Cation selectivity of activated carbon and nickel hexacyanoferrate electrode materials in capacitive deionization: A comparison study.

In this study, the electrosorption selectivity of porous activated carbon (AC) and nickel hexacyanoferrate (NiHCF), which represent two working mechanisms of capacitive electrosorption and redox intercalation, was investigated to separate cations in capacitive deionization (CDI). The cyclic voltammetry diagrams of AC showed the rectangular shape of double-layer charging, while that of NiHCF showed separated peaks associated with redox reactions. The specific capacitance of NiHCF was 143.6 F/g in 1 M NaCl, which was almost two times higher than that of AC. Cation selectivity experiments were conducted in single-pass CDI for a multi-cation solution. The electrosorption preference of the AC cathode was determined by a counterbalance between the ionic charge and hydrated size, reflecting the selectivity coefficient of different cations over Na+ in the range of 0.86–2.63. For the NiHCF cathode, the cation selectivity was mainly dominated by the hydrated radius and redox activity. Notably, high selectivities of K+/Na+ ≈ 3.57, Na+/Ca2+ ≈ 9.97, and Na+/Mg2+ ≈ 18.92 were obtained. A significant improvement in the electrosorption capacity and monovalent ion selectivity can be achieved by utilizing the NiHCF electrode. The study demonstrates the fundamental aspects and promising opportunities of CDI in regard to ion selectivity. [Display omitted] • NiHCF was fabricated as an intercalating electrode for cation separation. • CDI was conducted to examine selective cation separation in a multi-ionic solution. • Cation selectivity of AC was restricted by a counterbalance of ion charge and size. • High selectivity of NiHCF was achieved by its ion-size affinity and redox activity. • CDI with NiHCF is a promising approach for selective monovalent ion removal. [ABSTRACT FROM AUTHOR]