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

Area of Science:

• Electrochemistry
• Materials Science
• Environmental Engineering

Background:

• Capacitive deionization (CDI) is a promising technology for water treatment, particularly for brackish water desalination and softening.
• Standard CDI cells utilize microporous carbon electrodes to remove ions via electric double-layer (EDL) formation.
• Existing CDI systems exhibit size-based ion selectivity, favoring the adsorption of smaller hydrated ions over larger ones.

Purpose of the Study:

• To investigate the enhancement of size-based ion selectivity in CDI through surface functionalization of microporous electrodes.
• To develop a theoretical model incorporating finite ion size and micropore chemical charge to explain selectivity enhancements.
• To experimentally validate the theoretical predictions using a functionalized CDI cell.

Main Methods:

• Development of a micropore electrical double-layer (EDL) theory accounting for finite ion size and surface chemical charge.
• Fabrication and testing of a capacitive deionization (CDI) cell with a surface-functionalized (oxidized) cathode.
• Electrosorption experiments using an electrolyte containing equimolar potassium (K+) and lithium (Li+) ions.

Main Results:

• Surface functionalization of the cathode significantly enhanced the electrosorption ratio of smaller K+ to larger Li+ ions (from ~1 to 1.84 at 0.4 V).
• The developed micropore EDL theory accurately predicts selectivity enhancements due to denser counterion packing in functionalized micropores.
• Experimental limitations included cathode chemical charge degradation at higher applied voltages (~1 V).

Conclusions:

• Surface functionalization is an effective strategy to substantially improve size-based ion selectivity in capacitive deionization (CDI).
• The theoretical model provides valuable insights into the mechanisms governing enhanced ion selectivity in functionalized CDI systems.
• Further research is needed to address material stability challenges for practical application of functionalized CDI at higher voltages.