InterPore2026

Ion selectivity with capacitive deionization

21 May 2026, 15:20

15m

Oral Presentation (MS02) Environmental Porous Media: Water, Agriculture, and Remediation MS02

Speaker

Volker Presser

Description

Capacitive deionization (CDI) is widely implemented as an electrosorptive desalination technology that is typically designed and operated to maximize overall salt removal rather than to achieve ion-specific selectivity. However, ion electrosorption in porous carbons is inherently governed by pore size, surface chemistry, ion hydration, and transport kinetics, so that selective behavior can be deliberately exploited. This presentation will show how appropriate control of carbon porosity and operating conditions enables robust ion selectivity, from classical CDI cells to continuously operated flow-electrode CDI (FCDI) systems.
First, the presentation will address ion sieving in ultramicroporous carbon cloth electrodes with a very narrow pore size distribution centered around approximately 0.6 nm. In mixed electrolyte solutions, sub-nanometer confinement imposes hydration-shell energy barriers that depend strongly on ion size and dehydration energy. As a consequence, heavier monovalent cations such as K+ and Cs+ are preferentially electrosorbed over Li+ and Na+, while divalent cations such as Mg2+ and Ca2+ are effectively excluded from the smallest pores. When the pore size is increased into the wider micropore range, the selectivity pattern gradually shifts back toward the classical preference for multivalent species, illustrating the competition between electrostatic attraction and dehydration penalties. The time-dependent uptake further highlights how equilibrium and transport jointly shape the observed selectivity.
In the second part, the focus will shift from cyclic CDI operation to continuous separation using FCDI with activated carbon slurry electrodes. Here, ion removal and selectivity are tuned via pore structure, applied voltage, slurry composition, and flow conditions. In multi-cation feed solutions, the FCDI cell exhibits a pronounced preference for the removal of Ca2+ and Mg2+ over monovalent cations, consistent with their higher charge-to-hydrated-size ratio, while maintaining high separation rates and charge efficiencies approaching 70 % at moderate cell voltages.
Taken together, these results define a materials- and process-based design space for selective CDI, from static ion sieving in well-defined sub-nanometer pores to continuous monovalent–divalent separation in flow-electrode systems. The presentation will emphasize how precise control and characterization of pore networks, in the spirit of the InterPore community, can be translated into targeted ion separation at the device level.

References:
P. Ren, B. Wang, J.G.A. Ruthes, M. Torkamanzadeh, V. Presser, Cation selectivity during flow electrode capacitive deionization, Desalination 592 (2024) 118161.
Y. Zhang, J. Peng, G. Feng, V. Presser, Hydration shell energy barrier differences of sub-nanometer carbon pores enable ion sieving and selective ion removal, Chem Eng J 419(1) (2021) 129438.
Y. Zhang, P. Ren, L. Wang, E. Pamete, S. Husmann, V. Presser, Selectivity toward heavier monovalent cations of carbon ultramicropores used for capacitive deionization, Desalination 542 (2022) 116053.