Switchable Silver Mirror Window without Bromide Ion
Authors: Aoki Atsushi , Ito Akira , Morita Hisaya 
Published: December 2020

The reversible electrochemical switching mirrors between mirror state and transparent state using silver bromide ion complex have been greatly investigated for application to smart windows and mirrors.1,2 There are some advantages to reflect light and not to absorb one in the high reflective silver metal state from the viewpoint of the thermal management. However, there are also some drawbacks to use bromide ion. Silver bromide ion complex is poor solubility in the common solvents, which also required strong polar solvents such as DMSO. In addition, the silver bromide ion complex is light-sensitive and not suitable for many applications. Moreover, Br3
- ion as the redox couple of Br- ion has absorption in the visible wavelength region. Therefore, it was important to find the other systems where Ag may be deposited in a mirror form without the use of bromide ion. In the previous study, a mixed solvent of ionic liquid and acetonitrile has been employed instead of DMSO and silver bromide was not used. This novel reversible silver switching mirror window system is quite simple, stable and safe because the materials used in this system are citric acid as the electrolyte additive and Pd nanoparticles (NPs) as the catalyst which are plated as nuclei on the ITO electrode for facilitating Ag plating and dissolution.3 In this study, the non-bromide ion switchable silver mirror window was prepared and its performance was investigated.
The window has the sandwiched structure between Pd NPs deposited on the ITO electrode and semitransparent silver mesh (0.15 mm diameter, 40 mesh size) as a counter electrode on the glass substrate through silicon rubber spacer with 1 mm thickness. Silver plating bath composed of 0.2 M silver nitrite and 80 mM citric acid as an additive dissolved in the mixture solvent of EMIMTFSI ionic liquid and acetonitrile. The window size was 3.0 x 3.0 cm2. The transmittance of the transparent state of the device was 39% at the wavelength of more than 400 nm, which was not very high because of low transmittance of 58% at the silver mesh counter electrode as shown in Fig.1(a). Our college emblem was seen through the transparent state of the window. When the voltage bias was applied at -0.3 V for 7 sec, silver was electroplated on the Pd NP deposited on ITO to become the mirror state as shown in Fig.1(b). When the voltage bias was reversed at +0.3 V for 13 sec, the plated silver was electrodissolved to return to the transparent state of the window. The small voltage bias and the short plating/dissolution time was achieved in the present window.
The stability of the mirror state of the device at the open circuit was investigated. The device firstly shows the transparent state at the no bias condition for 3 sec, indicating the transmittance of 39% at the wavelength of 500 nm. When the voltage bias was applied at -0.3 V for 7 sec, the transmittance decreases to 1%. The device was then changed at the open circuit and the transmittance was monitored for 1 hr. The value of the transmittance keeps at 1% after 1 hr. Therefore, our window shows good memory effect because of no use of bromide ion. Thus, the window has superior potential for meeting requirements for practical applications including smart windows.
References
[1] Araki, S.; Nakamura, K.; Kobayashi, K.; Tsuboi, A.; Kobayashi, N. Adv. Mater., 2012, 24, OP122.
[2] Cho, S.M.; et al. Sol. Energy Mater. Sol. Cells, 2018, 179, 161–168.
[3] Aoki, A.; Ito, A.; Watanabe, S. Sol. Energy Mater. Sol. Cells, 2019, 200, 109922.

Figure 1