Enhancing the Ion-Size-Based Selectivity of Capacitive Deionization Electrodes

Abstract

Capacitive deionization (CDI) is an emerging water treatment technology often applied to brackish water desalination and water softening. Typical CDI cells consist of two microporous carbon electrodes sandwiching a dielectric separator, and desalt feedwater flowing through the cell by storing ions in electric double layers (EDLs) within charged micropores. CDI cells have demonstrated size-based ion selectivity wherein smaller hydrated ions are preferentially electrosorbed over larger hydrated ions. We demonstrate that such size-based selectivity can be substantially enhanced through the addition of chemical charge to micropores via surface functionalization. We develop a micropore EDL theory that includes both finite ion size effects and micropore chemical charge, which predicts such enhancements and elucidates that they result from denser counterion packing in micropores. With our experimental CDI cell, we desalted an electrolyte consisting of equimolar potassium (K+) and lithium (Li+) ions. We show that use of a surface-functionalized (oxidized) cathode significantly increased the electrosorption ratio of smaller K+ to larger Li+ compared to a cell with a pristine cathode, for example, from ∼1 to 1.84 for a charging voltage of 0.4 V. Our model predicts yet-higher electrosorption ratios are attainable, but our experimental cell suffered from significant cathode chemical charge degradation at applied voltages of ∼1 V.

Recent advances in ion selectivity with capacitive deionization

“…Recently, Guyes et al presented a theory which predicted an enhancement of size-based selectivity towards K + over Li + and Na + , with increasing chemical charges in the micropore added by surface modification. 142 Dynamic calculations by Zhao et al using porous electrode theory for mono/di cation mixtures, with monovalent anions, showed that an electrode that initially selectively adsorbed monovalent cations, switched to the adsorption of divalent cations and desorption of the adsorbed monovalent cations later in the process, in line with experimental observations. Also, in Zhao et al, Gouy-Chapman-Stern (GCS) theory was used for mono/di cation mixtures containing the same monovalent anion, and combined with a model that describes ion transport to a planar charged wall.…”

“…In a mixture of ions of varying sizes, the excluded volume effect can result in a selectivity towards the smaller ion, captured by a difference in F exc,i between ions. We make use here of a very useful expression for the contribution to the chemical potential of a volume/excess term, m exc,i in the pore, which results in a contribution F exc,i = exp(Àm exc,i ) where we assume that this excess term is zero in bulk solution, which is approximately valid for a low salt concentration (order 10 mM) 142. For an ion inside a pore, m exc,i was derived to be155,156…”

Electrochemical approaches for selective recovery of critical elements in hydrometallurgical processes of complex feedstocks

“…For instance, size-based selectivity of microporous carbon electrodes could be further enhanced by adding negatively charged carboxyl groups in the micropores through oxidation with nitric acid. The separation factor of K + over Li + increased from ∼1 to 1.84 thanks to functionalization ( Guyes et al., 2019 ). Vanadium was recovered using resin-activated carbon (RAC) composite electrodes.…”

Partial Desalination of Saline Groundwater: Comparison of Nanofiltration, Reverse Osmosis and Membrane Capacitive Deionisation

“…In total, the ion specific adsorption and the interaction between ions are time dependent with regard to the ion specific valence, diffusion coefficient, hydrated radius, the applied electricity, the pH, and the ion concentration [53,[82][83][84]. The ion volume exclusion interactions resulting in a preferred adsorption of ions with a smaller hydrated radius, were described by Suss and Guyes et al [85,86]. However, as currently shown in the study of Tsai et al [83], the influence of ion exchange membranes can reduce the affinity and thereby the selectivity.…”