Nickel hexacyanoferrate electrodes for high mono/divalent ion-selectivity in capacitive deionization

Selective ion removal has been a point of focus in capacitive deionization because of its industrial applications such as water purification, water softening, heavy metal separation and resource recovery. Conventionally, carbon is used as electrode material for selectivity. However, recent developments focus on intercalation materials such as Prussian Blue Analogues, due to their size-based preference towards cations. Selectivity of nickel hexacyanoferrate electrodes from a mixture of Na⁺, Mg²⁺, and Ca²⁺ ions was studied in this work. Here, a CDI cell with two identical NiHCF electrodes was operated in two desalination modes: (a) cyclic, in which ions are removed from and released into the same water reservoir and thus, the ion concentration remains the same after one cycle, and (b) continuous, in which ions are removed from one water reservoir and released back in a different reservoir. An average separation factor of ≈15 and 25, reflecting the selectivity of the electrodes, was obtained for Na⁺ over Ca²⁺ and Mg²⁺ from an equimolar solution of Na⁺, Ca²⁺ and Mg²⁺ in both, cyclic and continuous desalination. It was concluded that NiHCF, used in a symmetric CDI cell, is a promising material for highly selective removal of Na⁺ from a multivalent ion mixture.