Speaker
Description
Prussian Blue Analogues (PBAs) have attracted considerable interest as cathode materials for next-generation sodium-ion and multivalent batteries due to their open framework, rapid ion diffusion, structural versatility, and low cost synthesis. However, the role of structural water and its influence on electrochemical performance remains incompletely understood. Here, we combine operando Fourier-transform infrared (FTIR) spectroscopy at ALBA's MIRAS beamline¹ with operando X-ray diffraction (XRD) to elucidate reaction mechanisms, structural evolution, and water dynamics in PBAs. Continuous monitoring of the cyanide (CN⁻) and water (OH⁻) stretching vibrations reveals distinct redox mechanisms across different hydrated PBA compositions. In Berlin Green (FeFe-PBA), the reversible shift of the CN⁻ stretching band from 2113 to 2102 cm⁻¹ directly tracks Fe redox activity and spin-state transitions during Na⁺ and Ca²⁺ intercalation.² Structural water also exhibits composition-dependent behavior. Commercial MnFe-Prussian White undergoes progressive, irreversible water release coupled to structural phase transitions, whereas FeFe-Prussian Blue and Berlin Green seem to exhibit some reversible water dynamics that closely follow the Fe²⁺/Fe³⁺ redox process and associated lattice-volume changes.³ These findings showcase operando synchrotron FTIR as a unique tool to complement to XRD and XAS and reveal structural-water dynamics largely hidden from conventional operando techniques.