Improved performance and long-term stability of activated carbon doped with nitrogen for capacitive deionization

Nitrogen-doped activated carbon (NAC) is prepared by a combination of acid pre-treatment and thermal nitrogen doping for the positive electrode of asymmetric capacitive deionization (a-CDI) cells. The oxygen content in AC controlled by the acid pre-treatment significantly affects the doping amount of N atoms from melamine, which enhances the surface negative charge in NACs to promote the salt adsorption capacity (SAC). Here NAC with 30% HNO3 pre-treatment (NAC30) possesses a highly negatively charged surface to exhibit a fast ion desorption rate during discharging. The asymmetrical NAC30//AC cell shows the maximum reversible SAC of 24.7 +/- 1.6 mg g(-1). In addition, the negative surface charge of NAC30 is further promoted and the reversible SAC of NAC30//AC are greatly enhanced to ca. 55 mg g(-1) when pH of the 8 mM NaCl solution is adjusted from 5.4 to 7.5. In the long-term stability test, NAC30//AC remains 40% of its maximum reversible SAC after 100 charging-discharging cycles, indicating that our nitrogen doping is able to effectively reduce the oxidation of activated carbon, confirmed by the electrochemical impedance analysis.