Enhanced Removal of Cu ²⁺ and Pb ²⁺ Ions from Wastewater via a Hybrid Capacitive Deionization Platform with MnO ₂ /N-Doped Mesoporous Carbon Nanocomposite Electrodes

Integrating MnO₂ with carbon is a reliable strategy to improve capacitive deionization (CDI) performance by leveraging the unique properties of both components (i.e., MnO₂ and carbon). However, the influences of preliminary functionalization of carbon (e.g., nitrogen doping, KOH activation) and pairing of cathodes and anodes on the CDI performance have yet to be systematically explored. Herein, we prepared a group of MnO₂-decorated mesoporous carbon composites with nitrogen as a dopant (i.e., MK-NMCS, K-NMCS, NMCS, and CS), and systematically evaluated the desalination performance of various cathode//anode pairs in a hybrid capacitive deionization (HCDI) for capturing Na⁺, Cu²⁺, and Pb²⁺, respectively. Of all electrodes, the MK-NMCS//K-NMCS pair demonstrates the optimum desalination performance based on salt adsorption capacity (SAC) and cycling stability, offering a SAC of 25.4 mg g^-1 and a SAC retention of 102.4% after 50 consecutive charge-discharge cycles at 1.2 V in 500 ppm of NaCl solution. In addition, the MK-NMCS//K-NMCS electrodes also show the maximum ion adsorption capacity (IAC) toward Cu²⁺ and Pb²⁺ ions compared to other cathode//anode pairs, attaining an IAC of 37.0 and 30.0 mg Cu²⁺ per gram electrode materials at 1.2 V in 500 and 200 ppm of Cu²⁺ solutions, respectively (cf. 32.2 mg of Pb²⁺ per gram of electrode materials in 200 ppm of Pb²⁺ solution). Besides, these electrodes exhibit excellent cycling stability when applied in removing each heavy metal ion separately, with IAC retentions of 90.0 and 98.5% after 50 cycles toward Cu²⁺ and Pb²⁺ ions, respectively. Mechanical analysis reveals that both heavy metals are likely to be sequestered via capacitive electrosorption by carbon, intercalation with MnO₂, and surface complexation at the external surface of the [MnO₆] octahedral layers. Our results demonstrated a great potential of the MnO₂-decorated N-doped carbon//prefunctionalized carbon pairs, in particular, the MK-NMCS//K-NMCS electrode pair for capturing heavy metal ions via HCDI platforms. Such prefunctionalization and pairing strategies are very promising for screening high-performance composite electrodes for wastewater remediation.