Size-Based Selectivity of Capacitive Deionization Electrodes in Mixtures of Monovalent Ions

Eric Guyes, Faculty of Mechanical Engineering, Technion - Israel Institute of Technology, Haifa, Israel
Matthew Suss, Faculty Of Mechanical Engineering, Technion - Israel Institute Of Technology, Haifa, Israel

Capacitive deionization (CDI) is a fast-developing technology with primary applications in wastewater remediation, brackish water desalination, and water softening. Electric double layer (EDL) models have been extensively used to predict characteristics of CDI electrodes, such as salt storage capacity, salt adsorption rate, and charge storage.¹ Such models typically treat ions as point particles and thereby neglect ion volume exclusion interactions in the electrode micropores. Therefore, these models do not predict preferential adsorption in monovalent ion systems, despite experimental evidence that smaller ions are selectively adsorbed.² An understanding of selectivity would be useful for wastewater and agricultural systems to remove undesirable species (e.g., excess sodium or heavy metals) while retaining minerals (calcium and magnesium).

Recent work by Suss incorporated volume exclusion interactions into a single-electrode equilibrium CDI model and fitted it to experimental data using the ion radius as an adjustable parameter.²  The work we present here extends the model of Suss to a two-electrode system and validates it for a range of charging voltages in a system of potassium chloride and lithium chloride salts. As shown the figure, our measured selectivity factor for (smaller) potassium vs. (larger) lithium at 1 V matches closely the theoretical results for an adjusted ion diameter d_i = 1.3d_h,i, where d_h,i is the hydrated ion diameter (diamonds). Data from three other experimental works are also shown (refs. 23, 27, 31 in Suss). Further, novel mechanisms for boosting ion-size based selectivity will be presented, and possible applications in wastewater removal and liquid phase separations will be analyzed and discussed.

References:

1. Biesheuvel, P. M., Porada, S., Levi, M. & Bazant, M. Z. Attractive forces in microporous carbon electrodes for capacitive deionization. J. Solid State Electrochem. 18, 1365–1376 (2014).

2.  Suss, M. E. Size-based ion selectivity of micropore electric double layers in capacitive deionization electrodes. J. Electrochem. Soc. 164, E270–E275 (2017).