Additive-Free,\nIn Situ Rapid Repair of Vacancies in\nFe[Fe(CN)<sub>6</sub>] Electrodes for Efficient Capacitive Deionization

Fe[Fe(CN)<sub>6</sub>] (FeHCF) is considered a promising\nmaterial\nfor capacitive deionization–desalination of saline wastewater\ndue to its excellent structure. However, additives are usually introduced\nduring the synthesis of FeHCF in order to avoid [Fe(CN)<sub>6</sub>]<sup>3–</sup> vacancy defects filled by ligand water, which\ncan result in the appearance of harmful byproducts and additional\nwater treatment costs. In this study, an additive-free in situ vacancy\nrepair strategy is proposed for the rapid synthesis of high-quality\nFeHCF in a saturated K<sub>3</sub>Fe(CN)<sub>6</sub> solution. During\nthe process of synthesizing FeHCF in solution, a high concentration\nof [Fe(CN)<sub>6</sub>]<sup>3–</sup> is found to facilitate\nthe binding of Fe<sup>3+</sup> to [Fe(CN)<sub>6</sub>]<sup>3–</sup> and hinder the hydrolysis and coordination reaction of Fe<sup>3+</sup>. After undergoing repair, FeHCF4 demonstrates an increased capacity\nand highly reversible electrochemical performance due to the robust\nstructure. When utilized as Faraday cathodes in hybrid capacitive\ndeionization (HCDI) systems, FeHCF4 exhibits a higher salt removal\ncapacity (65.67 mg g<sup>–1</sup>) and lower energy consumption\n(0.68 kWh kg<sup>–1</sup>-NaCl) compared to unrepaired FeHCF1,\nwhile still maintaining excellent cycling performance. This environmentally\nfriendly approach of repairing vacancies serves as a source of inspiration\nfor the advancement of high-performance Prussian Blue analogues as\ncapacitive sodium-removing materials.