Structural/Compositional-Tailoring of Nickel Hexacyanoferrate Electrodes for Highly Efficient Capacitive Deionization

Prussian blue analogs (PBAs) represent a crucial class of intercalation electrode materials for electrochemical water desalination. It is shown here that structural/compositional tailoring of PBAs, the nickel hexacyanoferrate (NiHCF) electrodes in particular, can efficiently modulate their capacitive deionization (CDI) performance (e.g., desalination capacity, cyclability, selectivity, etc.). Both the desalination capacity and the cyclability of NiHCF electrodes are highly dependent on their structural/compositional features such as crystallinity, morphology, hierarchy, and coatings. It is demonstrated that the CDI cell with hierarchically structured NiHCF nanoframe (NiHCF-NF) electrode exhibits a superior desalination capacity of 121.38 mg g⁻¹, a high charge efficiency of up to 82%, and a large capacity retention of 88% after 40 cycles intercalation/deintercalation. In addition, it is discovered that coating of carbon (C) film over NiHCF can lower its desalination capacity owing to the partial blockage of diffusion openings by the coated C film. Moreover, the hierarchical NiHCF-NF electrode also demonstrates a superior selectivity toward monovalent sodium ions (Na⁺) over divalent calcium (Ca²⁺) and magnesim (Mg²⁺) ions, allowing it to be a promising platform for preferential capturing Na⁺ ions from brines. Overall, the structural/compositional tailoring strategies would offer a viable option for the rational design of other intercalation electrode materials applied in CDI techniques.