Electrochromic Devices Using Prussian Blue Electrochromic Layer and HPC-Based Electrolyte

Electrochromic devices (ECDs) have been studied for technological application. The ECDs with Prussian blue (PB)/hydroxypropyl cellulose (HPC) electrolyte/CeO₂-TiO₂ configuration were assembled and characterized. The films of PB were obtained by galvanostatic electrodeposition applying -0.45 μA for 300 s on ITO glass immersed in a solution of 0.05 M K₃Fe(CN)₆, 0.05 M FeCl₃×6H₂O and 0.05 M HCl. The electrolyte was prepared by mixing 0.3 g of HPC with 30 mL of acetone and addition of 10 % m/m of LiClO₄. To this mixture, polyethyleneglycol (PEG) also was added. The ECDs were assembled by combining the HPC membrane between the counter-electrode consisting of CeO₂-TiO₂ and the Prussian Blue electrochromic layer_. The devices were characterized by chronocoulometry coupled with UV-Vis transmittance, open circuit properties, and voltammetric cycling. The ECD was switched transparent and blue states applying -2.6 and 2.0 V, respectively. The charge density measurements showed an increase from ~-10 to -14 mC/cm² when the potentials where applied for 15 s and 60 s, respectively. While this potential is applied, we observe a variation of transmittance in the UV-Vis spectra region, revealing that switching the ECD between -2.6 and 2.0 V for 15 s/15 s and 60 s/60 s resulted in 27 % and 42 % of difference between both states. Changes of transmittance in open circuit after applying a potential of -2.6 V for 15 s revealed a decrease from 50% to 27 % in 24 h. The cyclic voltammetry measurements performed at 100 and 200 mV/s showed better resolutions of the oxidation and reduction peaks for higher scan rates. Moreover, it was observed that after 1,000 cycles, the charge density decreased from -10 to -6.9 mC/cm² and the difference of transmittance between colored and bleached state decreased from 41 to 30 %. The obtained results revealed that Prussian blue with HPC electrolyte are very promising materials for ECD applications.