Enhanced capacitive deionization of a low-concentration brackish water with protonated carbon nitride-decorated graphene oxide electrode

Jian Yu ^a, Yue Liu ^a, Xumei Zhang ^a, Rumeng Liu ^a, Qi Yang ^a, Shen Hu ^a, Haiou Song ^{b, c, *}, Pengcheng Li ^d, Aimin Li ^c, Shupeng Zhang ^{a, c, *}

Abstract

Freshwater resources are one of the core elements that affect the harmonious development of mankind and society. Capacitive deionization (CDI) technology is one of the effective methods to transform brackish water into fresh water. The choice of material for a CDI electrode is critical to its electrosorption performance, which directly affects the electrosorption performance through interface optimization. Herein, protonated carbon nitride (H-C₃N₄)-modified graphene oxide (H-C₃N₄-mGO_1/8) is fabricated by a simultaneous nucleophilic addition and amide reaction in order to enhance capacitive deionization of a very low concentration brackish water. Using activated carbon (AC) as the positive electrode and H-C₃N₄-mGO_1/8 as the negative electrode, H-C₃N₄-mGO_1/8 || AC asymmetric CDI devices are used to remove ions from a NaCl aqueous solution. The CDI test results indicate that the system has a high electrosorption capacity of 8.36 mg/g in the 50 mg/L NaCl solution with a low applied voltage of 1.2 V, which is 1.40 times than AC || AC symmetric one. The salt electrosorption capacity, electrosorption rate of the H-C₃N₄-mGO_1/8 || AC asymmetric electrodes improve with increasing applied voltage due to the stronger Coulombic interaction between the electrode and charged ions with the formation of a more sufficient electric double layer principle.

Keywords: Capacitive deionization; Electrosorption; Organic functionalization; Asymmetric electrode

In this work, a novel nanomaterial made up of GO modified by H-C₃N₄ through a simple one-pot method has been designed successfully. An asymmetric CDI electrode was assembled with H-C₃N₄-mGO_1/8 and AC, and the initial concentration of the NaCl solution was 50 mg/L for CDI adsorption testing using a working voltage of 1.2 V. The results showed that the electrical adsorption capacity of the asymmetric electrode was 8.36 mg g^-1, which is 1.40 times that of commercial AC || AC. It also performs fast maximum adsorption rate of 0.1879 mg ( g·min) ^-1 and low power density of 45.89 J g^-1. The adsorption capacity remained above 94% after six cycles, indicating its good stability of cycling. Above all, H-C₃N₄-mGO_1/8 || AC electrodes have a huge potential in desalination for low-concentration NaCl solution.