Anomalously enhanced moisture-vapor-transport rate in vertically aligned 0.8nm-diameter carbon-nanotube membranes

Membranes with small-diameter, vertically aligned carbon-nanotube (VA-CNT) pores provide a promising path toward highly permeable membranes for applications such as breathable yet protective garments, desalination membranes, and highly efficient filters. However, it has not been previously possible to fabricate VA-CNT membranes with sub-nanometer pores because of the difficulty in growing aligned forests of CNTs of such small diameters. Here, we describe the transport properties of macroscopic VA-CNT membranes made from 0.8nm diameter CNTs grown in bulk with a specialized catalyst. The membranes are made by dispersing nanotubes in a liquid prepolymer and aligning them with an electric field before locking them in place in a polyurethane matrix by UV curing. After oxygen-plasma etching to open the CNT pores, the VA-CNT membranes were permeable and demonstrated cation selectivity in KCl solution, and sublinear conductance scaling with salt concentration at low molarities. These characteristics indicate that the membranes were defect-free, with small-diameter CNTs serving as the primary pathway for transport. Moisture-vapor-transport-rate (MVTR) measurements revealed that water vapor diffused many orders of magnitude faster than either Knudsen diffusion, or the rates reported in literature for larger-diameter VA-CNT membranes. These scalably fabricated VA-CNT membranes with sub-nanometer pores may present new opportunities for fundamental studies of transport in 1D pores, and to engineer highly permeable membranes for a variety of applications.