Flexible and Tailorable Alkylviologen/Cellulose Nanocrystals Composite Films for Sustainable Applications in Electrochromic Devices

The new, sustainable, and pro-environmental electrochromic devices (ECDs) have attracted tremendous attention due to the current issues regarding energy sources consumption and environmental pollution. A free-standing composite film, consisting of cellulose nanocrystals (CNCs) as biological matrix, viologen, and KCl, was prepared by a casting method. Such a composite film is flexible and can be tailored into various shapes for patterned applications. The viologen/KCl/CNCs film was sandwiched between two indium-tin oxide (ITO) glasses to construct viologen-based ECDs. The performance of the viologen-based ECDs did not depend on the organic solvent. ECDs based on four kinds of viologens with different substituent groups exhibited distinct coloration response. Besides the nature of the substituent groups attached to the bipyridine rings, the presence of CNCs is a crucial factor that affects the electrochromic performance of ECDs. The dimethyl-substituted viologen ECD has a fast response time (10 s for bleaching and 7.3 s for coloring). And for the dibenzyl-substituted viologen, a low voltage of -2.2 V can drive the coloration of its ECDs. The diethyl-substituted viologen ECD exhibits 30% transmittance variation and good coloring/bleaching stability. The ability to be tailored and degradability of the composite film provides a feasible strategy for the sustainability of electrochromic displays.