Selective Mono/divalent Ion Separation in Bifunctional Ionomeric Capacitive Deionization Incorporated with Counter-Charge Ionomer Layer

In this study, a carbon-based bifunctional ionomeric capacitive deionization (BICDI) system was successfully developed by applying a counter-charge ionomer layer to the BICDI electrode without the need of a monovalent ion exchange membrane (M-IEM) for the selective separation of mono/divalent ions. The counter-charge ionomer layer facilitates the selective separation of mono/divalent ions through a combination of mechanisms involving electrostatic repulsion, steric hindrance, and hydrophobic interactions. Meanwhile, the BICDI electrode matrix serves as an ion transport channel, enabling seamless ion transport and improving the electrosorption capacity of the electrode. SEM and EDS analyses have confirmed the presence of a counter-charge ionomer layer encapsulating the BICDI electrode. Notably, BICDI-C cell with counter-charge ionomer layer exhibits higher Li+ ion adsorption capacity and selectivity (321.6 mu mol/g, beta(Li+)(Mg2+) = 6.6) compared to commercial monovalent cation exchange membrane CDI (MCDI-C) cell (206.4 mu mol/g, beta(Li+)(Mg2+) = 1.4). The selectivity of the BICDI-C cell for mono/divalent ions can be optimized by adjusting various testing parameters, including flow rate, voltage, etc. More importantly, this versatile method is not only applicable for the separation of monovalent cation, but can also enhance the monovalent anion adsorption capacity and selectivity of BICDI-A (43.6 mu mol/g, beta(Cl-)(SO42-) = 7.3) cell by incorporating a counter-charge ionomer layer onto the BICDI cathode. This approach allows for the efficient separation of both cationic and anionic species, broadening the range of potential applications for BICDI technology.