Novel Electrochromic Devices Composed of Viologens and Polymer Electrolytes

In this thesis, we investigated the electrochromic (EC) performance of two novel viologens-based electrochromic devices (ECDs), including vinyl benzyl viologen (VBV) and nonyl viologen (NV) through controlling the curing degrees of the gel electrolytes, namely, the immobilization of viologens. Two different methods were used to examine this phenomena, thus enabling us to establish the relationship between the EC performance of the two novel viologens, curing degrees of the gel electrolytes, and the diffusion coefficients of redox species in the electrolytes. Firstly, to study the influence of immobilization of viologens on their EC performance. A novel viologen, VBV, was synthesized and utilized in an ECD. The polymerizable vinyl moieties on VBV allows it to perform self-immobilization by UV-curing method. Immobilized VBV (I-VBV) was obtained on the electrode and grafted with polymer electrolyte through simultaneously applying UV irradiation and potential bias toward an ECD. The ECD (PB/Fc/I-VBV ECD) was fabricated in which ferrocene (Fc) acted as a redox mediator, while Prussian blue (PB) and I-VBV served as anodic and cathodic coloring materials, respectively. With the utilization of polymer electrolyte, Fc, and I-VBV, the proposed PB/Fc/I-VBV ECD with a UV-curing time of 40 s (PB/Fc/I-VBV-40) exhibits the best EC performance among different UV-curing times in terms of long-term stability. It gives 60.6% transmittance change (∆T%) at 615 nm initially when switched between 1.2 V and -0.8 V. Short bleaching and coloring times of 1.32 s and 2.13 s were observed respectively. Moreover, a good long-term stability was obtained, maintaining 86.5% of its original ∆T% after 10,000 cycles. The PB/Fc/I-VBV ECD also exhibits a unique memory characteristic among all viologens-based ECDs reported in literatures and allows a tunable multi-electrochromism. In addition, the diffusion coefficients (apparent diffusivities (Dapp)) of the redox species in PB/Fc/I-VBV ECD were calculated based on Cottrell equation, establishing a relation between the diffusion rates of VBV and curing degrees. Thus, the reasons attributed to different EC performance of PB/Fc/I-VBV ECD were verified. In the second part, we investigated three kinds of viologens-based electrochromic device (ECD) (heptyl viologen (HV), octyl viologen (OV), and nonyl viologen (NV) utilizing ferrocene (Fc) as a redox mediator. Among them, NV-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, NV-based ECD shows a desirable initial transmittance change (∆T% = 56.7% at 605 nm), and long-term stability (∆T% = 45.4% after 4,000 cycles). According to the result from the first part, we introduced an UV-cured polymer electrolyte and varied its curing degrees in the NV-based ECD. UV-cured polymer electrolyte containing polymeric ionic liquid (PIL, 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and ethoxylated trimethylolpropane triacrylate (ETPTA). 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-based ECD because of the immobilization of NV. This phenomena was evaluated by calculating the apparent diffusivities (Dapp) of the redox species in the NV-based ECD under various curing degrees. In addition, increasing the amount of PIL leads to a lower driven energy of NV-based ECD, following the same trend as the Dapp. Among all NV-based ECD, 20 wt% of PIL addition (20-PIL ECD) exhibits a large transmittance change (∆T% = 55.2% at 605 nm), short switching times (2.13 s in coloring process and 2.10 s in bleaching process), high coloration efficiency (60.4 and 273.5 (after excluding the current at steady-state) cm2/C), and exceptional cycling stability (∆T% = 53.8% after 10,000 cycles (97.5% remained)).