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 ​m² ​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.