Electrochromic device with Prussian blue and HPC-based electrolyte

Given the importance of the development of electrochromic devices (ECDs) for technological applications small devices with Prussian blue (PB)/hydroxypropyl cellulose (HPC) electrolyte/CeO₂-TiO₂ configuration were assembled and characterized. The films of PB were obtained galvanostaticaly by applying -0.45 μA for 300 s. The cyclic voltammetry, in 1 mol L^-1 KCl solution, of these films showed a redox pair peaks at well-defined potential of 0.16 V for oxidation and 0.4 V for reduction processes. The ECDs were assembled by combining the HPC-based membrane between the counter-electrode consisting of CeO₂-TiO₂ and the PB electrochromic layer_. The devices were characterized by chronocoulometry coupled with UV-vis transmittance spectroscopy, open circuit properties, and voltammetry. The combination of PB with the transparent HPC electrolyte membrane resulted in an electrochromic device that switched between transparent and blue states by applying -2.6 and 2.0 V, respectively. The charge density measurements showed an increase from -6.0 to -8.4 mC cm^-2 when the potentials where applied for 15 and 60 s, respectively. The UV-vis spectra of the ECDs revealed that switching the ECD between -2.6 and 2.0 V for 15 s/15 s and 60 s/60 s resulted in 28 and 32% of transmittance difference between both states, respectively. The memory test, i.e., the changes of transmittance in open circuit after applying a potential of -2.6 V for 15 s revealed a decrease of transmittance from 60 to 59% in 3 h. The cyclic voltammetry measurements performed from 20 to 500 mV s^-1 showed better resolutions of the oxidation and reduction peaks for higher scan rates. Moreover, it was observed that after 2000 cycles, the charge density decreased from -6.0 to -1.5 mC cm^-2 and the difference of transmittance between colored and bleached state decreased from 28 to 5%.