Giant osmotic power generation in subnanometer-diameter single-wall carbon nanotubes

In nano- or angstrom-scale 1-D pores, surface charge significantly influences ion transport, and enables potential applications in desalination, ultrafiltration, and energy conversion. Ion transport through 1D nanopores with homogeneously distributed charge, such as boron nitride nanotubes, has been recently studied, with such nanotubes demonstrating the capacity to generate "giant" osmotic power under a salt gradient. However, the mechanisms of ion transport in inhomogeneously charged 1D nanopores remain intriguing. Here, we explore the use of 0.8-nm and 3-nm-diameter single-wall carbon-nanotubes (SWCNTs) with highly charged entrance regions and comparatively uncharged center regions. The SWCNTs reveal selective ion transport and exceptionally high osmotic-power densities comparable to and exceeding that of boron nitride nanotubes. To explain this phenomenon, we use analysis and finite-element modelling to compare diffusio-osmotic flow in homogeneously and inhomogeneously charged nanotubes. The evidence indicates that the highly charged entrance region dominates ion selectivity and generates electrical current induced by salinity gradients. Understanding the ion-transport mechanisms in inhomogeneously charged, small-diameter CNTs may enable ion-selective nanotube membranes optimized for different energy-conversion and separation processes, including desalination and osmotic-power harvesting.