Impact of Asymmetric\nMicrostructure on Ion Transport\nin Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> Membranes

Consolidation or densification of low-dimensional MXene\nmaterials\ninto membranes can result in the formation of asymmetric membrane\nstructures. Nanostructural (short-range) and microstructural (long-range)\nheterogeneity can influence mass transport and separation mechanisms.\nShort-range structural dynamics include the presence of water confined\nbetween the 2D layers, while long-range structural properties include\nthe formation of defects, micropores, and mesopores. Herein, it is\ndemonstrated that structural heterogeneity in Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> membranes fabricated via vacuum-assisted\nfiltration significantly affects ion transport. Higher ion permeabilities\nare achieved when the dense “bottom” side of the membrane,\nrather than the porous “top” side, faces the feed solution.\nCharacterization of the membrane reveals distinct differences in flake\nalignment, surface roughness, and porosity across the membrane. The\ndirectional dependence on permeability suggests that one region of\nthe membrane experiences stronger internal concentration polarization,\npotentially suppressing permeability through the porous side of the\nmembrane.