Potential‐Dependent, Switchable Ion Selectivity in Aqueous Media Using Titanium Disulfide

Abstract

The selective removal of ions by an electrochemical process is a promising approach to enable various water-treatment applications such as water softening or heavy-metal removal. Ion intercalation materials have been investigated for their intrinsic ability to prefer one specific ion over others, showing a preference for (small) monovalent ions over multivalent species. In this work, we present a fundamentally different approach: tunable ion selectivity not by modifying the electrode material, but by changing the operational voltage. We used titanium disulfide, which shows distinctly different potentials for the intercalation of different cations and formed binder-free composite electrodes with carbon nanotubes. Capitalizing on this potential difference, we demonstrated controllable cation selectivity by online monitoring the effluent stream during electrochemical operation by inductively coupled plasma optical emission spectrometry of aqueous 50 mm CsCl and MgCl . We obtained a molar selectivity of Mg over Cs of 31 (strong Mg preference) in the potential range between -396 mV and -220 mV versus Ag/AgCl. By adjusting the operational potential window from -219 mV to +26 mV versus Ag/AgCl, Cs was preferred over Mg by 1.7 times (Cs preference).

Recent advances in ion selectivity with capacitive deionization

“…123 On a system level, dependence of cell selectivity on operation parameters such as applied current, cell voltage, ion concentration, and pH among others will have to be systematically studied to find optimum conditions that enhance selectivity for the CDI cell. Intercalation electrodes such as TiS 2 show switchable preference depending on the potential of the electrode, as shown by Srimuk et al 43 Such insights will be useful in realizing the full potential of existing (and the search for new) electrode materials.…”

“…This technique of determination has been used in CDI literature for selective separation from cationic mixtures. 43,44 2. Electrodes for ion selectivity…”

“…7A, as they specifically interact with Na and Li ions. Srimuk et al paired a TiS 2 intercalation electrodes 43 with an AC anode in a CDI setup to study selective removal of Mg 2+ vs. Cs + . The TiS 2 electrodes were found to be more selective towards Mg 2+ (r E 30) in a specific applied electrode potential range (vs. Ag/AgCl).…”

Structure and Potential‐Dependent Selectivity in Redox‐Metallopolymers: Electrochemically Mediated Multicomponent Metal Separations

“…A swapping of ion preference (α A,B crossing 1) was not observed for all anion pairs, however, these results correlate with prior observations that electrode charge density can impact ion‐preference at different potentials. [ 32a,35 ]…”

“…Previous electrosorption work has shown that the effect of surface change density on the preference of ions adsorbed to that surface is dependent on several factors including ion size, hydrated ion size, and pore size. [ 32,35 ] Near‐neighbor effects, solvation, and ion‐crowding within pores may play a key role in dictating selectivity. Future work using in situ characterization methods will aid in the elucidation of more complex mechanisms for ion selectivity, and shed light into the interplay between structural and potential‐dependent selectivity.…”