Appropriate additives to dynamic windows based on reversible metal electrodeposition allow aqueous-based electrolytes to be used that support minute-long switching times across a wide temperature range.

Thin films that electronically switch between optical states are of interest for a number of applications including dynamic windows, switchable sunglasses, and optical gating devices. We report thin films based on the reversible electrodeposition of Cu and Au on tin-doped indium oxide (ITO) electrodes modified with Pt nanoparticles that switch between transparent and opaque states. By systematically studying the voltammetry of electrolytes with various compositions, we determine that Cu and Au synergistically facilitate optically reversible metal electrodeposition and that comparable electrolytes with either metal alone do not give rise to reversible systems. Furthermore, we demonstrate that halides in the electrolyte promote optically reversible metal electrodeposition by accelerating both the metal electrodeposition and dissolution processes.

Dynamic windows with electronically tunable transparency have applications in buildings, automobiles, and switchable sunglasses. By modulating solar lighting and heating flux, well-functioning dynamic windows improve the energy efficiency, comfort, and aesthetics of spaces. While most switchable windows utilize electrochromic materials, we have developed dynamic windows based on the reversible electrodeposition of metals. These windows possess clear-to-black transitions with ~80% optical contrast in 1 minute. 25 cm2 prototypes of these devices switch thousands of times without detectable deterioration. The windows function through the electrochemical movement of metal between a transparent conducting working electrode and a counter electrode metal frame. To facilitate uniform electrodeposition over a large working electrode area, transparent conducting oxides are modified with a self-assembled monolayer of Pt nanoparticles, which serves as an inert metal seed layer for metal nucleation and improves electrodeposit morphology. An aqueous electrolyte separates the two electrodes and consists of Ag+, Cu2+, Pb2+, and/or Bi3+ ions. We investigated several electrodeposition chemistries and established relationships between electrolyte composition and device performance. In addition, by selecting appropriate electrolyte additives, we have constructed dynamic windows that switch reversibly at temperatures as low as -40°C without electrolyte freezing. Lastly, we consider how improvements in electrode design will increase the scalability and optical functionality of these devices. Photograph of Mt. Rose, Nevada through a 25 cm2 dynamic window based on reversible metal electrodeposition as it switches in subfreezing weather: Figure 1