Probing Ion Solvation and Transport in Single-Digit Nanopores with First-Principles Simulations

Carbon nanotubes underpin a large array of materials systems and technological applications, including supercapacitors and water desalination. In these devices, understanding of the ion behavior is essential for predicting and optimizing the performance; however, many mechanistic details remain enigmatic. These include transport and solvation properties of the ions, and how they are governed by the degree of confinement. Here, we employ first-principles simulations to unravel key features of the solvation structure of several common ions at the interface with graphitic interface and under confinement within single-digit nanopores. We find that polarizable ions generally exhibit a stronger adsorption at graphitic interfaces and these effects are found to be significantly enhanced under confinement. In addition, we find that confinement significantly influences ion selectivity and transport, i.e., ions with a small radius are found to yield a notably larger energy barrier to reach the pore entrance. Our study therefore points to the complex interplay between confinement and specific ion effects, which has broad implications in optimizing CNTs for ion selectivity and energy storage.