Abstract
As the demand for efficient, large-scale energy storage solutions grows, redox flow batteries (RFBs) emerge as a key technology for integrating renewable energy systems. However, improving energy efficiency and performance of RFBs remains a significant challenge. This study investigates metal hexacyanometallates as solid booster materials for redox-targeting flow batteries (RTFBs), leveraging their tunable redox properties, structural stability, and mixed valence states. Using density functional theory with the Perdew-Burke-Ernzerhof functional, this work explores the electronic structure, reduction potential, and vibrational properties of Cu²⁺, Co²⁺, Fe²⁺, and Mn²⁺ hexacyanometallates. Simulated infrared spectra revealed key vibrational modes, including shifts in CN stretching frequencies upon alkali metal intercalation. Notably, intercalation induced redshifts and increased intensity in CN modes, demonstrating significant electronic structure alterations. Band structure analysis showed distinct electronic properties, with Co₃[Fe(CN)₆]₂ exhibiting half metallic behavior and Fe₃[Fe(CN)₆]₂, Mn₃[Mn(CN)₆]₂, and Cu₃[Fe(CN)₆]₂ displaying metallic characteristics. Potassium intercalation shifted conduction bands closer to the Fermi level, resulting in semiconducting behavior for K₂Fe₃[Fe(CN)₆]₂ and a band gap of 1.2 eV. Reduction potential analysis highlighted Fe₃[Fe(CN)₆]₂ and Mn₃[Mn(CN)₆]₂ as top candidates for enhancing redox activity in RTFBs. These findings demonstrate the potential of metal hexacyanometallates as tunable booster materials. Future research will explore the impact of alkali metal intercalation, including potassium, sodium, and lithium ions, on reduction potentials to further optimize their performance in RTFBs.