Carboxyl-functionalized covalent organic framework for high-performance capacitive deionization

Covalent organic frameworks (COFs) are emerging as promising candidates for high-performance capacitive deionization (CDI) for their ordered pore architectures, tunable functionalization, and abundant active sites. Here, we demonstrate a carboxyl-functionalized COF (TpPa-COOH-COF) for enhanced CDI performance. Comprehensive structural characterizations reveal that the microporous channels of TpPa-COOH-COF, decorated with carboxyl groups exhibit superior electrochemical performance and high salt adsorption capacity (SAC) compared to its non-functionalized counterpart (TpPa-COF). The integration of carboxyl groups within the material enhances the efficient adsorption of Na+, leveraging a combination of electrostatic forces and ligand coordination. In a hybrid CDI (HCDI) system, TpPa-COOH-COF electrode demonstrates remarkable desalination performance, achieving a SAC of 70.49 mg g−1 in a 2,000 mg L−1 NaCl solution under 1.6 V. Furthermore, the pH-dependent ionization behavior of carboxyl within the channel endows the electrode with tunable ion selectivity. Selective Na+ adsorption dominates under weakly acidic to neutral conditions (pH 4 and 7), while K+ selectivity increases under mildly alkaline condition (pH 9). The successful integration of carboxyl with ordered porous architectures provides a strategic pathway for developing next-generation desalination systems with improved efficiency and tunable ion selectivity.