Electrolyte design for dynamic windows based on reversible metal electrodeposition
Keywords: Chemistry.

Throughout human history, innovation has always met society's needs. In most recent decades, the human population has turned its attention indoors, where people spend almost 90% of their time, over half of which is spent looking at a screen. During this time spent inside, the only true connection that people have to the outside world is through the window. Exposure to nature lighting is crucial for human wellness; however, too much light can be an issue through unwanted glare and improper heat management. Unfortunately, to date, static technologies such as blinds or shades are incapable of managing the light while maintaining the view. This forces people to either sacrifice their connectivity to natural daylighting by shutting their blinds or waste ~30% of their energy usage by unnecessarily running the heat and air conditioning. Dynamic, "smart", windows with electronically controlled transmission reduce glare without obstructing views while increasing the energy efficiency of buildings via lighting, heating, and cooling savings. Electrochromic materials, which change color with voltage, are widely explored for use in dynamic windows, but they have not been extensively commercialized due to their inability to simultaneously achieve fast, color neutral tinting over a wide optical dynamic range at a low cost. Reversible metal electrodeposition (RME) is a promising alternative that addresses the drawbacks of existing technologies. The usage of metals is an attractive option for dynamic, "smart", window applications due to their high optical extinction coefficients, color neutral metallic state, chemical inertness, and high solubility in "green" solvents such as water. Additionally, RME-based dynamic windows have superior color neutral tinting and are able to go darker than other technologies, which is especially attractive for the residential market where privacy and sleeping are very important. These windows operate by the reversible, electrochemical movement of metallic ions dissolved in an electrolytic medium on a transparent conducting electrode. The electrolyte is the source metal ions, which induces reversible tinting, and is a crucial component of this electrochemical device. In order to meet the demand for widespread implementation of RME dynamic windows, the electrolyte must demonstrate fast, color-neutral switching with high durability and scale at a low cost. These parameters serve as the foundation for this thesis. To enable color-neutral, fast switching, Bi and Cu ions are implemented as the main source for metal electrodeposition, due to their synergistic relationship enabled by galvanic displacement. I then take a fundamental electrochemical approach to address durability by studying various anion and pH combinations in the electrolyte. I conclude that an acidic perchlorate-based electrolyte permits fully reversible metal electrodeposition without harming the substrate or introducing irreversible side reactions. A concern with a liquid-based electrolyte in a large-scale, vertically facing window is the hydrostatic head pressure build-up. I address this scaling concern by developing a transparent, cross-linked PVA hydrogel electrolyte to prevent any potential leaking from seal breakage. This combination of color-neutral, fast switching enabled by Bi and Cu, highly durable cycle- and shelf-life due to the acidic ClO4- medium, and practical implementation at scale from the PVA hydrogel has paved the way towards the realization for universal adoption of next generation RME dynamic windows