesses\na higher proton dissociation constant. Thus, at 20% RH and 80 °C,\nAlBDC–COOH attains a higher proton conductivity of 13.6 mS\ncm<sup>–1</sup> than AlBDC–SO<sub>3</sub>H (11.9 mS\ncm<sup>–1</sup>). In contrast, with an incremental content\nof water, the available diffusion space of water molecules progressively\ndiminishes, leading to a reduced diffusion ability and thus a lower\ncontribution of vehicle transfer. Accordingly, jump transfer becomes\nthe dominant proton conduction process, and the abundant hydrogen\nbond networks in the AlBDC–SO<sub>3</sub>H membrane provide\nmore proton transfer paths and thus a higher proton conductivity of\n73.1 mS cm<sup>–1</sup> at 80 °C and 100% RH, over 1 order\nof magnitude higher than that of the pristine Al-BDC membrane (6.2\nmS cm<sup>–1</sup>). This study may shed light on the functional\ngroup selection of PEMs targeting different operation conditions.