Double-Walled Carbon Nanotubes Enable Breakdown of the Trade-off between Ion Selectivity and Water Permeability

Although evidence has been presented for desalination potentials in single-walled carbon nanotubes (SWCNTs), it is still very challenging to overcome the trade-off between ion selectivity and water permeability by simply tuning the carbon nanotube (CNT) size. In this work, we prove that double-walled carbon nanotubes (DWCNTs) can make it. Employing a series of molecular dynamics simulations, we find a striking phenomenon that tuning the combination architecture of DWCNTs can significantly improve the desalination performance, with the salt rejection rate even reaching 100% in some cases while maintaining high levels of water flux. Specifically, under a certain outer CNT (20,20), with the increase in inner CNT radius, the salt rejection rate reaches a maximum for the CNT (9,9), attributed to the small size of the inner CNT and the space between the two CNT walls that significantly impedes the ion passage; however, it still allows the passage of massive water. Furthermore, as the pressure difference increases, the water flux greatly increases, while the salt rejection rate only slightly decreases for the CNTs (8,8) and (9,9), effectively addressing the trade-off between ion selectivity and water permeability. As a result, optimizing the architecture of DWCNTs should be an effective strategy for designing an efficient desalination membrane, which is still a challenge for SWCNTs.