Citric acid-assisted synthesis of FeFe(CN)6 with reduced defects and high specific surface area for aqueous zinc–sodium hybrid batteries

Abstract

Due to their open structure, numerous reactive sites, and eco-friendly characteristics, Prussian blue-based electrode materials find widespread application in aqueous zinc-ion batteries. However, the Prussian blue materials face challenges such as low specific surface area and high defect rate, resulting in slow ion transport and poor cycling performance. This study focuses on synthesizing ferric ferricyanide (FeFe(CN)₆) material with reduced vacancies and a higher specific surface area. The synthesis process incorporates citric acid as a chemical assistant, resulting in the as-synthesized C-FeFe(CN)₆ featuring a significantly larger specific surface area of 406.625 m² g⁻¹, compared to FeFe(CN)₆ without citric acid (28.143 m² g⁻¹). Moreover, C-FeFe(CN)₆ has a lower vacancy rate of 20% compared to FeFe(CN)₆ at 31%. The addition of citric acid effectively slows down the nucleation rate of C-FeFe(CN)₆ crystals. When utilized as cathode materials in aqueous zinc–sodium hybrid batteries, C-FeFe(CN)₆ demonstrates highly reversible Na⁺ storage behavior, exhibiting a remarkable reversible capacity of 162.5 mA h g⁻¹ at 0.1C and a good cycling performance with 88% capacity retention over 500 cycles (at a high rate of 45C). These superior performance characteristics make C-FeFe(CN)₆ a promising candidate for extensive energy storage applications.