Molecular insights into capacitive deionization mechanisms inside hydrophobic and hydrophilic carbon nanotube channel electrodes

Abstract

In capacitive deionization (CDI) processes, the deionization and charge compensation mechanisms and desalination performance are highly influenced by confined electrolytes' microscopic properties rendered by electrodes' porous structures and chemical composition. Single-wall carbon nanotubes (SWCNTs) have become fascinating materials in research and industry for the unique and anomalous properties of their confined water molecules. Here, we present a computational molecular dynamics (MD) study to investigate various desalination mechanisms of CDI systems with hydrophilic or hydrophobic armchair SWCNT electrodes with chiral indices of 9, 10, and 15. The water cluster structure inside the uncharged and charged confinement is shown to be directly influenced by the SWCNT wall wettability and effective pore diameter. The different charge compensation mechanisms of each system are shown to result in significantly different desalination performances. Hydrophilicity increased the ion transport rate, while it is shown that this does not necessarily result in better desalination performance at larger pores. We investigate the variation in the solvation structure of ionic species upon confinement. In addition, the capacitance of the systems is calculated and no relationship between this metric and desalination capacity is shown to exist. The charge efficiency of the systems as a linking factor between desalination capacity and capacitance is also investigated. The static dielectric constant of confined water molecules and its tensor components are calculated and it is shown that they deviate from the bulk value upon confinement. This study provides clear molecular insights into the relationship between desalination mechanisms and performance and can be useful for CDI electrode design studies.