Recent Advances in Ion Selectivity with Capacitive Deionization Prof. Louis C.P.M. de Smet Wageningen University & Wetsus, the Netherlands

Within the last decade, in addition to water desalination, capacitive deionization (CDI) has been used for resource recovery and selective separation of target ions in multicomponent solutions.  This presentation will provide an overview of the mechanisms of selective ion removal from a mixture of salt solutions utilizing different electrode materials, carbon, non-carbon, and composites, together with or without membranes.

Special focus will be put on recent developments from our labs in applying polymers, particularly polyelectrolytes, and using intercalation materials to pursue ion selectivity in CDI.  First, we investigated the selective separation of Na and Mg ions by coating polyelectrolyte multilayers (PEMs) on a standard grade cation-exchange membrane, used in combination with porous, carbon-based electrodes. Consequently, the Na/Mg ion selectivity value changed from 0.5 to 2.8, that is from a Mg ion preference to a Na ion preference.

Next, we studied a non-carbon system, containing nickel hexacyanoferrate (NiHCF) intercalation electrodes, which involves the energy-efficient, (reversible) redox reaction-based insertion of cations in the NiHCF lattice. We found a preferential uptake of monovalent Na over divalent Mg and Ca ions with selectivity values ³ 20, which remained largely independent of the Na ion concentration.

We consider modular functionalization methods such as utilizing PEMs on membranes and/or electrodes in both carbon and non-carbon systems, a vital step in widening the ion selectivity capabilities of CDI. Such capabilities also have the potential to further enable the use of applied energy efficiently in removing target ions.