Bionic inspired electrostatic-complexed reduced graphene oxide/covalent organic framework nanosheets for high-performance nanofiltration membranes

Reduced graphene oxide (rGO) membranes show great potential in nanofiltration due to their stable sub-nanometer interlayer channels; however, their tortuous pathways result in low fluxes. Current intercalation methods to improve fluxes usually weaken interlayer interactions and enlarge interlayer channels, leading to unfavorable selectivity and stability. Inspired by sandcastle worms, we introduced positively charged covalent organic framework nanosheets (CONs) into negatively charged rGO to form heterostructured laminar membranes through electrostatic complexation. The rGO/CON membranes were obtained by vacuum filtration of mixed dispersions of rGO and CONs on macroporous substrates. CONs endow the rGO/CON membranes with vertically short pathways and enhanced hydrophilicity, while the electrostatic complexation between rGO and CONs contributed to the formation of tighter interlayer structures. Therefore, simultaneously improved permeances and rejections were achieved. Through systematic optimization of the rGO/CON mass ratio and component loading, the composite membrane exhibited exceptional separation performance, achieving a permeance of 110.6 L m-2 h-1 bar-1 while maintaining an outstanding rejection of 99.8% for Congo red (CR). This performance surpassed that of most reported graphene-based nanofiltration membranes. In addition, the rGO/CON membranes exhibited good stability during long-term operations and under harsh acidic/alkaline conditions. This study provides an alternative approach for designing laminar nanocomposite membranes with high permselectivity and stability for nanofiltration.