Abstract
AbstractSelective removal of ions from water via capacitive deionization (CDI) is relevant for environmental and industrial applications like water purification, softening, and resource recovery. Prussian blue analogs (PBAs) are proposed as an electrode material for selectively removing cations from water, based on their size. So far, PBAs used in CDI are selective toward monovalent ions. Here, vanadium hexacyanoferrate (VHCF), a PBA, is introduced as a new electrode material in a hybrid CDI setup to selectively remove divalent cations from water. These electrodes prefer divalent Ca2+ over monovalent Na+, with a separation factor, βCa/Na ≈3.5. This finding contrasts with the observed monovalent ion selectivity by PBA electrodes. This opposite behavior is understood by density functional theory simulations. Furthermore, coating the VHCF electrodes with a conducting polymer (poly‐pyrrole, doped with poly‐styrenesulphonate) prevents the contamination of the treated water following the degradation of the electrode. This facile and modular coating method can be effortlessly extended to other PBA electrodes, limiting the extent of treated water contamination during repeated cycling. This study paves the way for tunable selectivity while extending the library of electrodes that can be successfully used in (selective) CDI.
Highlights
Selective removal of ions from water via capacitive deionization (CDI) is or voltage.[1,2] Once removed, these ions relevant for environmental and industrial applications like water purification, are stored inside or at the surface of the softening, and resource recovery
The substitution of nickel with vanadium in the lattice of a Prussian blue analogs (PBAs) switches its preference in CDI experiments from monovalent ions toward divalent ions
This finding provides the first correlation between the substituent transition metal ion in a PBA and the preference of the resulting PBA toward different ions, and opens vast possibilities for tuning PBA selectivity according to the desired application, within the framework of ion-selective CDI
Summary
The synthesis of VHCF-active particles was performed according to a modified coprecipitation adopted from literature.[32]. A 20 × 10−3 m solution of Na4[Fe(CN)6]·10H2O (Sigma Aldrich) was prepared in 200 mL of water containing 1% (v/v) HCl (Sigma Aldrich). These acidified precursors were dropwise added to a 200 mL of 10% (v/v) HCl solution. The crystallinity of the VHCF particles was assessed by powder X-ray diffraction (XRD), performed using a copper source for diffraction angles in the range of 10° < 2θ < 70°. The X-ray photoelectron spectroscopy (XPS) spectrum of the VHCF particles was obtained by a JPS-9200 photoelectron spectrometer (JEOL, Japan) under ultrahigh vacuum, using a monochromatic Al Kα source at 12 kV and 20 mA and was processed by CASA XPS software (version 2.3.16) with Shirley background fitting correction. The water content in the VHCF powder was estimated by thermo-gravimetric analysis (TGA), performed under N2 environment at the rate of 5 °C min−1 from 25 to 700 °C
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