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.

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Prussian Blue and its analogues (PBAs) are promising cation intercalation materials for energy storage and environmental applications. Here, we investigate Na + diffusion in porous electrodes comprised of nickel hexacyanoferrate (NiHCF) PBA nanoparticles (NPs), conductive carbon additive, and polymer binder. We combine experimental characterization, an electronically limited version of porous electrode theory, and simulation to link rate limitations in galvanostatic cycling to electron conduction through NP agglomerates. Using potentiostatic intermittent titration (PITT), we find that the apparent diffusion coefficient of Na + within NiHCF electrodes varies non-monotonically between 10 −11 cm 2 /sec (at 50% degree of intercalation, DOI) and 10 −10 cm 2 /sec (at DOIs of 0% and 100%). Galvanostatic cycling of electrodes with different average NP-agglomerate sizes reveals that two-fold higher rate capability is achievable when agglomerate radius reduces two-fold, despite having the same NP size distribution. We subsequently introduce and validate theory that explains the variation of diffusion coefficient with DOI, yielding a simple expression for the apparent diffusion coefficient that is proportional to the effective electronic conductivity through NP agglomerates. Finally, using DOI-dependent PITT data we model galvanostatic (dis)charge through electroactive spheres and show agreement with experimental results, confirming that electron conduction through NP agglomerates limits the rate capability of NiHCF electrodes.

Partial substitution of Ni2+ in the host lattice of nickel hexacyanoferrate by Mg2+ or Ca2+ from aqueous electrolytes leads to rapid capacity fade during galvanostatic cycling, while capacity is retained by intercalation into interstitial sites.

NASICON (sodium superionic conductor) materials are promising host compounds for the reversible capture of Na + ions, finding prior application in batteries as solid-state electrolytes and cathodes/anodes. Given their affinity for Na + ions, these materials can be used in Faradaic deionization (FDI) for the selective removal of sodium over other competing ions. Here, we investigate the selective removal of sodium over other alkali and alkaline-earth metal cations from aqueous electrolytes when using a NASICON-based mixed Ti−V phase as an intercalation electrode, namely, sodium titanium vanadium phosphate (NTVP). Galvanostatic cycling experiments in three-electrode cells with electrolytes containing Na + , K + , Mg 2+ , Ca 2+ , and Li + reveal that only Na + and Li + can intercalate into the NTVP crystal structure, while other cations show capacitive response, leading to a material-intrinsic selectivity factor of 56 for Na + over K + , Mg 2+ , and Ca 2+ . Furthermore, electrochemical titration experiments together with modeling show that an intercalation mechanism with a limited miscibility gap for Na + in NTVP mitigates the state-of-charge gradients to which phase-separating intercalation electrodes are prone when operated under electrolyte flow. NTVP electrodes are then incorporated into an FDI cell with automated fluid recirculation to demonstrate up to 94% removal of sodium in streams with competing alkali/alkaline-earth cations with 10-fold higher concentration, showing process selectivity factors of 3−6 for Na + over cations other than Li + . Decreasing the current density can improve selectivity up to 25% and reduce energy consumption by as much as ∼50%, depending on the competing ion. The results also indicate the utility of NTVP for selective lithium recovery.

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