Quasi-solid-state all-in-one electrochromic device enabled by I₃^-/I^- mediated viologen-functionalized hydrogel

Electrochromic devices (ECDs) have garnered significant attention due to their promising applications in smart windows, display technologies, and related fields. Viologen compounds demonstrate excellent electrochromic performance; however, their use in liquid-state systems is limited by challenges such as irreversible oxidation and dimerization. 1-butyl-4,4'-bipyridinium, a butyl-substituted viologen (BSV) is synthesized and effectively dispersed within a poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) double-network hydrogel electrolyte to fabricate a quasi-solid-state all-in-one ECD. The resulting device exhibits a large optical modulation of 79.6%, a fast coloration time of 6.5 s and a bleaching time of 28.6 s at 600 nm. We propose a strategy of incorporating an I₃^-/I^- redox couple into the gel electrolyte, which significantly enhances the charge transfer kinetics and thus reduces the bleaching time to 12.2 s. More importantly, the cycling stability is significantly improved, where 83.8% of its initial optical modulation is retained after 1200 cycles, increasing by more than twice compared to the device without KI. This work presents an effective strategy for developing high-performance quasi-solid-state all-in-one ECDs by integrating viologen-based hydrogelation with redox couple engineering.