MXene/COFs membrane with sandwich structure for rapid salt/dye separation by layer-by-layer electrophoretic deposition

Covalent organic frameworks (COFs) have evolved into a transformative category of nanofiltration materials due to their intrinsic pore channels and high porosity, sparking a burgeoning interest in fully harnessing the potential of COFs for applications in nanofiltration. Herein, a sandwich structured MXene/COFs membrane was fabricated by the technique of layer-by-layer electrophoretic deposition (LBL EPD), with COFs nanospheres (COFs NPs) serving as the intermediate layer. During the EPD process, controlling the deposition time allows for convenient regulation of each layer's thickness, enabling rapid production of defect-free MXene/COFs membranes. The ultra-thin MXene layer offers excellent film-forming properties for COFs NPs, whereas the intermediate layer of COFs NPs plays a primary sieving function. Moreover, the pore-rich COFs NPs, the ultra-thin MXene layer, and the COFs NPs layer with abundant slits guarantee short and successive mass transport pathways. Under optimal conditions, the MXene/COFs membrane provided a permeance as high as 200.2 L‧m-2‧h-1‧bar-1 and a rejection rate of Congo red (CR) as high as 98.9%. This exceptional performance holds steadfast even within pH ranges of 7 to 11 and during long-term operation. Meanwhile, MXene/COFs membranes exhibit commendable salt/dye separation performance, achieving an excellent separation factor of 121.1 for NaCl/CR. Moreover, the membrane's exceptional robustness and mechanical strength further facilitate its stable utilization under high transmembrane pressure (1∼5 bar). To summarize, MXene/COFs membranes, characterized by their excellent nanofiltration and salt/dye separation performance, highlight the advantages of COFs within the nanofiltration field. This study introduces an innovative alternative approach to designing COFs membranes for highly efficient and high-precision molecular separations.