Electrochromic windows and flexible devices based on viologen-containing hydrogel electrolytes

Abstract

Electrochromic devices (ECDs) with reversible optical modulation have great potential for smart window and flexible device applications. In this study, we propose low-power consumption, all-in-one-type ECDs based on LiCl-containing polyacrylamide (PAAm) hydrogel as a transparent and flexible gel electrolyte. Hydrophilic 1,1′-bis(3-hydroxypropyl) viologen dibromide (V-OHP) was served as the electrochromic (EC) material, hydroquinone (H2Q) was used as the anodic species, and H2Q/V-OHP/LiCl molar ratio was adjusted to obtain a balanced charge for the desired redox states of V-OHP. The basis for using V-OHP is derived from its hydroxyl groups, which enhance the hydrophilic characteristics of the radical cation (V-OHP*) and, as a result, the stability of the colored radical cations. This leads to enhanced stability, longer cycle life, and improved device dynamics. Investigations were also conducted into the EC behavior of viologen in solution. Following the utilization of various ECDs with various ratios of LiCl and H2Q, the most optimal sample was chosen and its efficiency in the application of EC windows and flexible devices was assessed. The created EC windows demonstrated distinct features including a significant contrast, outstanding stability, and a substantial cycle life (preserving over 94 % of the original transmittance after 800 cycles). Distinct device dynamics were also evident at a voltage of − 1.4 V with a time constant of about 10 s for coloring and 33 s for bleaching in open circuit condition.