Capacitive deionization (CDI) has become a promising method to solve the shortage of freshwater resources recently. However, the co-ion expulsion effect obviously hinders electrosorption capacity and charge efficiency of CDI. In this work, an asymmetric CDI cell is assembled in which Na+-intercalated Ti3C2Tx (NaOH-Ti3C2Tx) serves as a cation-selective cathode, while the activated carbon (AC) serves as the anode. The NaOH-Ti3C2Tx with negatively charged surface groups (-OH, -O, and -F) is adopted to weaken the co-ion expulsion effect. Benefited from the synergistic effect of the reduced co-ion expulsion effect and expanded interlayer space, the asymmetric CDI cell achieves a higher electrosorption capacity of 12.19 mg g-1 and a higher charge efficiency of 0.826 compared with the symmetric one composed of AC (4.55 mg g-1 and 0.306) in 100 mg L-1 NaCl solution. High cyclic stability of the as-prepared asymmetric CDI cell is also observed. The improved desalination performance indicates that NaOH-Ti3C2Tx is a promising alternative as cation-selective cathode material for asymmetric CDI cells. The desalination mechanism is discussed in detail to lay the foundation for further improvement of the CDI performance of other 2D materials like MXene.
Keywords: MXene (Ti3C2Tx); asymmetric capacitive deionization; cation-selective cathode material; co-ion expulsion effect; negatively charged group.