Size-Based Ion Selectivity of Micropore Electric Double Layers in Capacitive Deionization Electrodes

Abstract

Capacitive deionization (CDI) is a fast-emerging technology most commonly applied to brackish water desalination. In CDI, salt ions are removed from the feedwater and stored in electric double layers (EDLs) within micropores of electrically charged porous carbon electrodes. Recent experiments have demonstrated that CDI electrodes exhibit selective ion removal based on ion size, with the smaller ion being preferentially removed in the case of equal-valence ions. However, state-of-the-art CDI theory does not capture this observed selectivity, as it assumes volume-less point ions in the micropore EDLs. We here present a theory which includes multiple couterionic species, and relaxes the point ion assumption by incorporating ion volume exclusion interactions into a description of the micropore EDLs. The developed model is a coupled set of nonlinear algebraic equations which can be solved for micropore ion concentrations and electrode Donnan potential at cell equilibrium. We demonstrate that this model captures key features of the experimentally observed size-based ion selectivity of CDI electrodes.

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.…”

“…The preferential adsorption of K + over Na + , or more general, of smaller over bigger ions, also in equilibrium conditions, has been reported in other work as well, see Refs. [32,96,189]. In the EDL models used in this work, the i-mD and amphoteric Donnan (amph-D) model, it is assumed that ions are volume-less points, and selectivity based on size is thus not included [32].…”

“…[32,96,189]. In the EDL models used in this work, the i-mD and amphoteric Donnan (amph-D) model, it is assumed that ions are volume-less points, and selectivity based on size is thus not included [32]. 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]. Suss showed that, in equilibrium, theory including Long-term operation and pH changes 157…”

“…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).…”