Origin of Black Coloration in Reversible Metal Electrodeposition Dynamic Windows

Dynamic windows with electronically controlled transmission are a promising technology to increase the energy efficiency of buildings because they allow for the control of heat and light transmittance from the outside. Recently, our group has investigated dynamic windows based on reversible metal electrodeposition and designed devices that possess high contrast ratios and excellent cycleability. By modulating the structure and reactivity of the electrolyte and the electrode, uniform thin films of metals can be electrodeposited on transparent conducting oxides over a relatively large area, opening up the possibility of using reversible metal electrodeposition for practical smart window applications.[1]<br/>To generate a black opaque state with privacy level transmission (&lt; 0.1%), various metals have been investigated including Ag [2, 3], Cu-Bi [4], and Zn [5]. Reflection and transmission of the electrodeposits must be minimized to generate a dark black appearance. However, a deep understanding of the coloration principle in terms of the micro- and nanostructures of the electrodeposited metals has not been described. For example, Ag is a plasmonic material, and Zn is non-plasmonic material in the visible light region, but both metals can generate black colors. In this work, we explain the origin of black coloration in the reversible metal electrodeposition systems using electromagnetic simulation-based (finite-difference time-domain; FDTD) approaches.<br/>As the size of the window increases, the coloration efficiency (CE) of device tinting is an important issue to consider. We therefore have designed nanostructures that maximize coloration efficiency. By considering the deposits as arrays of isolated nanoparticles, the nanoparticle size effect in terms of transmittance and reflectance is investigated after fixing the amount of metal in a unit cell. Next, optical characteristics are analyzed by modeling the experimentally-determined three-dimensional structure of the electrodeposits. Finally, the effects of metal oxides and other side products in the electrodeposits are carefully examined from the perspective of optical resonance modulation.<br/><br/>References<br/>[1] Islam, S. M., Hernandez, T. S., McGehee, M. D., & Barile, C. J. (2019). Hybrid dynamic windows using reversible metal electrodeposition and ion insertion. Nature Energy, 4(3), 223-229.<br/>[2] Araki, S., Nakamura, K., Kobayashi, K., Tsuboi, A., & Kobayashi, N. (2012). Electrochemical optical modulation device with reversible transformation between transparent, mirror, and black. Advanced Materials, 24(23), OP122-OP126.<br/>[3] Jeong, K. R., Lee, I., Park, J. Y., Choi, C. S., Cho, S. H., & Lee, J. L. (2017). Enhanced black state induced by spatial silver nanoparticles in an electrochromic device. NPG Asia Materials, 9(3), e362-e362.<br/>[4] Islam, S. M., Palma, A. A., Gautam, R. P., & Barile, C. J. (2020). Hybrid dynamic windows with color neutrality and fast switching using reversible metal electrodeposition and cobalt hexacyanoferrate electrochromism. ACS Applied Materials & Interfaces, 12(40), 44874-44882.<br/>[5] Islam, S. M., & Barile, C. J. (2021). Dynamic Windows using reversible zinc electrodeposition in neutral electrolytes with high opacity and excellent resting stability. Advanced Energy Materials, 11(22), 2100417.