Revisiting Nickel Hexacyanoferrate Derivatized from Nickel and Nickel Hydroxide: Crystallinity as the Key to Enhanced Heterogeneous Electron-Transfer Rate

Abstract Nickel hexacyanoferrate (NiHCF) has attracted considerable interest for electrochemical applications; however, the relationship between its formation pathway, atomic structure, and charge-transfer kinetics remains poorly understood. Here, we investigate the electrochemical derivatization of NiHCF from deposited nickel (ND-NiHCF) and nickel hydroxide (NHOD-NiHCF), focusing on the structural origins that govern electron-transfer efficiency. Electrochemical analyses reveal that NHOD-NiHCF exhibits approximately 2-fold higher heterogeneous electron-transfer rate constant (kS) and substantially faster K+ diffusion than ND-NiHCF. Structural characterization by TEM, SAED, and grazing-incidence wide-angle X-ray scattering (GIWAXS) shows that NHOD-NiHCF consists of smaller crystallites within a polycrystalline framework. Operando infrared reflection-absorption spectroscopy (IRRAS) provides mechanistic insight into NiHCF formation, revealing dynamic structural evolution during electrochemical derivatization and its impact on electron-transfer kinetics. X-ray absorption spectroscopy (XAS) further demonstrates local coordination environments that ND-NiHCF possesses elongated C≡N bonds and a pronounced XANES pre-edge feature, indicative of local symmetry distortion. A structural model involving π back-donation from the underlying metallic Ni0 to cyanide ligands is proposed, leading to lattice expansion, hindered K+ intercalation, and inferior electron-transfer performance.