Enhancing\nthe Ion-Size-Based Selectivity of Capacitive\nDeionization Electrodes

Abstract

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