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.