Co-CoO encapsulated in nitrogen-doped carbon nanotubes for capacitive desalination: Effects of nano-confinement and cobalt speciation.

Capacitive deionization (CDI) has gained increasing attention as an environmentally friendly and energy-efficient technology for brackish water desalination. However, traditional CDI electrodes still suffer from low salt adsorption capacity and unsatisfactory reusability, which inhibit its application for long-term operations. Herein, we present a facile and effective approach to prepare Co and CoO nanoparticles co-incorporating nitrogen-doped (N-doped) carbon nanotubes (Co-CoO/N-CNTs) via a pyrolysis route. The Co-CoO nanoparticles were homogeneously in-situ encapsulated in the inner channels of the conductive CNTs to form a novel and efficient CDI electrode for the first time. The encapsulation of Co-CoO nanoparticles in CNTs not only inhibits the Co leaching but also significantly enhances the desalination capacity. The morphology, structure, and capacitive desalination properties of the Co-CoO/N-CNTs were thoroughly characterized to illuminate the nano-confinement effects and the key roles of the interaction between cobalt species in the CDI performance. The co-existing metallic cobalt and cobalt oxides act as the roles of effective active sites in the CDI performance. As a consequence, the optimum Co-CoO/N-CNTs electrode displays an outstanding desalination capacity of 66.91 mg NaCl g at 1.4 V. This work provides insights for understanding the nano-confinement effects and the key roles of the interaction between cobalt species on the CDI performance.