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 up to 20% compared to traditional, static low-emissivity windows.^[1] Furthermore, worker productivity can be increased by 2% due to reductions in eye strain, headaches, and drowsiness.^[2] 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 dissolved in a nearly colorless aqueous electrolyte between a transparent conducting working electrode and a metal mesh or ion-intercalation counter electrode (see Figure). 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 most recently demonstrated metal-based dynamic windows that boast a dark state below 0.001% visible light transmittance in less than 10 minutes, an ultrawide range for optical and solar modulation (ΔT_vis=0.76 and ΔSHGC=0.56) with uniform and color neutral (C*<5) tinting in prototypes >900 sq. cm.^[3]

To achieve a dark state transmission <0.1%, which has been defined as privacy state, ~150 mC/sq. cm of charge must be shuttled between the working electrode and counter electrode. We first outline the counter electrode design parameters for these high-contrast RME dynamic windows: high transparency, capacity, and surface area while maintaining low haze, sheet resistance, and cost. We show that metal meshes are a leading candidate for the counter electrode material based on these design parameters. Metal meshes also keep the design simple by having the same reversible electrochemical reaction on both electrodes (in opposite directions).

We then analyze the material composition of the metal mesh to ensure cycling durability in a Cu-Bi electrolyte. This analysis shows that previously used Cu mesh in as the counter electrode material increases [Cu²⁺] over cycling (~200 privacy cycles) which results in degradation optically and electrochemically. We then develop a transparent mesh using an inert core with a thin metal coating that enables a high capacity (1.5 C/sq. cm) Cu-Bi layer for a metal mesh counter electrode. Finally, we incorporate this custom designed mesh in an RME dynamic window and demonstrate 250 privacy cycles.

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, particularly with deep cycling.

References:

(1) Windows, V. D. Energy Consumption by Buildings; 2015.

(2) Hedge A; Nou D. Ergonomics International Journal Worker Reactions to Electrochromic and Low e Glass Office Windows. Ergon. Int. J. 2018, 2 (4), 1–12. https://doi.org/10.23880/eoij-16000166.

(3) Strand, M. T.; Hernandez, T. S.; Danner, M. G.; Yeang, A. L.; Jarvey, N.; Barile, C. J.; McGehee, M. D. Polymer Inhibitors Enable >900 Cm2 Dynamic Windows Based on Reversible Metal Electrodeposition with High Solar Modulation. Nat. Energy 2021, No. 6, 546–554. https://doi.org/10.1038/s41560-021-00816-7.