Design of Reversible Electroplating Cells for Energy-Saving Windows
Authors: Yeang Andrew L. , Hernandez Tyler S. , Strand Michael T. , Barile Christopher J. , McGehee Michael D. 
Published: February 2020

Dynamic windows, which allow electronic control over visible light and solar irradiation, enable responsive environments free of glare and unwanted heat. Electronic control over visible light transmittance and solar heat gain coefficients will allow dynamic windows to be integrated with algorithms to optimize for energy efficiency and comfort. The implementation of this technology increases building energy efficiency by 10% compared to traditional, static low-emissivity windows. Dynamic windows based on reversible metal electrodeposition (RME) represent an exciting, new class of electrochromic devices. These RME windows function like a battery by electrochemical movement of metallic ions between a transparent conducting working electrode and a metal mesh or ion-intercalation counter electrode. Metals are ideal light-modulators for dynamic windows because they can be color neutral, inert, photostable, and opaque at 20-30 nanometer thicknesses. Our team has demonstrated metal-based dynamic windows that boast cheap processing, fast switching (<1 minute), and stable performance over thousands of cycles on the 100 cm2 scale.
Our current RME windows use an acidic, aqueous halide-based liquid electrolyte that presents technical barriers to device scalability. Due to the inherent voltage drop across the indium tin oxide (ITO) electrode under operating conditions, the electrolyte must exhibit a wide voltage range for uniform metal deposition without undesired side reactions. In an acidic aqueous system, the hydrogen evolution reaction (HER) and ITO oxidation potential set a firm boundary on the voltage range in which the windows can operate. Finally, a liquid electrolyte limits the size of the window due to hydrostatic pressure build up in a vertically oriented window.
These design constraints set parameters for developing an electrolyte for RME windows with improved scale and durability. We explore a non-aqueous, non-halide electrolyte that meets these design constraints and is capable of reversibly depositing metals (e.g. Ag and Cu) for RME dynamic windows. The addition of various gel-inducing polymers and electrolyte additives allows us to design windows with reduced hydrostatic pressure build up and improved durability. Utilizing a combination of electrochemical and materials characterization techniques, we present a detailed study of the kinetic and transport properties as well as the morphological and optical effects of this electrolytic system. From these results, we successfully engineered a 400 cm2 window demonstrating fast, uniform transmission change with high optical contrast (~60%), color neutrality, and high cycle life.
Developing a strong understanding of reversibly electroplating metals using multivalent metallic ions with variable sizes yields valuable insight to batteries where reversible plating is critical for operation and durability. Further, insights gained from electrolyte-electrode interfaces and interactions in these RME windows can be used to improve fuel cell and battery performance.
Reversible Metal Electrodeposition Operating Principle and Schematic:

Figure 1