Additive-Free, In Situ Rapid Repair of Vacancies in Fe[Fe(CN)₆] Electrodes for Efficient Capacitive Deionization

Fe[Fe(CN)₆] (FeHCF) is considered a promising material for capacitive deionization-desalination of saline wastewater due to its excellent structure. However, additives are usually introduced during the synthesis of FeHCF in order to avoid [Fe(CN)₆]^3- vacancy defects filled by ligand water, which can result in the appearance of harmful byproducts and additional water treatment costs. In this study, an additive-free in situ vacancy repair strategy is proposed for the rapid synthesis of high-quality FeHCF in a saturated K₃Fe(CN)₆ solution. During the process of synthesizing FeHCF in solution, a high concentration of [Fe(CN)₆]^3- is found to facilitate the binding of Fe³⁺ to [Fe(CN)₆]^3- and hinder the hydrolysis and coordination reaction of Fe³⁺. After undergoing repair, FeHCF4 demonstrates an increased capacity and highly reversible electrochemical performance due to the robust structure. When utilized as Faraday cathodes in hybrid capacitive deionization (HCDI) systems, FeHCF4 exhibits a higher salt removal capacity (65.67 mg g^-1) and lower energy consumption (0.68 kWh kg^-1-NaCl) compared to unrepaired FeHCF1, while still maintaining excellent cycling performance. This environmentally friendly approach of repairing vacancies serves as a source of inspiration for the advancement of high-performance Prussian Blue analogues as capacitive sodium-removing materials.