ZIF-67 metal-organic frameworks and CNTs-derived nanoporous carbon structures as novel electrodes for flow-electrode capacitive deionization

Capacitive deionization (CDI) has been attracting great interest as a promising desalination technology during the last decade because of its energy, cost effectiveness, and eco-friendly process. In particular, CDI using flowable electrodes (FCDI) offers a continuous desalination stream without the need for discharging and much better salt removal than that of conventional CDI desalination technologies. Nevertheless, FCDI desalination still suffers from the inherent low electrical conductivity of its liquid slurry electrode, which results in high internal and interfacial resistances and limits the salt-removal performance of the FCDI desalination system. In this study, in order to improve the electrical conductivity of flow electrodes, we synthesized zeolitic imidazolate frameworks (ZIF-67), made of the widely investigated Zn/Co-based MOFs, onto CNT templates (ZIF-67@CNTs) and investigated their FCDI desalination performance. The salt removal rate for ZIF-67@CNTs included flow electrodes reached 1.09 mmol/m2s, which is a 57% increase over that of pristine AC (0.69 mmol/m2s), with a saltremoval efficiency of 37.3%. Electrochemical analyses including CV and EIS measurements confirmed that such improved salt removal performance originates from the enhanced electrical conductivity by the formation of a conducting bridge between the suspended AC particles in the slurry electrodes.