Divalent ion selectivity in capacitive deionization with vanadium hexacyanoferrate: Experiments and quantum-chemical computations

Selective 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 analogues (PBAs) are proposed as an electrode
material for selectively removing cations from water, based on their size. So far, PBAs
used in CDI have been selective towards monovalent ions. Here, we introduce
vanadium hexacyanoferrate (VHCF), a PBA, as a new electrode material in a hybrid
CDI setup, to selectively remove divalent cations from water.
These electrodes preferred divalent Ca 2+ over monovalent Na + , with a separation
factor, β Ca/Na ≈ 3.5. This finding contrasts the observed monovalent ion selectivity
by PBA electrodes. This opposite behavior was rationalized by Density Functional
Theory (DFT) simulations. Furthermore, the coating of the VHCF electrodes with a
conducting polymer (poly-pyrrole, doped with poly-styrenesulphonate, PPy/PSS)
prevented 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 in CDI. This study paves the way for tunable selectivity while extending the
library of electrodes that can be successfully used in (selective) CDI.