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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% HNO 3 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. Unlabelled Image • A new method improving the stability of an AC-based CDI cell was provided. • NAC shows a highly negatively charged surface and fast ion desorption rates. • NAC//AC promotes reversible SAC compared to AC//AC. • NAC30//AC exhibits a maximum reversible SAC of 24.7 ± 1.6 mg g−1. • N doping effectively reduces the AC oxidation in the long cycling test. [ABSTRACT FROM AUTHOR]

Abstract In capacitive deionization (CDI) systems, more Faradaic reactions generally occur at a higher cell voltage, resulting in the poor ion-removal efficiency and high energy consumption, but a low cell voltage may lead to a low deionization capacity. In this study, an integrated strategy, consisting of (1) selecting the acceptable charge/discharge time ratio, (2) finding the potential windows of both positive and negative electrodes, and (3) using the charge-balanced method for determining the optimal mass ratio between the positive and negative electrodes (denoted as (m+)/(m−)), is proposed to find the optimal cell voltage of a CDI system with a high desalination efficiency and low energy consumption. According to this strategy, two newly designed cells using two types of activated carbon (AC) show excellent CDI performances; cell 1 with AC = ACS679, (m+)/(m−) = 1:1.4, and cell voltage = 1.4 V: salt adsorption capacity of 12 mg g−1, charge efficiency of 62%, and energy consumption of 109 kJ mol−1 and cell 2 with AC = ACS25, (m+)/(m−) = 1:1, and cell voltage = 1.2 V: salt adsorption capacity of 13 mg g−1, charge efficiency of 81%, and energy consumption of 72 kJ mol−1 although the potential window of water decomposition for such AC-coated electrodes in 8 mM NaCl is about 2 V. This universal strategy is applicable to all CDI systems utilizing various electrode materials. Graphical abstract Image 1 Highlights • We demonstrate a universal strategy for constructing the AC//AC cell with high CDI performances. • The same charge/discharge time ratio with high reversibility on both electrodes is the key. • Influences of two-electrode mass ratio on the desalination performances are studied. • The pH change is affected by the Faradaic reactions from the potential windows of electrodes. [ABSTRACT FROM AUTHOR]