Enhancing the Total Salt Adsorption Capacity and Monovalent Ion Selectivity in Capacitive Deionization Using a Dual-Layer Membrane Coating Electrode

Capacitive deionization (CDI) is highly attractive for seawater desalination, while the trade-off between total salt adsorption capacity (SAC) and monovalent ion selectivity is a big challenge for current electrodes, especially facing solution containing organics. Herein, a simple approach was used to prepare a novel dual-layer membrane coating electrode (D-MCE) by depositing ion exchange polymers onto activated carbon fiber (ACF) to overcome this issue. Performances of the ACF, single-layer MCE, anion exchange membrane ACF, and D-MCE were investigated. Compared to ACF, the single-layer MCE showed an increased total SAC from 435.7 to 593.0 μmol/g with a reduced Cl–/SO42– selectivity from 0.9 to 0.4. The introduction of the membrane increased total SAC to 524.3 μmol/g and decreased Cl–/SO42– selectivity to 0.7. The D-MCE exhibited an improved total SAC to 679.5 μmol/g by inhibiting the co-ion repulsion and Faradaic reaction. Meanwhile, the D-MCE enabled fast Cl– diffusion and presented a high selectivity (3.8). After 50 cycles, the D-MCE exhibited an almost unchanged total SAC and selectivity even in the presence of organics. The narrowed pores and negative outermost layer endowed the D-MCE with an outstanding anti-fouling behavior, which well explained its stable performance. These findings confirmed the high potential of the D-MCE in addressing the trade-off and provided valuable insights into the design of the CDI electrode for desalination.