Voltage-Tunable Multicolor, Sub-1.5 V, Flexible Electrochromic

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Voltage-Tunable Multicolor, Sub-1.5 V, Flexible Electrochromic Devices Based on Ion Gels Hwan Oh, Dong Gyu Seo, Tae Yong Yun, Chan Young Kim, and Hong Chul Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00624 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on January 30, 2017

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ACS Applied Materials & Interfaces

Voltage-Tunable Multicolor, Sub-1.5 V, Flexible Electrochromic Devices Based on Ion Gels

Hwan Oh,† Dong Gyu Seo,† Tae Yong Yun, Chan Young Kim and Hong Chul Moon*

Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea

†

These authors equally contributed to this work.

* Corresponding authors. E-mail: [email protected] (H.C.M.)

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ACS Paragon Plus Environment


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Abstract Voltage-tunable multicolor electrochromic devices (ECDs) are fabricated based on flexible ion gels consisting of copolymers and ionic liquids as an electrolyte layer. Dimethyl ferrocene (dmFc) is incorporated into the gel, which serves as an anodic species. In this study, two electrochromic (EC) materials, monoheptyl viologen (MHV+) and diheptyl viologen (DHV2+), are employed, and show significantly different EC behavior despite the similar chemical structure. Both MHV+- and DHV2+-containing ECDs are slightly yellowish in the bleached state, whereas the colored states are magenta and blue, respectively. All devices show good coloration efficiency of 87.5 cm2/C (magenta) and 91.3 cm2/C (blue). In addition, the required power of ~248 μW/cm2 (magenta) and ~72 μW/cm2 (blue) to maintain the colored state put the ion gelbased ECDs in a class of ultralow power consumption displays. Based on the distinct difference in the coloration voltage range between MHV+ and DHV2+, and the rubbery character of the gel, flexible ECDs showing three different colors (slight yellow, blue, and maroon) are demonstrated. These results show that voltage-tunable multicolor ECDs based on the gel are attractive to functional electrochemical displays.

Keywords: Electrochemical Displays, Ion Gels, Electrochromism, Flexible Electronics, Lowpower Displays

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Introduction Electrochemical displays have attracted considerable interest owing to their simple device configuration (electrode/electrochemically active layer/electrode), wide variety of electrodes to employ irrespective of the work function, low voltage operation, and good solution processability.1-30 One representative electrochemical display is an electrochromic device (ECD), which changes the optical transmittance or absorbance according to the applied voltage.11-30 To demonstrate flexible or stretchable ECDs, a mechanically robust solid-state electrolyte is required. Although conventional polymer electrolytes, such as a composite of polyethylene oxide (PEO) and Li salts, have high mechanical toughness, they are unsuitable materials on account of their low ionic conductivity (10−3 ~ 10−1 mS/cm).31 Therefore, many research groups have developed polymer gel electrolytes (PGEs).31-37 In particular, ion gels composed of room temperature ionic liquids and ABA triblock copolymers have favorable characteristics, including outstanding ionic conductivity (1 ~ 10 mS/cm) at room temperature, extremely low vapor pressure, a wide electrochemical window, and solution processability.32 In addition, the mechanical properties of the ion gel can be selectively enhanced by chemical crosslinking of the A blocks as long as excellent ionic conductivity is not sacrificed.36 Therefore, ion gels have been employed in a range of electrochemical electronics. In general, ECDs with conjugating polymers11-17 or metal oxide18-20 show rapid dynamics based on fast electron-transfer reactions between the electrode and EC film. However, such Type III ECDs with solid-state bleached and colored films require a multilayered device structure, including both primary and secondary EC layers.38 In contrast, Type I ECDs having bleached and colored EC species soluble in an electrolyte correspond to relatively slow diffusioncontrolled devices, but consist of only an EC electrolyte layer with two electrodes.21-26 In addition, they exhibit high coloration efficiency (η), large transmittance contrast (ΔT), and excellent flexibility, when the ion gels are employed as the electrolyte.21,22 Type II ECDs are 3



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intermediate between Types I and III, where the bleached species are only soluble in the electrolyte.38 One of widely used Type I electrochromic (EC) materials is the 1,1’-substituted-4,4’bypyridium salt (called viologen).21-30 When N-substituents are short alkyl chains (e.g. methyl or ethyl), the colored state including the viologen cation radical shows a blue color. As the alkyl length increases (e.g. heptyl) and electrolytes have hydrophilic character, colored species tend to dimerize.22,38 As a result, the colored state is in magenta, but the quasi-reversible oxidation of the dimerized viologen cation radical degrades the device performance.39,40 Various aryl-substituents have been also reported to obtain other colors such as green.22,27-30 Based on these numerous viologens, multicolor ECDs based on EC ion gels have been demonstrated. However, only a single color can be realized from each cell, according to the employed viologens. In this study, voltage-tunable multicolor, flexible ECDs on plastic were successfully demonstrated. Two EC materials, monoheptyl viologen (MHV+) and diheptyl viologen (DHV2+), were employed in the gel, and the ECDs were fabricated by sandwiching the resulting EC gel between two ITO-coated substrates (glasses or polymer sheets). Although MHV+ and DHV2+ have a similar chemical structure, their EC behavior was significantly different. While the blue colored state was observed from the ECDs containing DHV2+ at −0.6 V, the magenta coloration of the ECDs including MHV+ occurred at −1.1 V. Both devices were characterized in terms of η and ΔT. The coloration of MHV+-containing ECDs for magenta was efficiently conducted with η of 87.5 cm2/C, and the device exhibited a large ΔT ~ 90 % at 546 nm. Blue ECDs showed similar performance: η ~ 91.3 cm2/C and ΔT ~ 91 % at 602 nm. Another important character of gelbased ECDs is the low power consumption to maintain the color.22 When the power required to keep the transmittance level less than ~30 % was measured, the power consumption (~248 μW/cm2 (magenta) and ~72 μW/cm2 (blue)) was low enough to put these ECDs in a class of ultralow power displays.41 To utilize the coloration voltage difference between MHV+ (−1.1 V or 4


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higher) and DHV2+ (−0.6 V or higher), and take advantage of the rubbery character of the gel, we fabricated flexible ECDs on plastic based on a mixture of MHV+ and DHV2+. The resulting ECD showed diverse colors when the applied voltage was judiciously adjusted: slightly yellowish (bleached state), blue (colored state I at −0.8 V), and maroon (colored state II at −1.3 V).

Experimental Section Materials: All materials used in this study were purchased from Sigma-Aldrich and used as received. Diheptyl viologen bis(hexafluorophosphate) [DHV(PF6)2] and monoheptyl viologen hexafluorophosphate [MHV(PF6)] were prepared by an anion exchange reaction. For example, diheptyl viologen dibromide [DHV(Br)2] (1.0 g, 1.94 mmol) was dissolved in DI water (120 mL). The separately prepared aqueous solution (10 mL) containing ammonium hexafluorophosphate (NH4PF6) (0.697 g, 4.28 mmol) was added dropwise to the solution. After reacting at 30 oC for 24 h, the DHV(PF6)2 was obtained as a precipitate, which was collected, washed with DI water, and dried at 50 oC for 24 h in a vacuum. MHV(PF6) was prepared using the same procedure with monoheptyl viologen bromide [MHV(Br)]. An ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMI][TFSI]) was also synthesized by an anion exchange reaction between 1-ethyl-3-methylimidazolium bromide [EMI][Br] and excess LITFSI in DI water.5 The ITO-coated glass (sheet resistance : 10 Ω/sq, Asahi Glass Co.) and ITO-coated PET sheet (sheet resistance : 60 Ω/sq, Sigma-Aldrich) were sequentially cleaned with acetone (5 min), methanol (5 min), and 2-propanol (5 min) under sonication, and treated further with UV/ozone for 10 min prior to use. EC Gel Preparation and Device Fabrication: The ECDs were fabricated in ambient air as follows. First, a mixture of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) in a 4 : 36 weight ratio (typical polymer mass : ~80 mg) was dissolved completely in acetone at 50 oC. 5



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Premeasured DHV(PF6)2 (or MHV(PF6)) and dmFc in a 3 (or 1.87) : 1 weight ratio were added to the acetone solution. The resulting solution was cast onto the ITO-coated substrate, and dried at room temperature. Another ITO-coated substrate was then placed on top of the gel, giving ITO/EC gel/ITO configuration of the device (Scheme 1). Two electrodes were fixed firmly using a 60 µm-thick double-sided tape.

Scheme 1. Schematic diagram of the fabrication process for gel-based ECDs and chemical structure of EC materials employed in this study.

Characterization: Mechanical properties of the gel were evaluated by using Advanced Rheometric Expansion System (TI Instrument) with parallel plates of 25 mm diameter. Dynamic isothermal frequency sweeps (ω = 0.1 ~ 100 rad/s) of storage (G’) and loss moduli (G’’) were conducted at a strain amplitude and temperature of 0.05 and 20 oC, respectively. Ionic 6


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conductivity of the gel was measured by electrochemical impedance spectrometer (IM6, ZAHNER) at 20 oC, in which the frequency range was 0.1 to 106 Hz with an AC amplitude of 10 mV. The redox potentials of DHV(PF6)2 and MHV(PF6) were measured by cyclic voltammetry (CV). The cyclic voltammograms for the EC gels containing dimethyl ferrocene (dmFc) and either DHV(PF6)2 or MHV(PF6) were acquired using a potentiostat (Wave Driver 10, Pine Instrument) at a potential sweep rate of 20 mV/s. A platinum disk, Ag wire, and ITO-coated glass were employed as the working, reference, and counter electrodes, respectively. It is noted that dmFc served as both an anodic species and internal standard. The UV-vis spectra of the ECDs at various voltages were recorded on a UV-vis spectrometer (V-730, Jasco) from 400 nm to 1100 nm with a 1.0 nm resolution. The scan speed was 400 nm/min. To investigate the device dynamics, transient transmittance and device current profiles of ECDs at a fixed wavelength were obtained by a combination of the same UV-vis spectrometer and potentiostat. The square wave and DC voltage were supplied from a potentiostat (Wave Driver 10, Pine instrument) and a source meter (Keithley 2400, Tektronix), respectively.

Results and Discussion Figure 1a shows frequency dependence of storage (G’) and loss moduli (G’’) of the gel employed in this work. G’ was larger than G’’, and did not indicate remarkable frequency dependency over the entire investigated frequency range (0.1 – 100 rad/s). These results imply the gel behaves as an elastic solid, and its network structure is nearly invariant. The 10 wt % P(VDF-co-HFP)-based gel had high G’ values (> 104 Pa), which is ~10 times larger than those (~ 103 Pa) of conventional ion gels based on ABA-type block copolymers such as polystyrene-bpoly(methyl

methacrylate)-b-polystyrene

(PS-b-PMMA-b-PS,

SMS)

or

polystyrene-b-

poly(ethylene oxide)-b-polystyrene (PS-b-PEO-b-PS, SOS).34 As a result, the mechanically robust free-standing gel that can be applied to ‘cut-and-stick’ protocol was obtained (see Figure 7



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S1 in the Supporting Information). To fabricate low-voltage and rapid-response electrochemical devices, high ionic conductivity of the gel is essentially required due to lower voltage drop and facilitated mass transport of electrochemically active species. The frequency dependence of Z’ is given in Figure 1b. The bulk gel resistance (R) can be extracted from the plateau Z’ value at high frequency where the impedance is purely resistive: the measured value was ~235 Ω. With consideration of gel thickness (h) of 2 mm and 4 mm diameter, the ionic conductivity (σ) of ~6.7 mS/cm was calculated according to σ = h / AR where A corresponds to the cross-sectional area. This value is favorably comparable with that (ca. 5 ~ 6 mS/cm) of other ion gels including 10 wt% SMS or SOS. In short, the gel containing 10 wt % high toughness P(VDF-co-HFP) in this work has enhanced mechanical property in addition to similar ionic conductivity compared to conventional ion gels, which is suitable for flexible ECDs.

Figure 1. (a) Dynamic isothermal frequency sweeps of G’ and G’’ at a strain amplitude of 0.05, and (b) plots of Z’ versus frequency for the gel consisting of 10 wt% P(VDF-co-HFP) and 90 wt% [EMI][TFSI]. Inset of (b) indicates the metal-insulator (ion gel)-metal (MIM) configuration used in the electrochemical impedance (EIS) study. Experimental temperature was 20 oC for both.

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Figure 2 displays the cyclic voltammograms (CVs) of the EC gels including either MHV+ or DHV2+ with dmFc. The redox reactions of the MHV+/MHV• (Figure 2a) and DHV2+/DHV+• (Figure 2b) couples occurred at −1.2 V and −0.75 V (vs. dmFc+/dmFc), respectively. Because dmFc served as an anodic species as well as the internal standard material, the difference in redox potential between the EC material and dmFc correspond to the voltage required for the coloration of ECDs. Therefore, the coloration at a higher voltage is anticipated in the MHV+containing ECDs.

Figure 2. Cyclic voltammograms (CVs) of (a) MHV+ and (b) DHV2+ in EC gels containing dmFc at a scan rate of 20 mV/s, where a platinum disk, Ag wire, and ITO-coated glass were employed as the working, reference, and counter electrode, respectively. The dmFc serves as both anodic species and internal standard. 9



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Figure 3. UV-vis absorption spectra at various voltages of (a) MHV+ and (d) DHV2+-containing ECDs. Photographs of the bleached and colored states of MHV+-containing ECD [(b) and (c)], and DHV2+-containing ECD [(e) and (f)].

To examine the voltage dependence of the optical properties of ECDs, UV-vis absorption spectra were recorded at various applied voltages. Figure 3a shows the variation of the absorption spectra of MHV+-containing ECDs according to the external applied voltages. A broad and weak peak at ~450 nm originated from dmFc dissolved in the gel, through which the ECD indicated a slightly yellow color in the bleached state (Figure 3b). While there was no change in the spectra up to −1.0 V, a characteristic absorption peak (𝜆𝜆max ~546 nm) arising from 10


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the produced MHV• was observed at −1.1 V. As the applied voltage was increased, stronger absorption was detected because of the higher MHV• concentration. Previously, a magenta color was achieved by utilizing DHV2+ in hydrophilic ion gels with [BMI][BF4], but the color arose from electrochemically unstable radical cation dimers.22 On the other hand, for ECDs with MHV+, there is no significant absorption at a wavelength range longer than ~750 nm (see Figure 3a), which reveals that the origin of a magenta color is not unstable radical dimers. A dramatic color change in the ECD was clearly shown in Figure 3c (colored state). On the other hand, two strong absorption peaks at ~602 nm and ~553 nm were detected from the colored state of the ECDs including DHV2+ (Figure 3d), although the slightly yellowish bleached state (Figure 3e) was the same. As a result, the blue colored state of the DHV2+-containing device was observed (Figure 3f). As expected in electrochemical analysis (Figure 2), the coloration occurred at a lower voltage of −0.6 V than that of the MHV+-containing ECDs. To investigate the coloration and bleaching kinetics for the ECDs containing MHV+ and DHV2+, the transient transmittance profiles were recorded at 546 nm (Figure 4a) and 602 nm (Figure 4b), respectively. In Figure 4a, the requisite time for the change in the maximum transmittance of 90 % (Δt90%) was 11 s for the coloration of the ECD with MHV+. On the other hand, bleaching was tested under two different conditions: short-circuit and open-circuit conditions. When an external voltage was applied, the reduced EC species (namely, MHV• or DHV+,•) and dmFc+ are produced at the cathode and anode, respectively. The resulting concentration gradient induces the diffusion of colored EC species and dmFc+ from the electrode to the center of the gel. These two species eventually encounter and spontaneously react due to the negative ΔG for the reaction, corresponding to the bleaching process under open-circuit conditions. If the circuit is shorted, the direct re-oxidation of colored species occurs near the electrode in addition to the bleaching route for the opened circuit.21,22 Thus, a much faster bleaching process (Δt90% ~ 22 s) of the shorted device than that (Δt90% ~ 125 s) under open-circuit 11



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conditions is quite reasonable (see Figure 4a). The DHV2+-containing ECDs showed similar EC behavior (Figure 4b): Δt90%for coloration at −0.7 V was ~17 s, and Δt90% for bleaching was ~32 s (short-circuit condition) and ~146 s (open-circuit condition).

Figure 4. Variation of the transient transmittance profiles for ECDs containing (a) MHV+ and (b) DHV2+ at the wavelength of maximum absorption (λmax) upon the application of −1.3 V for (a) and −0.7 V for (b), followed by bleaching under either short-circuit or open-circuit conditions.

Another important metric to evaluate the ECD performance is the coloration efficiency (η), which is expressed as38 12


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𝜂 =

∆𝑂𝐷 𝑙𝑜𝑔(𝑇𝑏 /𝑇𝑐 ) = ∆𝑄 ∆𝑄

where ΔOD is the change in optical density defined by the logarithm of the ratio between the transmittance at the bleached state (Tb) and at the colored state (Tc), and ΔQ is the injected charge for the corresponding ΔOD. The η can be extracted from the slope of a linear fit of ΔOD vs. ΔQ plots, by which the η values for the ECDs including MHV+ and DHV2+ were determined to be ~87.5 cm2/C at −1.3 V (Figure 5a) and ~91.3 cm2/C at −0.7 V (Figure 5b), respectively, which are favorably comparable to previously reported viologen-based Type I ECDs.21-23

Figure 5. Plots of the optical density versus injected charge density for ECDs with (a) MHV+ and (b) DHV2+ upon the application of −1.3 V and −0.7 V, respectively. 13



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Figure 6. Transient variations of device current density and transmittance at λmax for ECDs including (a) MHV+ and (b) DHV2+, in which the external voltage was applied whenever the transmittance became higher than 30 % after the first coloration followed by cutting the power source.

In contrast to the emissive displays, such as OLEDs or ECL devices, ECDs can provide information, even after cutting the applied voltage because colored species are still present in part. In addition, the realization of low power consumption devices is a very important feature 14


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for practical applications. To estimate the power required to maintain the colored-state, an external voltage was applied to the fresh device for coloration. Then, the voltage was cut when the transmittance reached ~10 %. Subsequently, the voltage was applied again when the transmittance reached ~30 %, which was our benchmark for the colored state (see Figure S2 in the Supporting Information). This process was repeated for 3600 s. Figure 6a and 6b show the transient current density and transmittance profiles at λmax for ECDs based on MHV+ and DHV2+, respectively. To extract the average current density across the device, the total amount of injected charge was calculated by integrating the transient current density profile, which was divided by the experimental duration (3600 s), giving 191 μA/cm2 for the MHV+-containing ECD (magenta) and 90 μA/cm2 for the DHV2+-containing ECD (blue). The power consumption of the ECDs to maintain a transmittance lower than ~30 % was calculated as ~248 μW/cm2 for magenta and ~72 μW/cm2 for blue with consideration of experimental voltages of −1.3 V and −0.8 V, respectively. This suggests that the gel-based ECDs in this work belong to ultralow power consumption displays, such as e-paper displays (~2 mW/cm2).41 It is noted that a distinct difference in the coloration voltage range was observed: −1.1 V or higher for magenta and −0.6 V or higher for blue ECDs (see Figure 2 and 3). This means if the device includes both MHV+ and DHV2+, a multicolor can be realized with judicious voltage control. To verify this hypothesis, we fabricated ECDs including mixtures of EC materials with three different molar ratios of MHV+/DHV2+: 20/80, 50/50, and 80/20. Figure 7 displays photographs of ECDs at various applied voltages and compositions of the EC mixtures. Irrespective of the composition, all devices showed blue colored state at –0.8 V. However, the second colored state highly dependent on the composition. For example, intense maroon color was induced at –1.3 V as MHV+ fraction increased. This result is quite reasonable because we fixed total amount of EC materials. Therefore, we selected 80/20 molar ratio of MHV+/DHV2+ to demonstrate flexible voltage-tunable ECDs exhibiting distinct two colored states. 15



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Since the second reduction of DHV2+ (namely, DHV+,• to DHV0) can simultaneously occur at –1.2 V (Figure S3 in the Supporting Information), we examined variation of UV-vis spectra for DHV2+ only ECDs at high voltages (> –0.9 V) (Figure S4 in the Supporting Information). We realized that absorption intensity of DHV0 is much weaker than that of DHV+,•, implying interference of blue-color in coloration of MHV+ is reduced. As a result, the distinct change in color can be facilitated. Therefore, we consider the second reduction is advantageous feature for the concept of voltage-tunable multicolor ECDs.

Figure 7. Photographs of ECDs based on mixed EC materials at various conditions: molar ratios of 20/80, 50/50 and 80/20 (MHV+/DHV2+), and applied voltages of 0.0 V, –0.8 V and –1.3 V.

Figure 8a shows voltage dependence of absorption spectra for ECDs consisting of MHV+ (80 mol%) and DHV2+ (20 mol%). At relatively low voltages (less than −1.0 V), the spectra were the same as that of the ECDs containing only DHV2+. As the applied voltage exceeds the reduction potential of MHV+, the characteristic absorption of MHV• (λmax ~ 546 nm) was also observed. 16


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Although MHV• and DHV+,• simultaneously affect the absorption spectra, a peak at ~546 nm was dominant because MHV+ (and thus MHV•) is a major EC material in the gel. Using this blending system and rubbery character of the gel, flexible multicolor ECDs were demonstrated on plastic. The device clearly exhibited voltage-tunable multicolor behavior from the slightly yellowish (bleached state), blue (colored state I at −0.8 V), to maroon (colored state II at −1.3 V) (see Figure 8b).

Figure 8. (a) Applied voltage dependence of the absoprtion spectra and (b) photographs of the voltage-tunable EC behavior for ECDs based on mixed EC materials.

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Conclusions We have demonstrated sub-1.5 V, flexible, voltage-tunable multicolor ECDs based on EC gels on plastic. Two EC materials, MHV+ and DHV2+, with a similar chemical structure were employed for the magenta and blue ECDs. Both devices indicated high coloration efficiency, large transmittance contrast, and low-power consumption to maintain the colored state. Because the coloration voltage ranges for MHV+ and DHV2+-containing ECDs were distinctly different, voltage-tunable multicolor EC behavior was anticipated with a mixture of MHV+ and DHV2+. Indeed, the resulting ECDs including mixed EC materials changed their color from slightly yellowish, blue, to maroon, as the applied voltage was carefully increased. Overall, these results imply that gel-based ECDs are attractive functional electrochemical devices for ultralow power consumption reflective displays.

Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2016R1C1B2006296).

Supporting Information Available. Photograph of the free-standing EC gel consisting of DHV(PF6)2, dmFc, P(VDF-co-HFP) and [EMI][TFSI] in weight fraction of 3:1:4:36, photographs of ECDs at 30 % transmittance containing (a) MHV+, and (b) DHV2+, extended cyclic voltammogram of DHV2+ in the EC gel including dmFc, variation of UV-vis spectra of ECDs including DHV2+ at voltages higher than –0.9 V. These materials are available free of charge via the Internet at http://pubs.acs.org.

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“Table of Contents”

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Voltage-Tunable Multicolor, Sub-1.5 V, Flexible Electrochromic

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