Formation of electrically conductive hollow fiber membranes via crossflow deposition of carbon nanotubes – Addressing the conductivity/permeability trade-off

Electrically conductive membranes (ECMs) have significant potential for enhanced membrane separations. While most of the existing studies on ECMs have focused on flat sheet membranes, it is the hollow fiber membrane format that is the preferred format for large-scale treatment applications. While ECMs in hollow fiber format are emerging, existing approaches tend towards extremes of membrane conductivity or permeability without considering the trade-off between these metrics. We aim to understand and optimize this trade-off by studying the effect of the normal (pressure, lift) and tangential (shear) forces associated with conductive coating deposition. A suspension of functionalized single walled/double walled carbon nanotubes (SW/DWCNTs) in solution containing sodium dodecyl sulfate and polyvinyl alcohol was pressure deposited in crossflow along the active lumen surface of polyethersulfone microfiltration membranes. The impact of the crossflow deposition parameters affecting surface forces (feed pressure, crossflow velocity) were quantified according to both membrane permeability and conductivity in a 22 design-of-experiments study. Unexpectedly, the highest membrane permeability (~2900 LMH/bar) and conductivity (~670 S/m) and were obtained at the high feed pressure and high crossflow velocity condition. A Robeson-like plot was generated that demonstrates a trade-off between composite membrane permeability and conductivity for 29 CNT-coated hollow fiber membranes fabricated at various SW/DWCNT concentrations, crossflow velocities, and applied pressures. Measured conductivities (up to 2500 S/m) and permeabilities (up to 6000 LMH/bar) for the CNT-coated hollow fiber membranes were several orders of magnitude higher than previous conductive hollow fibers, indicating the substantial benefits of crossflow deposition. The electrically conductive hollow fiber membrane fabrication technique developed herein will be useful in a variety of conductive membrane applications including fouling prevention, enhanced removal of charged particles, and integrated membrane sensing capabilities.