Dynamic Windows Based on Reversible Silver Electrodeposition with Improved Resting Stability and Cyclability

Dynamic windows based on reversible metal electrodeposition are an attractive way to enhance the energy efficiency of buildings and show great commercial potential. Reversible metal electrodeposition is a promising approach for constructing dynamic windows because of its excellent light-modulating capabilities. Our team has 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 with uniform and color neutral (C*<5) tinting in prototypes >900 cm².

Resting stability is the chemical and mechanical stability of the electroplated metal thin films in dark-state windows when the power is cut off. Though the resting stability of the electroplated thin film is crucial for window applications, a limited amount of research has been done regarding this problem. Mc Andrew et al. observed cracks in Cu-Bi co-deposition after 12 h in the presence of Cu²⁺ and presented a Bi-based device with over 2 months of resting stability. However, because of the finger-like structure of Bi electrodeposition, Bi RME windows lack good performance in stripping speed, coloration efficiency, and cyclability.

Silver is an attractive metal because only one electron is needed to plate its cations. The resulting nanoparticles have a strong localized surface plasmon resonance (LSPR) that results in high coloration efficiency. However, reversible silver electrodeposition devices with better than 24 h resting stability and cyclability over 1000 cycles have not been reported. Using dimethyl sulfoxide (DMSO) as the solvent, we develop Ag electrolytes that support electrochemically and optically reversible Ag electrodeposition with high optical contrast (~75% clear-state transmission and <0.1% dark-state transmission). An acid additive in the electrolyte suppresses Ag oxidation, enabling consistent optical properties of the Ag windows after 7500 cycles. Cu is often used to promote cyclability because it assists in stripping a metal film. We show, however, that it causes the silver films to dissolve in a few hours when the window is left in a dark state. By not using Cu and avoiding the formation of Br₃^-, we demonstrated that electroplated silver films could stay in the privacy state (<0.1% transmission) for more than 80 days while soaking in the degassed electrolyte at room temperature. Lastly, the trifluoracetic acid (TFA) additive helps build a more color-neutral privacy state Ag film.

This work illustrates that devices based on reversible Ag electrodeposition in DMSO electrolytes are promising for windows with excellent reversibility (99.9 % Coulombic efficiency) and coloration efficiency (49.7 cm²/C for the 10% T, 2-3 times higher than Cu-Bi aqueous windows). In contrast to previous Bi-Cu films in RME windows produced from aqueous electrolytes that can suffer from stress corrosion cracking, the Ag DMSO windows possess long-term stability in their opaque state without the application of external power for 80 days at room temperature. Furthermore, the improved cycle life was reached by adding TFA additive to remove metal residues and keep the film from whiting by maintaining uniform NP morphology.