Structure-performance tailored covalent organic framework for superior capacitive deionization removal of heavy metal ions

Covalent organic frameworks (COFs) are highly regarded for their tunable pore structures, high specific surface areas, and functionalizable active sites, making them promising candidates for heavy metal removal through capacitive deionization (CDI). However, their application in CDI faces inherent challenges, such as low electrical conductivity and insufficient utilization of redox-active sites. To address these limitations, a high-performance COF-based electrode material was synthesized by integrating COFs with carbon nanotubes (CNTs) via in situ growth (COF@CNT). By optimizing the crystallinity, charge distribution, and accessibility of active sites in the COF@CNT framework, the resultant sulfonic acid-functionalized TpPa (Tp: 1,3,5-triformylphloroglucinol and Pa: 1,4-phenylenediamine) COF (S-TpPa@CNT) exhibited an exceptional Cd²⁺ adsorption capacity of 165.23 mg/g at 1.2 V with an initial concentration of 80 mg/L, representing state-of-the-art performance and the highest reported value among CDI electrodes. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that the synergistic roles of sulfonic acid groups and the β-ketoenamine structure within the COF framework regulated the charge distribution within the COF framework and created a lower binding energy state. These findings demonstrate the potential of functionalized COF@CNT composites as high-performance electrode materials for efficient and sustainable water purification, paving the way for next-generation CDI technologies.