Initially, carbon-based electrodes were combined with surface modifications to address (and tune) ion selectivity. In most cases ion-exchange membranes were added to a (membrane) capacitive deionization (CDI) system (MCDI). Polyelectrolyte (PE) multilayers (PEMs) on a standard grade cation-exchange membrane (CXM) were investigated in MCDI for the selective separation of Na+ and Mg2+. The Na+/Mg2+ selectivity was found to change from 0.5 to 2.8 upon the addition of a PEM and be stable during a 40-cycle experiment. Also, different pairs of (functionalized) PEs and different operational CDI conditions have been investigated to improve and/or impart selectivity.
Furthermore, CDI electrodes made from carbon and conductive polymers showed an increased anion removal capacity (up to 65 mg Cl/g_anode) as well as an almost full selectivity towards chloride removal in binary anion solutions, with either monovalent phosphate or sulphate. Phosphate selectivity in CDI via such a rejection mode was further studied with various anion-exchange membranes (AXMs).
In addition, intercalating CDI electrodes have been explored to study the selectivity to cations. Experimentally, the focus was on transition metal hexacyanoferrates. The salt removal capacity of nickel hexacyanoferrate electrodes was found to be ~35 mg/g (at 20 mM NaCl) and the PBA electrodes were found to consume 50% less energy than carbon electrodes for a similar degree of desalination. High inherent selectivities were obtained: Na+/Ca2+ ≈15 and Na+/Mg2+ ≈25.
Interestingly, the preference of divalent ion was observed when using vanadium instead of nickel (Ca2+/Na+ ≈3.5). This opposite behavior was understood by density functional theory simulations. Furthermore, coating of the vanadium-based electrodes with a conducting polymer prevents contamination of the treated water following the electrode degradation.
Furthermore, the use of multi-layered, polyelectrolyte coatings was also combined with intercalation materials. This enabled the simultaneous selectively of cations (by the electrodes) and anions (by the coated membranes).
Crown-ethers (CE), organic compounds known to interact with cations selectively, were chemically bond to various polyelectrolytes from fossil- and bio-based origin. The resulting functionalized polymers were used to build multi-layered coatings. The build-up process, their thickness and viscoelasticity, as well as their interactions with various cations were studied with quartz crystal microbalance with dissipation monitoring (QCM-D). When changing from Cs+ to K+ and Na+ solutions, the QCM-D responses were found to be larger for the CE-containing coatings, indicating that these building blocks can facilitate the further development of ion separation applications.
Finally, together with a strong, international team of CDI experts a review was written on recent advances in ion selectivity with capacitive deionization.
