MOF-templated synthesis of nitrogen-doped carbon for enhanced electrochemical sodium ion storage and removal

Water scarcity has become a prominent global challenge in the twenty-first century, prompting the rapid advancement of desalination technology . Capacitive deionization (CDI) stands out as a cost-effective solution for sustainable water purification . The electrode material plays a pivotal role in capacitive deionization, impacting the salt ion removal and charge storage capacity. Carbon-based materials, characterized by high surface area and electrical conductivity , are ideal materials for capacitive deionization. However, their effectiveness in salt ion removal is hindered by unclear pore structures and poor wettability , limiting salt ion transport and storage. In this study, nitrogen-doped hierarchical porous carbon is successfully synthesized through the carbonization of MOF-5 and melamine mixtures, wherein melamine serves as both a nitrogen source and porogenic agent. Through optimization of carbonization temperature, the resulting MOF-5-derived nanoporous carbon (referred to as NPC-800) retains the cubic morphology of MOF-5, possesses a large surface area (754.34 ​m 2 ​g −1 ), high nitrogen content (10.13 ​%), and favorable wettability . Electrochemical analysis reveals that the NPC-800 electrode demonstrates specific capacities of 91.8, 76.1, 66.3, 51.0, 28.0, and 15.2 mAh ​g −1 ​at current densities of 0.2, 0.5, 1.0, 2.0, 4.0, and 6.0 ​A ​g −1 , respectively, outperforming NPC-700 (26.3, 19.7, 13.1, 6.90, 2.30, and 1.30 ​mAh ​g −1 ) and NPC-900 (46.0, 37.8, 30.4, 21.3, 11.7, and 7.50 mAh ​g −1 ). The superior electrochemical performance of NPC-800 can be attributed to its maximal specific surface area, abundant pore structure, and optimal wettability, facilitating increased active sites for salt ion adsorption and diffusion. Moreover, NPC-800 exhibits low intrinsic resistance, rapid ion transfer kinetics, and exceptional cycling stability (50,000 cycles) with 100 ​% capacity retention at 5 ​A ​g −1 . Further investigation into the CDI performance of NPC electrodes under different applied voltages (0.8, 1.0, and 1.2 ​V) and initial NaCl solution concentrations (100, 300, and 500 ​mg ​L −1 ) demonstrates the superior adsorption capacity of the NPC-800 electrode compared to the other two electrodes. Specifically, at 1.2 ​V in a 500 ​mg ​L −1 salt solution, NPC-800 exhibits a faster salt adsorption rate (2.8 ​mg ​g −1 ​min −1 ) and higher salt adsorption capacity (24.17 ​mg ​g −1 ) compared to NPC-700 and NPC-900. Consequently, the melamine-assisted synthesis of N-doped porous carbon materials holds promise as an optimal choice for capacitive deionization.