Development of Low Power Consumption Electrochromic Device Using Spontaneous Discoloration Reaction

Abstract

Recently, dimming functional materials have been attracted attention as one of the solutions to energy problems. Dimmable materials are capable of continuously adjusting the transmittance of light according to the intensity of external light, which is expected to reduce cooling and heating loads of buildings such as houses and office buildings and realize energy saving. Prussian blue (PB, KFe3+[Fe2+(CN)6]) is known as one of the dimming functional materials that shows reversible color changes upon Fe2+/Fe3+ redox reaction [1], and has been used for electrochromic devices (ECD) as an inexpensive and highly durable dimming function material. The conventional ECD is controlled by bipolar voltage applications for coloration and discoloration [2]. In contrast, , we developed here the ECD equipping a function of a spontaneous discoloration without applying voltage with the aim to reduce power consumption.ECD is consisted of indium-doped tin oxide (ITO) glass / PB / electrolyte / PB / ITO glass. The PB-coated ITO glasses were prepared by casting PB ink onto ITO glasses (15 x 15 mm) and spin-coated at 2000 rpm for 10 sec. The electrolyte used was a propylene carbonate containing potassium bis(trifluoromethanesulfonyl)imide and polymethyl methacrylate.By applying the voltage at 1.5 V to the ECD, the wavelength peak of a reflected light shifted from 460 nm (blue color) to 535 nm (yellow color) due to the reduction and oxidation of PB on the anode and the cathode, respectively. That is, KFe3+[Fe2+(CN)6] (blue) was reduced to K2Fe2+[Fe2+(CN)6] (colorless) on a cathode and KFe3+[Fe2+(CN)6] (blue) was oxidized to KFe3+[Fe3+(CN)6] (yellow) on an anode. After removing the voltage application, wavelength shifted back from 535 nm to 500 nm, indicating that PBs spontaneously changed to their original states. It is estimated that the color returns to the original color in about 3 hours. The spontaneous discoloration might be caused by the potential difference between the electrodes. The detailed discussion regarding the relationship between spontaneous discoloration and the elapsed time will be present.[1] K. Yoshino, “Prussian Blue; New applications and its nanoparticles,” THE CHEMICAL TIMES, 229(3):23-27, (2013)[2] H. Tanaka, (2010), “Development of electrochromic devices by printing processes,” NEW GLASS, 25(3): 60-63, (2010) Figure 1