(184j) Mosaic Ion-Exchange Resins for Reducing the Overpotential for Water Dissociation in Bipolar Junctions for Electodeionization Separations

Authors: 
Electrodeionization (EDI) is an electrochemical separation technology capable of removing salt

from water using an applied electric field, ion-exchange membranes, and ion exchange resin bed

in the diluent stream. [1] The porous ion-exchange resin bed reduces the purified stream

resistance, the largest resistance in the system, by conducting ions across the immobilized bed

and by dissociating water to generate H+ and OH- ions for current flow through the stack. Water

dissociation in the ion-exchange resin bed occurs in nanometer thin bipolar junctions in the resin

bed. [2-4] These junctions consist of oppositely fixed positive charges that generate an internal

electric field for dissociating water. Most ion-exchange resin beds for EDI employ commercially

available cation and anion ion-exchange resins mixed together, but such mixing of the different

beads results in underutilized resins because few fixed anionic and cationic groups form effective

bipolar junctions. The consequence of poorly formed bipolar junctions results in energy

inefficient water desalination via EDI.

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In this work, mixtures of thin cation and anion polymer electrolyte brushes were thermally

grafted to silicon wafer and fumed silica nanoparticle surfaces to realize thin film mosaic ion-

exchange materials for EDI. Grafting the oppositely charged ion conductors to the same substrate

leads to a greater probability of opposite fixed charges being in close proximity for water

dissociation. The process to prepare the mosaic materials started with thermally grafting mono-

hydroxy terminated non-ionic polymer brushes to the substrate via a condensation reaction under

an inert environment. Then, subsequent ionization reactions (e.g., Menshutkin, acetyl

sulfonation, or thermal decomposition/acid catalyzed hydrolysis) [5-7] converted the brushes

into anionic and cationic polymer electrolytes. Verification of brush grafting and introduction of

ionic moieties was made using water contact angle measurement and Fourier transform infrared

spectroscopy (FTIR). Ionic conductivity of the homo- and mixed-polymer electrolyte brushes to

interdigitated electrodes was conducted via electrochemical impedance spectroscopy. Future

studies aim to incorporate the mosaic materials for resin-wafer EDI.