Electric Double-Layer Structure in Primitive Model Electrolytes: Comparing Molecular Dynamics with Local-Density Approximations

Abstract

ABSTRACT:We evaluate the accuracy of local-density approximations (LDAs) using explicit molecular dynamics simulations of binary electrolytes comprised of equisized ions in an implicit solvent. The Bikerman LDA, which considers ions to occupy a lattice, poorly captures excluded volume interactions between primitive model ions. Instead, LDAs based on the Carnahan− Starling (CS) hard-sphere equation of state capture simulated values of ideal and excess chemical potential profiles extremely well, as well as the relationship between surface charge density and electrostatic potential. Excellent agreement between the EDL capacitances predicted by CS-LDAs and computed in molecular simulations is found even in systems where ion correlations drive strong density and free charge oscillations within the EDL, despite the inability of LDAs to capture the oscillations in the detailed EDL profiles.

Desalination with porous electrodes : Mechanisms of ion transport and adsorption

“…1.2, which are formed in the micropores inside the carbon particles [29][30][31][32]. These EDLs contain three types of charge: electronic, chemical and ionic.…”

“…Suss describes in Ref. [32] how the preferential adsorption based on ion-size can be considered, namely by adding an excess chemical potential difference between micro-and macropores, ∆µ [29,32,192]. The BMCSL equation, which we will not discuss in full detail in this work, describes ∆µ ex i as function of the hard-sphere diameter of ion i, which is used as a fitting parameter, and a factor that represents the volume fraction occupied by all finite sized ions [32].…”

“…For electrodes based on carbon, as will be considered in this Chapter, during the adsorption step, ions are stored in electrical double layers (EDLs) formed in the electrodes [29,30]. For every electron transported from one electrode to the other, in both electrodes a counterion can adsorb in these EDLs, or a co-ion can desorb, see Chapter 2.…”

“…In CDI with porous carbon electrodes, ions are adsorbed into electrical double layers (EDLs) that are formed in the micropores of the electrodes [29][30][31][32]. For each electron transported from one electrode to the other, either a counterion can be adsorbed into the EDL (desired), or a co-ion can be desorbed (undesired).…”

“…Consequently, feed water flowing through the cell is desalinated. Ions are adsorbed in the micropores of the electrodes, where electrical double layers (EDLs) are formed [29][30][31]. After the electrodes are saturated with salt, they are discharged and ions desorb.…”