Unraveling the chloride ion capture capability of nitrogen-doped porous carbon for capacitive deionization and desalination battery

Abstract

Developing carbonaceous materials with low cost and high chloride ions capture ability is highly desired but great challenging for the practical application of capacitive deionization (CDI). Herein, we use cheap petroleum coke as precursors to synthesize N-doped activated porous carbon (N-a₄pC), which exhibited excellent Cl^- capture capability. The N-a₄pC electrode possesses a larger capacitance and more pronounced pseudocapacitive characteristics. We then assemble N-a₄pC//activated carbon (AC) cell with N-a₄pC as the anode and AC as the cathode, and the N-a₄pC(Cl^-)//AC(Na⁺) cell demonstrated high desalination capability of 38.1 mg g⁻¹, which is significantly higher than the desalination capability of the AC(Cl^-)//AC(Na⁺) and AC(Cl^-)//N-a₄pC(Na⁺) cells. Density functional theory (DFT) calculations show that N-containing graphene structures have a greater adsorption energy for Cl^-, and the differential charge density maps indicate that N facilitate the accumulation of weak positive charges. Moreover, CDI cell as desalted batteries to light one light-emitting diode (LED) shows a continuous and stable discharge process. The desorption voltage of the N-a₄pC(Cl^-)//AC(Na⁺) cell is influenced by the adsorption voltage and the concentration of the NaCl. Meanwhile, the desorption voltage increases proportionally with the number of cells, demonstrating the stability as a desalting battery. This work demonstrates the significant potential of industrial waste in the development of low cost carbon electrodes, reveals the mechanism behind the enhanced Cl^- capture capability due to N-doping and pore structure optimization, and systematically showcases the desalting battery performance using a CDI cell during the desorption process.