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

Kaustub Singh,Zexin Qian,P.M Biesheuvel,Han Zuilhof,Slawomir Porada,Louis C.P.M De Smet

doi:10.1016/j.desal.2020.114346

Abstract

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.

Highlights

Capacitive Deionization (CDI) is an electrochemical technique in which polarized electrodes remove ions from water [1,2]
We demonstrate the performance of electrodes fabricated using nickel hexacyanoferrate (NiHCF) in CDI experiments to selectively remove monovalent (Na+) over divalent (Ca2+, Mg2+) ions
The surface morphology of the NiHCF particles, observed under scanning electron microscopy (SEM) (Fig. S1 Supporting Information), shows inter-particle agglomeration, and BET analysis shows a small surface area (≈13 m2/g). While such a small area would be highly detrimental for carbon-based electrodes in CDI, it does not hamper the performance of intercalationbased materials, in view of the different mode of ion storage, i.e. inside the lattice of the intercalation material rather than only near its surface

Summary

Introduction

Capacitive Deionization (CDI) is an electrochemical technique in which polarized electrodes remove ions from water [1,2]. In some intercalation materials, such as Prussian Blue Analogues (PBAs) [16,17,18], the ion insertion is accompanied by the electrochemical reduction of a redox-active element in the lattice These intercalation materials retain the attributes of porous carbon-based electrodes, such as a non-toxic nature, easy preparation, and fast charge transfer kinetics [16], while improving upon the charge adsorption capacity [19] and eliminating co-ion expulsion from the electrode [4]. It has been studied with electrodes based on intercalation materials as well [25,26,27] Much of this focus has been on the separation between cations with the same valence, and very few studies are available on the selective removal of a cation from a mixture of ions with different valency.

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Conclusion