Stable viologen-based electrochromic devices: Control of Coulombic interaction using multi-functional polymeric ionic liquid membranes

Newly designed polymeric ionic liquid, poly(vinylidene fluoride-co-difluorovinylidene aminooxoethyl-1-butylimidazolium-co-vinylidene aminooxoethyl-1-butylimidazolium tetrafluoroborate) (denoted as PFI-BF₄), were successfully synthesized with an intention to form the stable electrospun membranes as quasi-solid-state electrolytes for the use in the electrochromic devices (ECDs). The fluorine atoms in the PFI-BF₄ membranes could restrain viologen from being quenched on the cathode. The incorporation of imidazolium entities in the PFI-BF₄ membranes could enhance the electrochemical stability and lower the charge transfer resistance at the electrolyte/cathode interface due to the suppression of viologen which are being trapped on the membranes. With an optimal molar ratio of imidazoliums to fluorines on the PFI-BF₄ membranes, the viologens would be nearly free from aggregation since the Coulombic attraction towards fluorines and the Coulombic repulsion towards imidazoliums reach equilibrium. After a systematic investigation, the PFI-BF₄_1.5 ECD exhibited a high transmittance change (ΔT) of 73.0%, with an effective coloration efficiency (η_e) of 460 cm² C^-1 at 600 nm and fast response times (bleaching time of 3.9 s and coloring time of 1.7 s). Most importantly, the PFI-BF₄_1.5 ECD retained 95.9% and 88.3% of its initial ΔT after 10,000 and 50,000 continuous switching cycles, respectively.