Achieving Low-Energy Driven Viologens-Based Electrochromic Devices Utilizing Polymeric Ionic Liquids

Herein, three kinds of viologens-based electrochromic devices (ECDs) (heptyl viologen (HV­(BF₄)₂), octyl viologen (OV­(BF₄)₂), and nonyl viologen (NV­(BF₄)₂)) were fabricated utilizing ferrocene (Fc) as a redox mediator. Among them, the NV­(BF₄)₂-based ECD exhibits the highest coloration efficiency (36.2 cm²/C) owing to the lowest driving energy. Besides, switching between 0 and 1.2 V, the NV­(BF₄)₂-based ECD shows a desirable initial transmittance change (Δ<i>T</i> = 56.7% at 605 nm), and long-term stability (Δ<i>T</i> = 45.4% after 4000 cycles). Furthermore, a UV-cured polymer electrolyte containing polymeric ionic liquid (PIL, 1-allyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide) and ethoxylated trimethylolpropane triacrylate (ETPTA) was introduced to the NV­(BF₄)₂-based ECD. By controlling the weight percentage of the PIL, different curing degrees of the polymer electrolytes were obtained and led to an improved stability of the NV­(BF₄)₂-based ECD because of the immobilization of NV­(BF₄)₂. This observation was explained by calculating the apparent diffusivity (<i>D</i>_app) of the redox species in the NV­(BF₄)₂-based ECD under various curing degrees. In addition, increasing the amount of PIL leads to a lower driven energy needed for the NV­(BF₄)₂-based ECD, following the same trend as the value of <i>D</i>_app. Among all NV­(BF₄)₂-based ECDs, 20 wt % of PIL addition (20-PIL ECD) exhibits large transmittance change (Δ<i>T</i> = 55.2% at 605 nm), short switching times (2.13 s in coloring and 2.10 s in bleaching), high coloration efficiency (60.4 and 273.5 cm²/C at 605 nm, after excluding the current density at the steady state), and exceptional cycling stability (Δ<i>T</i> = 53.8% after 10,000 cycles, or retained 97.5% of its initial Δ<i>T</i>)