MnO2 decorated N-doped mesoporous carbon electrodes boost enhanced removal of Cu2+ and Pb2+ ions from wastewater via a hybrid capacitive deionization platform

Integrating Faradaic (charge transfer) materials with carbon has been proven extensively to be a valid strategy to prepare highly efficient electrodes for electrochemical desalination or removal of heavy metal ions from wastewater via a capacitive deionization (CDI) platform. However, the influences of preliminary functionalization of the carbon component (e.g., nitrogen doping, hydroxyl grafting) and pairing of cathodes and anodes on the desalination performance have yet to be thoughtfully 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 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 Cu2+ solutions, respectively (cf. 32.2 mg Pb2+ per gram electrode materials in 200 ppm Pb2+ solution). Besides, these electrodes exhibit excellent cycling stability when applied in removing each heavy metal ion separately, with an IAC retention 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 the MnO2, and surface complexation at the external surface of the [MnO6] octahedral layers. These findings underscore the pivotal roles of pre-functionalization of the original carbon and pairing of cathodes and anodes in the configuration of HCDI cells with improved performance and provide valuable insight into the optimization of Faradaic/carbon composite electrodes for remediation of wastewater with heavy metals via CDI platforms.