Insight into electrosorption behavior of monovalent ions and their selectivity in capacitive deionization: An atomic level study by molecular dynamics simulation

Abstract

Nanopore in capacitive deionization (CDI) plays a key role in selective removal of target ions. In this work, we investigate the transport behavior of ions in the electrosorption process on an atomic level using molecular dynamics (MD) simulations, and explore the selectivity between two typical ions, namely Cl− and NO3− characterized by identical valence but distinct molecular geometry. We show that dehydration effect enables smaller pores to accommodate more ions relative to its inner volume, but it requires higher electronic potential. However, high electronic potential decreases energy efficiency by enhancing the competence between anions and waters as highly polar molecules. It is also found that ion dehydration is crucial for selectivity with complicated dependency on pore size and electronic potential, as well as ion geometry. Planar geometry of NO3− helps removing the surrounding solvated waters while these waters are weakly coordinated around the O atom of NO3−. NO3− also promotes its entrance into the nanopore by altering its angle of the structive plane towards the longitudinal direction of the nanotube. With strong affinity to carbon, NO3− are preferred even when the carbon is not charged. In addition, a maximal selectivity (above 8 for 1.0-nm, 1.5-nm and 2.0-nm pores) can be obtained with certain electronic potential, but further increasing of the electronic potential will lead to lower selectivity.