Scalable synthesis of electrically conductive polyethersulfone (PES)-carbon nanotube (CNT) membranes by sequential casting

Electrically conducting membranes (ECMs) are generally prepared by coating porous polymer membranes with conductive nanomaterials, however, this coating synthesis reduces permeability of the membranes by up to 90 % as the pores are blocked by conductive materials. Additionally, there are several challenges inherent in the manufacturing scalability of ECMs, as the commonly used vacuum-, pressure-, or spin-coating-based syntheses are not scalable roll-to-roll methods. In this research, highly conductive membranes were fabricated by a simple linear sequential casting (SC) process using a doctor blade. This research identifies dope solution properties and casting conditions that enable ECM synthesis using scalable casting processes. The SC process also enables tuning of ECM water permeance which is not possibly using current coating methods. A 15 wt% polyethersulfone (PES) polymer dope was cast as a 200 μm thick support layer. Subsequently, multi-walled carbon nanotubes (CNTs) dissolved in N-Methyl-2-pyrrolidone (NMP) and propylene glycol (PG) were cast atop the polymer to form a thin conductive skin layer. Two SC synthesis processes were examined by casting CNTs on either uncoagulated or pre-coagulated PES substrates. The two-layer cast solution was immersed in a coagulation bath to make the ECMs by nonsolvent-induced phase separation (NIPS). The formation of a defect-free CNT layer on PES was found to be governed primarily by solvent–nonsolvent diffusion dynamics, rather than their mutual solubility, as evidenced by measured solubility parameters and the observed migration of CNTs into the coagulation bath. ECMs synthesized by the SC process could be made with looser porous structures (pure water permeance of 570 ± 80 vs. 210 ± 20 L/(m 2 .h.bar)) at comparable electrical conductivities (1200 ± 200 vs. 1400 ± 200 S/m) and comparable electrochemical activity (94 % methyl orange (MO) removal within 15 minutes at an applied potential of 3 V vs. Ag/AgCl as compared to control ECMs synthesized by vacuum deposition (VD). The SC process provides a scalable method for fabricating high-performance ECMs with tunable separation and electrochemical properties, expanding the industrial viability of ECMs.