Ammonia Removal from Water by Capacitive Deionization Using N-Doped Carbon Derived from ZIF-8

Ammonia removal from wastewater is critical due to its toxicity and eutrophication impact, and capacitive deionization (CDI) offers a promising energy-efficient solution, with its performance being strongly influenced by the electrode materials used. In this study, nitrogen-doped carbon electrodes were synthesized via calcination of ZIF-8 precursors and evaluated in a flow-through (FT) capacitive deionization system for ammonia removal. Experimental results were complemented by density functional theory (DFT) and ab initio molecular dynamics simulations (AIMD) to elucidate the ammonia adsorption mechanisms. Materials calcined at 600, 700, and 800 °C were examined, with 800 °C identified as the optimal condition for CDI testing. Electrode performance was assessed using a 500 mg/L NH₄Cl feed in batch mode at a constant voltage of 1.20 and -1.20 V during charging and discharging. Additional tests in constant-current mode, applying 0.01-3 mA/cm², were performed to evaluate behavior under varying operational conditions. Despite a 9 mg/cm² loading, nitrogen-doped carbon electrodes showed significantly enhanced performance, achieving an adsorption capacity of 32 mg/g. To clarify the origin of this high performance, physicochemical, structural, and theoretical analyses were conducted along with cyclic voltammetry and electrochemical impedance spectroscopy to assess electrochemical properties. Additionally, DFT and AIMD analysis highlighted the advantageous adsorption and electronic features of NH₄⁺@N-doping configurations in graphene with and without interaction with water. The system demonstrated its energy-saving potential with a charge efficiency of 28-45% and an energy consumption of 0.0013 kWh/kg. Overall, nitrogen-doped carbons show strong promise for improving FT-CDI in energy-efficient ammonia removal.