Ion selective electrosorption by two pseudocapacitive intercalating nanocomposite electrodes

Two ion intercalating electrodes were in an ion selectivity study for capacitive deionization (CDI). One has MXene (Ti₃C₂T_x) and reduced graphene oxide (rGO) as MG. The second involved rod-shaped NTO (Na₈Ti₅O₁₄) and rGO as NG. An ion selectivity study of mono- and divalent cations in binary and ternary solutions was conducted. The results suggested that the ion selectivity was influenced by the hydrated radius, the valence state of the ions, and the intrinsic charge of the electrode materials. In a binary solution, both electrodes exhibited higher selectivity for K⁺ ions over Li⁺ ions due to their smaller hydration radius. In a ternary solution, the order of ion selectivity was Ca²⁺ ≈ Mg²⁺ > Li⁺ and Ca²⁺ ≈ Mg²⁺ > K⁺ respectively. The NG electrode demonstrated a higher selectivity for divalent ions over monovalent ions (Mg²⁺/Li⁺ and Ca²⁺/Li⁺) due to stronger intrinsic negative charge. The specific adsorption capacities (SACs) of MG electrode with well-organized interlayer spaces were found to be higher than NG electrode as 105, 130, 122 mg/g for MG, and 99, 102, 90 mg/g for NG. A pseudo-second-order model was found to describe the kinetics of diffusion-controlled intercalation. These findings are valuable for developing selective electrosorption water treatment systems.