Microstructure-Dependent Ion Selectivity in Graphene Oxide-Based Membranes

Graphene oxide (GO) membranes hold promise for precise separation due to their unique laminar structures and tunable separation properties. However, although water transport in GO membranes has been extensively investigated, the key mechanisms governing ion transport and selectivity remain poorly understood. In this work, we fabricated pristine and propylenediamine (PPD)/pentamethylenediamine (PTD)-cross-linked GO membranes via vacuum filtration and employed low-field nuclear magnetic resonance (LF-NMR) to elucidate their nanoscale pore architectures, including pinholes and structural defects. Our results show that cross-linking leads to a more ordered arrangement of the nanochannels and reduces the dimensions of structural defects. Ion diffusion experiments demonstrated that the membrane microstructure plays a crucial role in determining the tortuosity of the ion transport pathways. Notably, large structural defects dominate ion transport; when present, ions tend to bypass the interlayer nanochannels, leading to reduced ion selectivity. In contrast, intrinsic pinholes are too small to significantly contribute to ion transport. Molecular dynamics simulations showed that interlayer spacing, ion properties, and interactions with the membrane jointly govern ion diffusion within the GO-based nanochannels. More importantly, the simulation results deviated from experimental observations, further implying the pivotal role of large structural defects in ion transport. These findings provide valuable guidelines for designing next-generation GO-based membranes with improved ion selectivity and performance for precise separation.