Nitrogen-Doped Carbon Nanospheres for Capacitive Deionization

Capacitive deionization (CDI) based on the theory of a double electric layer has received much attention in the past two decades as a highly promising desalination technology, and carbon nanospheres are considered promising carbon-based electrode materials for CDI, which have been investigated by a large number of researchers. However, the isolation between the spheres greatly increases the contact resistance, limiting their application in CDI. To understand the effect of structure on the performance of CDI and to obtain a better CDI performance, solid carbon spheres and monodisperse hollow carbon spheres were prepared and compared with interconnected hollow carbon spheres (IHCSs). The resulting IHCSs, with their unique cavity structure and interconnected carbon walls, exhibited superior electrochemical performance and electrosorption capacity. In addition, we improved the electrical conductivity, surface wettability, and adsorption properties of the carbon nanospheres by incorporating nitrogen doping. By adjusting the amount of nitrogen functional groups in the carbon nanosphere precursors, we produced interconnected hollow carbon nanospheres with similar structures (particle size, specific surface area, and porosity) but different nitrogen contents and investigated the effects of different nitrogen contents on the CDI properties of hollow carbon nanospheres. The results indicated that interconnected hollow carbon nanospheres with a high nitrogen content (designated as NFs-2) demonstrated the best CDI performance, achieving an electrochemical adsorption capacity of 20.43 mg g–1 in a NaCl solution with a concentration of 500 mg L–1. Additionally, NFs-2 exhibited the largest specific surface area of 728 m2 g–1. Furthermore, NFs-2 displayed excellent cycle stability in a desalination cycle using a 100 mg L–1 NaCl solution, retaining up to 97.17% of its initial desalination capacity after 25 sorption–desorption cycles. These findings highlight the promising applications of NFs-2 in CDI technology.