Enhancing the Ion-Size-Based Selectivity of Capacitive Deionization Electrodes

Capacitive deionization (CDI) is an emerging water treatment technology often applied to brackish water desalination and water softening. Typical CDI cells consist of two microporous carbon electrodes sandwiching a dielectric separator, and desalt feedwater flowing through the cell by storing ions in electric double layers (EDLs) within charged micropores. CDI cells have demonstrated size-based ion selectivity wherein smaller hydrated ions are preferentially electrosorbed over larger hydrated ions. We demonstrate that such size-based selectivity can be substantially enhanced through the addition of chemical charge to micropores via surface functionalization. We develop a micropore EDL theory that includes both finite ion size effects and micropore chemical charge, which predicts such enhancements and elucidates that they result from denser counterion packing in micropores. With our experimental CDI cell, we desalted an electrolyte consisting of equimolar potassium (K⁺) and lithium (Li⁺) ions. We show that use of a surface-functionalized (oxidized) cathode significantly increased the electrosorption ratio of smaller K⁺ to larger Li⁺ compared to a cell with a pristine cathode, for example, from ∼1 to 1.84 for a charging voltage of 0.4 V. Our model predicts yet-higher electrosorption ratios are attainable, but our experimental cell suffered from significant cathode chemical charge degradation at applied voltages of ∼1 V.