Gel Polymer Electrolyte for Reversible Metal Electrodeposition Dynamic Windows Enables Dual-Working Electrodes for Faster Switching and Reflectivity Control

Dynamic windows based on reversible metal electrodeposition (RME) represent an exciting, new class of electrochromic devices. These RME windows function by the electrochemical movement of metallic ions dissolved in a nearly colorless aqueous electrolyte between a transparent conducting working electrode and a metal mesh 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 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*&lt;5) tinting in prototypes &gt;900 cm²¹.<br/><br/>Though aqueous liquid electrolytes often have higher ionic conductivities than ones with high polymer content², dynamic windows with liquid electrolytes face problems of leakage and short-circuiting, especially for large-scale windows. Gel polymer electrolytes (GPEs) are attractive due to their ability to act as a physical separator between the two electrodes to help avoid short-circuiting^3,4. For this study, a porous polyvinyl alcohol (PVA) GPE is synthesized as a physical separator where the polymer backbone provides the mechanical structure, and the pores are filled with BiCu ClO₄ liquid electrolyte. This highly transparent physically cross-linked PVA gel was implemented into RME dynamic windows, and the PVA GPE window can readily be tinted to below 0.1% visible light transmittance, exhibits color-neutral transmittance, and has similar coloration efficiency with liquid windows.<br/><br/>Importantly, the use of PVA GPE in windows provides opportunities for dual-working electrode (dual-WE) windows without short-circuiting. Dual-WE windows are made by incorporating a second piece of ITO-glass and integrating the Cu mesh counter electrode within the electrolytic matrix to the center layer of the window<b>.</b> Compared with single-WE windows, dual-WE windows are more susceptible to short circuits with a higher potential for the middle free-standing mesh to touch the WEs. Our dual-WE GPE windows switch about two times faster than single-WE liquid windows, i.e. the dual-WE window reaches the 0.1% transmittance state in 101 s while the single-WE liquid windows take 207 s. Additionally, each side of the dual-WE GPE window can be tinted individually to demonstrate varied optical effects (i.e., more reflective or more absorptive), providing users and intelligent building systems with greater control over the appearance and performance of the windows in single device architecture.<br/><br/>This work demonstrates the first application of GPE in the RME dynamic windows and strides toward commercialization by showing the dual-WE GPE architecture without short-circuiting concerns especially in large-scale windows, along with the benefits of twice the speed of switching and the controllable reflectivity states.<br/><br/>1. Hernandez, T.; Alshurafa, M.; Strand, M.; Yeang, A.; Danner, M.; Barile, C.; McGehee, M., Electrolyte for Improved Durability of Dynamic Windows Based on Reversible Metal Electrodeposition. <i>Joule </i><b>2020,</b> <i>4</i>, DOI:10.1016/j.joule.2020.05.008<br/>2. Alesanco, Y. A.-O.; Viñuales, A.; Rodriguez, J.; Tena-Zaera, R., All-in-One Gel-Based Electrochromic Devices: Strengths and Recent Developments. LID - 10.3390/ma11030414 [doi] LID - 414. (1996-1944 (Print)),<br/>3. Alipoori, S.; Mazinani, S.; Aboutalebi, S. H.; Sharif, F., Review of PVA-based gel polymer electrolytes in flexible solid-state supercapacitors: Opportunities and challenges. <i>Journal of Energy Storage </i><b>2020,</b> <i>27</i>, 101072, DOI:https://doi.org/10.1016/j.est.2019.101072<br/>4. Luo, W.-B.; Chou, S.-L.; Wang, J.-Z.; Kang, Y.-M.; Zhai, Y.-C.; Liu, H.-K., A hybrid gel–solid-state polymer electrolyte for long-life lithium oxygen batteries. <i>Chemical Communications </i><b>2015,</b> <i>51</i> (39), &lt;a href="tel:8269-8272"&gt;8269-8272&lt;/a&gt;, DOI:10.1039/C5CC01857A