Interplay of Composition, Crystallinity, and Chemical Structure in CoHCF and NiHCF Thin Films Prepared at Different Temperatures

Understanding how growth conditions govern structural order and ion transport in Prussian blue analogues (PBAs) thin films is essential for optimizing their electrochemical performance. Here, cobalt and nickel hexacyanoferrate (CoHCF and NiHCF) thin films were electrodeposited potentiostatically at temperatures between 20 and 60°C. A combination of cyclic voltammetry, scanning electron microscopy, X-ray diffraction, and Raman spectroscopy was employed to elucidate the interplay between composition, crystallinity, and chemical structure. Under identical conditions, CoHCF exhibits a maximum current density approximately 2.2 times higher than NiHCF, indicating significantly faster electrochemical kinetics. X-ray diffraction reveals temperature-dependent lattice expansion without phase transitions, with a maximum near 40°C, associated with structural relaxation and compositional variations. Raman spectroscopy further reveals temperature-dependent local structural evolution, where cyanide band narrowing at intermediate temperatures indicates improved short-range order, while band broadening at higher temperatures reflects increased defect density. These findings demonstrate that temperature-controlled defect redistribution governs both short- and long-range structural order in PBA thin films, directly influencing ion transport and electrochemical response. This work provides new insights into structure–property relationships and establishes deposition temperature as a key parameter for tuning electrochemical functionality in hexacyanoferrate-based electrodes.