Detecting the Conduction Property of Proton Carriers at Different Relative Humidity Based on Postsynthetic MOF Nanosheets

Exploring the transfer ability of proton carriers at different relative humidity (RH) is vital for the rational design and development of high-performance proton exchange membranes (PEMs). However, the highly humidity-dependent transfer channel and random carrier distribution disqualify most membrane materials. Herein, a series of MIL-53 metal-organic framework (MOF) nanosheets with stable, quantifiable pore structures and different conducting groups are prepared through postsynthetic ligand exchange, followed by spin coating to assemble lamellar membranes. We demonstrated that proper binding energy between the carrier group and water molecule is favorable for proton transfer based on the vehicle mechanism at low RH. Particularly, strong binding energy traps water molecules, hindering proton transfer even though the carrier possesses a higher proton dissociation constant. Thus, at 20% RH and 80 °C, AlBDC-COOH attains a higher proton conductivity of 13.6 mS cm^-1 than AlBDC-SO₃H (11.9 mS cm^-1). In contrast, with an incremental content of water, the available diffusion space of water molecules progressively diminishes, leading to a reduced diffusion ability and thus a lower contribution of vehicle transfer. Accordingly, jump transfer becomes the dominant proton conduction process, and the abundant hydrogen bond networks in the AlBDC-SO₃H membrane provide more proton transfer paths and thus a higher proton conductivity of 73.1 mS cm^-1 at 80 °C and 100% RH, over 1 order of magnitude higher than that of the pristine Al-BDC membrane (6.2 mS cm^-1). This study may shed light on the functional group selection of PEMs targeting different operation conditions.