Recent advances in selective ion separation using flow-electrode capacitive deionization

Abstract

The growing need for selective ion separation in water treatment and resource recovery has driven research beyond conventional desalination. Flow-electrode capacitive deionization (FCDI) exhibits the advantages of continuous operation and high capacity and is therefore well suited for these applications. This review focuses on the recent and rapid advances in achieving selective ion separation using FCDI. Initially, we discuss the fundamental mechanisms of achieving selectivity, broadly categorizing them into the engineering of flow-electrode materials (e.g., functionalization, intercalation hosts, nanostructured carbon), tailoring of ion-exchange membranes (e.g., polyelectrolyte coatings, nanofiltration membranes, carrier-facilitated transport), and tuning of operational parameters and system design (e.g., voltage, flow rate, cell architecture). Subsequently, we survey state-of-the-art applications according to target ions, including the separation of monovalent cations from divalent ones for water softening (Ca/Mg removal) and the recovery of critical resources such as lithium and ammonia. Furthermore, we cover the selective removal of anions, including nutrients (nitrate, phosphate), contaminants (fluoride, chromate), and valuable organic acids. Distinct from prior reviews on CDI selectivity, this work specifically highlights FCDI's unique features including continuous slurry-electrode operation, feed channels separated by ion-exchange membranes, and diverse flow/rocking-chair/redox modes, thus providing a focused synthesis of strategies and potential applications. Finally, key challenges related to system stability, fouling, and cell design are summarized, and future research directions are highlighted. This review demonstrates that through the sophisticated combination of materials science and system engineering, FCDI is evolving into a robust and versatile technology critical for the future of sustainable water treatment and the circular economy.