A Tetramethyl Viologen for Aqueous Electrochromic Devices

The poor electrochemical reversibility of the dimers limits the application of viologens in aqueous electrochromic devices (ECDs). Here, we report a tetramethyl viologen, 2,2',6,6'-tetramethyl-4,4'-bipyridine hydrobromide ([MVH][Br]), in which tetramethyl substitution provides significant steric hindrance to prevent dimerization. Our experiments show that this designed viologen performs excellently in an aqueous solvent, including excellent color variation, optical modulation, cycling stability, and radical inertness. Typically, an aqueous ECD fabricated with the viologen is able to retain 95.48% of its optical modulation after 5000 cycles. Furthermore, we found that this viologen is impervious to UV polymerization. Based on this property, we demonstrate a bichromatic ECD based on a hydro-ionic hybrid gel using in situ UV polymerization. This study not only extends the application of viologens in aqueous ECDs but also paves the way for the future development of high-mechanical-performance hydrogel-based ECDs.