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

MR Ceron and F Aydin and SA Hawks and DI Oyarzun and CK Loeb and A Deinhart and C Zhan and TA Pham and M Stadermann and PG Campbell, ACS APPLIED MATERIALS & INTERFACES, 12, 42644-42652 (2020).

DOI: 10.1021/acsami.0c07903

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 (S-Na/Ca >> 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.

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