(380an) Local Hydrophilicity Enables Macroscale and Molecular Structure Control of 2D MXene Membrane

The laminar separation membranes assembled from a two-dimensional (2D) nanomaterial, titanium carbide (MXene), exhibit a promising molecular separation performance and ultrafast water permeation. However, 2D MXene membranes undergo swelling issues in the aqueous solution owing to the intercalation with water molecules, which undermines membrane performance in a long-term operation. Here, with a combination of the three-dimensional imaging technique, density function theory (DFT) calculation, and molecular dynamics (MD) simulation, we discover that local hydrophilicity is the critical factor for controlling the structure of the laminar MXene membrane. First, with intercalated chaotropic cesium (Cs) ion, the new MXene membrane exhibited noticeably decreased amounts of water pockets and thus contained fewer defects. Second, both X-ray diffraction and DFT analysis revealed that Cs-intercalated MXene only includes a single layer of water, while other intercalants typically contain 2-3 layers of water. This shrunk nanochannel size can resist water-induced swelling as well. Third, the narrow nanochannel resulted in a highly competing ion transport pathway. MD analysis suggested a preferential transport of lithium, which is validated by an enhanced Li/Mg separation in the experiment. Moreover, with fewer defects and better-controlled nanochannel, the membrane exhibits a significantly decreased ion permeation rate for multiple ions, including lithium, sodium, and potassium, compared with the pristine MXene membrane. Our study implies that controlling local water in the nanochannel can be a powerful strategy for high-efficiency membrane separation.