Nickel Hexacyanoferrate Electrodes for Continuous Cation Intercalation Desalination of Brackish Water

Abstract

Using porous electrodes containing redox-active nickel hexacyanoferrate (NiHCF) nanoparticles, we construct and test a device for capacitive deionization in a two flow-channel device where the intercalation electrodes are in direct contact with an anion-exchange membrane. Upon reduction of NiHCF, cations intercalate into it and the water in its vicinity is desalinated; at the same time water in the opposing electrode becomes more saline upon oxidation of NiHCF in that electrode. In a cyclic process of charge and discharge, fresh water is continuously produced, alternating between the two channels in sync with the direction of applied current. We present proof-of-principle experiments of this technology for single salt solutions, where we analyze various levels of current and cycle durations. We analyze salt removal rate and energy consumption. In desalination experiments with salt mixtures we find a threefold enhancement for K + over Na + -adsorption, which shows the potential of NiHCF intercalation electrodes for selective ion separation from mixed ionic solutions.

Theory of Water Desalination with Intercalation Materials

“…Finally, local chemical equilibrium is assumed between cations in the intercalation material and in the electrolyte. This chemical equilibrium is described using the Frumkin isotherm for intercalation [8,23,24]…”

“…As a consequence, the fluxes J ions,m and J ch,m become J ions,m = −k m ∆c T,m − ωX ∆φ m J ch,m = −k m 〈c T,m 〉 ∆φ m (7) where 〈...〉 is an average between values at the extreme ends of the membrane, and ∆ refers to a difference between these positions (with fluxes defined from left to right, ∆ is defined as "right" minus "left"). The total ion concentration on each side of the membrane is related to the salt concentration just outside (i.e., in the spacer channel, at a position right next to the membrane), c * , as c T,m = X 2 + (2c * ) 2 (8) while the Donnan potential (change in electrical potential) across each membrane edge is given by…”

“…For the electrolyte we have c = 10 mM, D = 2.2 × 10 −10 m 2 /s (based on a porosity of 30% and tortuosity based on the Bruggeman equation), and L = 250 µm, while for the nanoparticles, c = 2.2 M (at ϑ = 0.5 for NiHCF, see ref. [8]), D = 6.9 × 10 −16 m 2 /s [31], and L = 5 nm (based on a particle with a size of 30 nm; note that for a sphere, a typical diffusion depth is equal to the volume/area-ratio, which is on sixth of the size of the sphere). These numbers give a critical flux for Na + in the nanoparticles that is about 35× larger than in the electrolyte phase in the electrode, thus underpinning the choice to focus on transport in the electrolyte.…”

Redox‐Polysilsesquioxane Film as a New Chloride Storage Electrode for Desalination Batteries

“…The Cl − ions can be mostly removed from the sea water by 1) the oxidation of Cl − ion into Cl 2 gas, which is highly toxic in nature and needs a high electrode potential of +1.36 V (higher than O 2 evolution; +1.23 V); 2) a membrane separation process, including RO, deionization, etc. ; and 3) an energy storage device such as a capacitive deionization and desalination battery . Except the last one, all others are energy feeding processes.…”

Desalination with porous electrodes : Mechanisms of ion transport and adsorption

“…These problems related to electrode stability limit applicability of MCDI as a desalination technology for water containing dissolved oxygen. In order to improve electrode stability, it is important to find and investigate electrode materials that are stable, have a high salt adsorption capacity and are electronically conductive [195]. Not only electrodes, but also membranes may degrade in time [196].…”