Air-Stable, Self-Bleaching Electrochromic Device Based on Viologen- and Ferrocene-Containing Triflimide Redox Ionic...

We demonstrate an electrochromic device with self-bleaching ability that uses ethyl viologen- ([EV]²⁺) and ferrocene-based redox ionic liquids ([FcNTf]^-) as the electroactive species. These electroactive compounds are insensitive to atmospheric O₂ and H₂O in both their oxidized and reduced states once dissolved in a typical ionic liquid electrolyte ([BMIm][NTf₂]), allowing for the device to be assembled outside a glovebox without any encapsulation. This device could generate a deep blue color by the application of a 2.0 V potential between two fluorine-doped tin oxide (FTO) substrates to oxidize the ferrocenyl centers to [FcNTf]⁰ while reducing viologen to [EV]^+•. Self-bleaching occurs at OCP as [EV]^+• and [FcNTf]⁰ undergo homogeneous electron transfer in the electrolyte. The mass transport of ethyl viologen and ferrocenylsulfonyl(trifluoromethylsulfonyl)imide ([FcNTf]^-) anion was evaluated by double potential step chronoamperometry to study the impact of the diffusion coefficient on the self-bleaching mechanism. The electrochromic device demonstrated here shows a contrast ΔT (610 nm) around 40% at 2.0 V as colored cell voltage, a switching time in the order of few seconds for coloration and bleaching, coloration efficiency of 105.4 to 146.2 cm² C^1- at 610 nm, and very high stability (94.8% ΔT after 1000 cycles) despite the presence of O₂ and H₂O in the electrolyte.