Spectroscopic and microscopic study of Prussian blue film for electrochromic device application

All solid electrochromic devices (ECDs) were prepared using Prussian blue (PB) as electrochromic layer, CeO₂-TiO₂ as counter electrode and poly(vinylidene difluoride) (PVDF) as ionic conductive membrane. The electrodeposited PB films were characterized by ellipsometry and the thickness, roughness and refractive index of 217 nm, 1.9 nm and 1.5808, respectively, were obtained. The ECDs were assembled by combining the PVDF-based electrolyte membrane between the ITO-glass/CeO₂-TiO₂ and PB/ITO-glass electrodes. From the electrochemical measurements, it was stated that applying potentials of +1.5 and -2.8 V promoted the color change of the ECD from blue to transparent. The UV-vis transmittance spectra revealed that applying 15 s/15 s and 60 s/60 s square wave of -2.8 and 2.0 V resulted in the difference of 15.9 and 39.6% between both states, respectively. Moreover, the charge density values increased from ∼7 to ∼10 mC cm^-2, after applying -2.8 V for 15 and 60 s, respectively. The memory test of the ECD performed in open circuit, after applying -2.8 V for 15 s, revealed a decrease of transmittance from 37.5 to 21% in 24 h. The cyclic voltammetry performed at 100 mV s^-1, evidenced the reduction at -0.1 V and oxidation at 0.4 V peaks, responsible for PB color change. Additionally, it was stated that after 500 chronoamperometric cycles, the charge density values decreased from -4.8 to -1.9 mC cm^-2. Finally, the presented results suggest the possibility of using PB as electrochromic coating and PVDF as solid electrolyte in new ECDs development.