Molecular understanding of aqueous electrolyte properties and dielectric effect in a CDI system.

Capacitive deionization (CDI) provides performance of ion removal via electrodes with high porosity and confined nanopores. Hitherto, the main emphasis has been on the enhancement of removal efficiency based on a lab-scale setup, neglecting anomalous dynamic and dielectric properties in confinement. This work was completed to reveal structural and dynamic properties of hydrated ions in a 3-nm nanopore of a CDI electrode using molecular dynamics simulation based on the nonpolarizable (SPC/E) and polarizable (SWM4-DP/-NDP) force fields. It was shown that all the force fields caused marginal differences in hydration structure but greater differences in ionic dynamics. Furthermore, dielectric constants obtained from all the force fields were lower in confinement than in bulk, although they were slightly higher in the polarizable models. Angular distributions of water bonds and dipole vectors revealed this decrease was caused by the restriction of water rotational dynamics and hence a depression of dipolar fluctuations. Hydrogen bond lifetimes indicated that higher availability of hydrogen bonding sites also facilitated increasing the dielectric constant as it allowed a more frequent "switch" of the fast hydrogen bond. Both the angular distribution and the hydrogen bonding contributed to the slightly higher dielectric constant in the polarizable model than in the nonpolarizable model. Our local sensitivity analysis of a ion transport model revealed that the dielectric constant greatly determined ion transport in the electrode, although the diffusion coefficients of typical ions had limited influences. This work provides fundamental theories and great implications for the theoretical analysis of ion electrosorption in a CDI electrode and enhances its application in water treatment. [Display omitted] • Ionic structure and dynamics were investigated on a molecular scale. • Anomalous dielectric constants were found based on the MD simulations. • Hydrogen bonding network clearly explains the anomalous dielectric constant. • The dielectric constant greatly influences on ion transport in a CDI system. [ABSTRACT FROM AUTHOR]