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Cation Selectivity in Capacitive Deionization: Elucidating the Role of Pore Size, Electrode Potential, and Ion Dehydration

Journal Article · · ACS Applied Materials and Interfaces
Capacitive deionization (CDI) is a promising water desalination technology that is applicable to the treatment of low-salinity brackish waters and the selective removal of ionic contaminants. In this work, we show that by making a small change in the synthetic procedure of hierarchical carbon aerogel monolith (HCAM) electrodes, we can adjust the pore-size distribution and tailor the selectivity, effectively switching between selective adsorption of calcium or sodium ions. Ion selectivity was measured for a mixture of 5 mM NaCl and 2.5 mM CaCl2. For the low activated flow-through CDI (fteCDI) cell, we observed extremely high sodium selectivity over calcium (SNa/Ca $$\gg$$ 10, no Ca2+ adsorbed) at all of the applied potentials, while for the highly activated fteCDI cell, we observed a selectivity value of 6.6 ± 0.8 at 0.6 V for calcium over sodium that decreased to 2.2 ± 0.03 at 1.2 V. Molecular dynamics simulations indicated that the loss in Ca2+ selectivity over Na+ at high applied voltages could be due to a competition between ion-pore electrostatic interactions and volume exclusion (“crowding”) effects. Interestingly, we also find with these simulations that the relative sizes of the ions change due to changes in hydration at a higher voltage.
Research Organization:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA)
Grant/Contract Number:
AC52-07NA27344
OSTI ID:
1678847
Report Number(s):
LLNL-JRNL--809022; 1015112
Journal Information:
ACS Applied Materials and Interfaces, Journal Name: ACS Applied Materials and Interfaces Journal Issue: 38 Vol. 12; ISSN 1944-8244
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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