Electrical Image Potential and Solvation Energies for an Ion in a Pore in a Metallic Electrode or in a Nanotube

TL;DR

This paper investigates how electrical image potentials in nanoscale pores and nanotubes significantly influence ion solvation energies, affecting ion transport in applications like capacitive desalination and supercapacitors.

Contribution

It provides new calculations of image potentials in spherical and cylindrical pores, showing their impact on ion solvation and transport in nanotubes and porous electrodes.

Findings

Image potentials are larger in pores due to dielectric effects.

Large image charge potentials oppose increased solvation energy in nanotubes.

Image potentials can facilitate ion flow into nanotubes.

Abstract

Electrical image potentials can be important in small spaces, such as nanoscale pores in porous electrodes, which are used in capacitive desalination and in supercapacitors. It will be shown here that inside pores in porous metallic materials the image potentials can be considerably larger than near flat walls, as a result of the fact that the dielectric constant for an electric field perpendicular to a wall is much smaller than the bulk dielectric constant of water. Calculations will be presented for the image potential in spherical and cylindrically shaped pores. The calculations for cylindrical pores can also be applied to nanotubes. It was believed for a long time, on the basis of molecular dynamics simulations, that in order to push a salt solution through a small radius nanotube, work must be done against the solvation energy of the ions, which is larger inside a narrow nanotube than it is in the bulk. The relatively large image charge potential energy in narrow nanotubes, however, tends to oppose this increase in the solvation energy. The degree to which the image potential facilitates the flow of the salt ions into nanotubes will be discussed.