Poly(ionic liquid) Electrolytes for Switchable Silver Mirror

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Poly(ionic liquid) Electrolytes for Switchable Silver Mirror Xiao Hou, Zhenyong Wang, Zhiqiang Zheng, Jiangna Guo, Zhe Sun, and Feng Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05001 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Applied Materials & Interfaces

Poly(ionic liquid) Electrolytes for Switchable Silver Mirror Xiao Hou,† Zhenyong Wang,† Zhiqiang Zheng, Jiangna Guo, Zhe Sun, and Feng Yan*

Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China. †Authors with equal contributions. E-mail: [email protected] Keywords: switchable mirror; poly(ionic liquid); smart window; semi-solid electrolytes; flexible display. Abstract: Imidazolium-type small molecule ionic liquids (ILs) and their corresponding poly(ionic liquid) (PIL) homopolymers were synthesized and applied to reversible electrochemical mirrors (REMs). The effects of alkyl chain length of carbon chains at the N3 position and cation charge density (mono- and bis-imidazolium) on the electrochromic properties of Ag-based REMs were investigated by analyzing their electrodeposition and spectral properties. Longer alkyl chains and higher charge densities decreased the size and resulted in a more uniform distribution of Ag nanoparticles. Compared with IL-based liquid electrolytes, the PIL-based gel electrolytes formed smaller and denser electrodeposited metallic Ag nanoparticles because of their higher viscosity. These findings were used to 1

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guide fabrication of a 50 cm2 mirror dynamic window and flexible display. Due to several unique properties of PILs, the PIL-based REM exhibits fast switching speeds, superb cycling durability, small particle sizes, and uniform electrodeposited Ag nanoparticle films. These results make dynamic windows based on PIL electrolytes promising and competitive alternatives to traditional electrochromic windows.

Introduction Electrochromic devices (ECDs) are devices composed of two transparent conducting electrodes-typically indium tin oxide (ITO) or fluorine-doped tin oxide (FTO)-coated glass slides – and an inserted electrochromic material (ECM)-based electrolyte layer.1, 2 ECDs are attracting increasing attention because they can change their optical properties in response to external electrical stimuli, such as transmittance and absorbance, and have potential applications in intelligent displays,3-5 smart windows,6-8 and optical mirrors.6 An ECM is a key ECD component that must be able to reversibly change color (either between two colored states, or to/from a transparent/colored state) when a small electric current or voltage is applied. Therefore, the basic requirement for an ECM is that it possess bis-table electrochemical redox properties, which allow its optical properties to be modulated under electrical stimuli. Recently, ECDs based on organic small molecules and polymers, such as viologen,9-11 phenothiazine,12,13 spiropyran derivatives, polyaniline,14,15 metallopolymers, and polythiophene16 have shown intelligent responses under the stimuli of 2


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electrical voltage. However, organic ECMs usually have limited colors because of their restricted redox states. Recently, inorganic ECMs, including transition metal oxides (such as WO3, V2O5, and NiO)7, 17-19 have been successfully used in ECDs. Metal oxides are coated onto transparent conductors prior to device assembly via vacuum evaporation,20 sputtering,21 electrodeposition,22 or spincoating.23 Light modulation occurs via charge/discharge and/or electron injection/extraction when the electrical potential is applied.7, 18 However, these techniques are relatively expensive and time-consuming. On the other hand, reversible electrochemical mirrors (REMs) based on metal salts (such as Ag, Cu, Au, Bi, and Ni) 24-27 do not require tedious precoating processes. The basic design of an REM device is a pair of transparent conductive electrodes and an electrolyte that contains metal salts.28, 29 Metals are reduced from their salts to form bright and smooth mirror surfaces on the conductive substrate when an electric current is passed through the electrolyte. If a reverse bias is applied, the formed metal films can be further dissolved in the electrolyte, and such REMs have potential applications in smart windows and intelligent displays. Recently, electrochemically-stable Cu-based (or Ag-based) REMs using copper (or silver) redox chemistry have been studied,30-32 and the results indicate that electrolyte formulation is crucial for REM device fabrication.33 For example, Park et al. selected several electrolytes to successfully create a deposited silver mirror with a long memory effect.34 To improve the electrochromic properties of REMs, ionic liquid-based 3



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electrolytes have recently been applied to achieve longer memory effects,33 which acts as an anion barrier that protects the electrodeposited metallic film during the voltage-off state. Ionic liquids (ILs) are organic salts with negligible vapor pressures, and high thermal stabilities and ionic conductivities.35-37 Poly(ionic liquids) (PILs) are polyelectrolytes that contain an IL species in their monomer repeating units.38, 39 PILs are interesting research subjects because they combine the properties of ILs with the mechanical stability of polymers.40-42 For example, PIL-based materials have been applied as quasi-solid electrolytes for lithium ion batteries,43, 44 alkaline anion exchange fuel cell membranes,45, 46

intelligent response polymer materials,47, 48 carbon materials for supercapcitors,49, 50 and

antibacterial materials.51 Herein, a series of mono- and bis-imidazolium ILs and PILs (Scheme 1) with various substituents were synthesized and used in silver REMs. The effect of the substituted carbon chain length at the N3 position and the cation charge density on the electrochromic properties of REMs was investigated by analyzing their electrodeposition and spectral properties. Longer alkyl chains and higher charge densities decreased the size and created a more uniform distribution of Ag nanoparticles. Compared with IL-based liquid electrolytes, the PIL-based gel electrolytes formed smaller and denser electrodeposited metallic Ag nanoparticles because of their higher viscosity. Due to unique properties of PILs, PIL-based REMs exhibit rapid switching speed and superb cycling durability. The resultant PIL-based semi-solid (gel) electrolyte also exhibited high viscosity and could 4


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have potential applications in intelligent and flexible displays.

Experimental Section Materials. Silver nitrate (AgNO3), copper chloride (CuCl2), 1-methylimidazole, 1vinylimidazole, 1-bromobutane, 1-bromohexane, 1-bromooctane, 1-buthylimidazole, 1heptylimidazole, 1-decylimidazole, 1,6-dibromohexane, azobis-(isobutyronitrile) (AIBN), acetone, ethyl acetate, acetonitrile, diethyl ether, dimethyl sulfoxide (DMSO), methylene dichloride, ethanol and methanol were purchased from Aldrich and used as received without further purification. Molecular sieves (5 Å) were purchased from Energy Chemistry (Shanghai, China). Dimethyl sulfoxide (DMSO) was purchased from Aldrich and used after being dehydrated with 5 Å molecular sieves throughout the experiments. Poly(vinyl alcohol) (PVA, Mw = 1870 g mol-1) was obtained from Adamas Reagent Co., Ltd. The flat ITO electrodes (ITO-glass, ITO-PET) were purchased from Zhuhai Kaiwei Optoelectronic Technology Co., Ltd. (Zhuhai, China) and used after be cleaned.

Synthesis of imidazolium type ionic liquids. 1-Alkyl-3-methylimidazolium bromide was synthesized as follows: a solution containing 1-methylimidazole (28.0 mmol) and bromoalkane (28.0 mmol) was stirred at 80 °C for 24 h. The product was washed with ethyl acetate and diethyl ether three times, and then dried under dynamic vacuum at 60 °C for 24 h. 5



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1-Butyl-3-methylimidazolium bromide (IL-C4) (yield: 85.2%). 1H NMR (400 MHz, DMSO-d6): δ 9.18 (s, 1H), 7.76 (d, 2H), 4.16 (d, 2H), 3.85 (s, 3H), 1.75 (d, 2H), 1.39 1.07 (m, 2H), 0.89 (d, 3H). 1-Hexyl-3-methylimidazolium bromide (IL-C6) (yield: 87.6%). 1H NMR (400 MHz, DMSO-d6): δ 9.14 (s, 1H), 7.74 (d, 2H), 4.14 (t, 2H), 3.84 (s, 3H), 1.90 - 1.59 (m, 2H), 1.25 (s, 6H), 0.85 (d, 3H). 1-Octyl-3-methylimidazolium bromide (IL-C8) (yield: 84.3%). 1H NMR (400 MHz, DMSO-d6): δ 9.13 (s, 1H), 7.73 (d, 2H), 4.13 (s, 2H), 3.83 (s, 3H), 1.75 (s, 2H), 1.23 (s, 10H), 0.84 (s, 3H).

Synthesis of bis-imidazolium type ionic liquids. 3-(6-Bromohexyl)-1-methylimidazolium bromide was synthesized via stirring 1methylimidazole (12.0 mmol) with 1,6-dibromohexane (36.0 mmol) in acetonitrile at 70 °C for 72 h. The raw product was washed with diethyl ether three times, and then dried at 30 °C overnight. 1-Alkyl-3-(1-methylimidazolium-3-hexyl)imidazolium

bromide

was

synthesized

through a reaction of 1-alkylimidazole (52.0 mmol) and 3-(6-bromohexyl)-1methylimidazolium bromide (40.0 mmol) in methanol at 70 °C for 48 h. The raw product was washed with diethyl ether three times, and then dried at 30 °C overnight.

6


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3-(6-Bromohexyl)-1-methylimidazolium bromide (yield: 75.1%). 1H NMR (400 MHz, DMSO-d6): δ 9.05 (s, 1H), 7.79 - 7.42 (m, 2H), 4.04 (t, 2H), 3.73 (s, 3H), 3.41 (t, 2H), 1.68 (dd, 4H), 1.21 (dd, 4H). 1-Butyl-3-(1-methylimidazolium-3-hexyl)imidazolium bromide (IL-C6-Im-C4) (yield: 80.7%). 1H NMR (400 MHz, DMSO-d6): δ 9.29 (d, 2H), 7.95 - 7.59 (m, 4H), 4.18 (s, 6H),

3.87 (s, 3H), 1.91 - 1.63 (m, 6H), 1.27 (s, 6H), 0.90 (t, 3H). 1-Heptyl-3-(1-methylimidazolium-3-hexyl)imidazolium bromide (IL-C6-Im-C7) (yield: 79.8%). 1H NMR (400 MHz, DMSO-d6): δ 9.29 (d, 2H), 7.95-7.60 (m, 4H), 4.18 (t, 6H),

3.87 (s, 3H), 1.80 (s, 6H), 1.27 (d, 12H), 0.85 (t, 3H). 1-Decyl-3-(1-methylimidazolium-3-hexyl)imidazolium bromide (IL-C6-Im-C10) (yield: 77.4%). 1H NMR (400 MHz, DMSO-d6): δ 9.28 (d, 2H), 7.96 - 7.56 (m, 4H), 4.18 (t, 6H),

3.87 (s, 3H), 1.79 (s, 6H), 1.25 (d, 18H), 0.85 (t, 3H).

Synthesis of N-vinylimidazolium ionic liquid monomers. 1-Alkyl-3-vinylimidazolium bromide was synthesized as follows: a solution containing 1-vinylimidazole (28.0 mmol) and bromoalkane (28.0 mmol) was stirred at room temperature for 72 h. The product was washed with ethyl acetate and diethyl ether three times, and then dried under dynamic vacuum at room temperature for 24 h.

7



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1-Butyl-3-vinylimidazolium bromide (VmIL-C4) (yield: 78.1%). 1H NMR (400 MHz, DMSO-d6): δ 9.57 (s, 1H), 8.10 (d, 2H), 7.31 (dd, 1H), 5.97 (dd, 1H), 5.43 (dd, 1H), 4.21 (t, 2H), 1.98-1.62 (m, 2H), 1.29 (d, 2H), 0.91 (t, 3H).

1-Hexyl-3-vinylimidazolium bromide (VmIL-C6) (yield: 79.4%). 1H NMR (400 MHz, DMSO-d6): δ 9.54 (s, 1H), 8.07 (d, 2H), 7.29 (dd, 1H), 5.95 (dd, 1H), 5.41 (dd, 1H), 4.19 (d, 2H), 1.80 (s, 2H), 1.26 (s, 6H), 0.85 (s, 3H).

1-Octyl-3-vinylimidazolium bromide (VmIL-C8) (yield: 76.2%). 1HNMR (400 MHz, DMSO-d6): δ 9.46 (s, 1H), 8.02 (d, 2H), 7.23 (dd, 1H), 5.90 (dd, 1H), 5.32 (dd, 1H), 4.13 (s, 2H), 1.76 (s, 2H), 1.21 (s, 10H), 0.81 (s, 3H).

Synthesis of poly(ionic liquid)s. The polymerization of N-vinyl-imidazolium IL monomers was realized through free radical polymerization at 70 °C overnight under a nitrogen atmosphere, with 0.5 wt% of AIBN as the initiator, and DMSO as the solvent (the mass ratio of solvent and monomer was 2:1). The raw product was precipitated and washed with acetone three times. The obtained product was then dialyzed against alcohol to remove the unreacted monomers. The resultant pure product was dried under dynamic vacuum at 60 °C for 24 h.

Poly(1-butyl-3-vinylimidazolium bromide) (PIL-C4) (yield: 55.2%). 1H NMR (400 MHz, DMSO-d6): δ 9.37 (d, 1H), 7.73 (d, 2H), 4.08 (s, 3H), 1.54 (d, 6H), 0.90 (s, 3H). 8


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Poly(1-hexyl-3-vinylimidazolium bromide) (PIL-C6) (yield: 53.8%). 1H NMR (400 MHz, DMSO-d6): δ 9.11 (d, 1H), 7.73 (d, 2H), 4.22 (d, 2H), 1.83 (s, 2H), 1.26 (d, 6H), 0.87 (s, 3H). Poly(1-octyl-3-vinylimidazolium bromide) (PIL-C8) (yield: 51.4%). 1H NMR (400 MHz, DMSO-d6): δ 10.09-8.93 (m, 1H), 8.50-7.21 (m, 2H), 4.09 (s, 3H), 1.84 (s, 3H), 1.27 (s, 10H), 0.86 (s, 3H). Preparation of Electrolytes. IL electrolytes, including 84.9 mg (0.5 mmol) AgNO3, 13.4 mg (0.1 mmol) CuCl2, and 2.5 mmol of IL (the amount of Br− is 2.5 mmol) were dissolved in 1.8 mL of DMSO at room temperature. Corresponding PIL electrolytes, containing 84.9 mg (0.5 mmol) AgNO3, 13.4 mg (0.1 mmol) CuCl2, and 2.5 mmol of PIL were also dissolved in 1.8 mL of DMSO. The host polymer-based electrolytes were prepared by dissolving 84.9 mg (0.5 mmol) AgNO3, 13.4 mg (0.1 mmol) CuCl2, and 10 wt% of PVA into 1.8 mL of DMSO and 688.2 mg (2.5 mmol) IL-C8 and 668.1 mg (the amount of Br− is 2.5 mmol) IL-C6-Im-C10, respectively. The electrolyte containing AgNO3 (84.9 mg, 0.5 mmol) and PIL-C8 (718.1 mg, 2.5 mmol) was dissolved in 1.8 mL of DMSO to demonstrate the effects of Cu ions on the eletrodeposition of Ag ions.

Preparation of Electrochromic Silver Mirrors. ITO coated glasses (or PETs) (7Ω /sq) were cut into 5.0 x 10.0 cm2 as the conducting substrates. Before mounting, the substrates were ultrasonically cleaned by deionized water, acetone, ethanol for 8 min, respectively, 9



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and dried under vacuum at 80 °C. The electrochromic silver mirrors were constructed by sandwiching the IL (or PIL)-based electrolytes between two ITO glasses. The distance between two electrodes was adjusted to about 400 µm by a double-sided adhesive tape.

Characterization. 1H NMR spectra of the synthesized products were recorded on a Varian 400 MHz spectrometer using DMSO-d6 as the solvent. Fourier transform infrared (FT-IR) spectra of the PILs were carried out on a Specode 75 model spectrometer in the range of 400-4000 cm−1. Contact angle of the electrolytes on the ITO electrode was examined by static contact angle measurements (Drop Shape Analysis System DSA10, KRUESS, Germany). The rheological behaviors of the electrolytes were tested through a viscometer (Haake Rheo Stress 6000, Germany). The transmittance of the REMs was recorded on UVvis absorption spectra from 350-800 nm using a TU-1800 SPC spectrophotometer. The morphology of Ag nanoparticles on the ITO electrode was observed by a field-emission scanning electron microscope (SEM, Hitachi Model S4700). The current-voltage measurements were performed on Autolab PGSTAT 302N electrochemical workstation.

Results and discussion. To investigate the electrochromic performance of IL electrolyte-based REMs, imidazolium-type ILs: 1-butyl-3-methylimidazolium bromide (IL-C4), 1-hexyl-3methylimidazolium bromide (IL-C6), 1-octyl-3-methylimidazolium bromide (IL-C8), 110


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butyl-3-(1-methylimidazolium-3-hexyl) imidazolium bromide (IL-C6-Im-C4), 1-heptyl-3(1-methylimidazolium-3-hexyl) imidazolium bromide (IL-C6-Im-C7), and 1-decyl-3-(1methylimidazolium-3-hexyl) imidazolium bromide (IL-C6-Im-C10) were synthesized (Scheme 1 and Figure S1, the N3 position of imidazolium cations was indicated in red). Their chemical structure and purity were confirmed by 1H NMR spectra.

Scheme 1. Chemical structures of imidazolium-type ILs investigated in this work.

Figure 1 depicts the fabrication process for Ag-based REMs where an IL (or PIL)-based electrolyte containing AgNO3 and CuCl2 (5:1 molar ratio) is sandwiched between two ITO electrodes (5.0 x 10.0 cm2).31 When a reduction potential (-2.5 V) was applied, a reflective Ag mirror with a smooth surface formed in about 5 s, which then switched back to a 11



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transparent state when an electrical voltage of either 2.5 V or 0 V was applied. It has been previously reported that the deposition of mirror-like Ag nanoparticles occurs via the electroreduction of silver halide complexes (AgXn(1-n)) formed in the electrolyte.33 The surface becomes transparent again due to the dissolution (oxidation) of metallic silver, which produces the soluble AgXn(1-n) again.31, 52 Ag+ + n X− → AgXn1-n

(1)

AgXn1-n + e− → Ag + n X−

(2)

where n can take a value between 2 and 4, X = Cl, Br, or I.

Figure 1. Schematic illustration of fabrication of Ag-based REM. The REM devices show transparent state (no potential), and reflective (mirror) state (-2.5 V).

Figure S2 shows cyclic voltammetry (CV) measurements of electrochromic electrolyte (IL-4 solution) fabricated with Ag+ on the ITO electrode. The initial sweep from an open circuit potential to a negative direction showed a cathodic current from -0.26 V to a typical 12


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cathodic current peak at -0.62 V. These two values were attributed to the electrochemical reduction of Ag+ and Cu2+, respectively, which led to electrodeposition of a metallic Ag mirror on the ITO electrode. When the potential was swept from -1.4 V to the positive direction, an anodic current appeared at -0.34 V, reaching a peak at -0.05 V. This corresponds to the dissolution of deposited Ag and Cu to Ag+ and Cu+ into the electrochromic solution, which increased the transmittance of the ITO electrode. An additional oxidation current appears at around +0.45 V due to the oxidation of Cu+ to Cu2+. Moreover, the oxidation current appears at + 0.63 V which is attributable to the oxidation of 2Br− to Br2 on the counter electrode. The Br2 dissociation forms the Br3− or Br42−. Similar results were observed for other IL-based liquid electrolytes and PIL-based semi-solid electrolytes (Figure S3).

Figure 2. Optical transmittance spectra of IL-based REMs: a) voltage-off state, b) voltageon state (-2.5 V for 60 s). 13



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The effects of the charge density and the alkyl chain length substituted at the N3 position of imidazolium cations on the electrochromic properties of Ag-based REMs were investigated. Figure 2a shows optical transmittance spectra of REMs in the transparent (voltage-off) state. The REMs based on IL-C6-Im-C10 and IL-C8 are more transparent than other mono- and bis-imidazolium analogues (IL-C6-Im-C10 reached 74.5% at 650 nm). A rapid increase in transmittance was observed from 400-600 nm, likely because of the dark color of the original electrolytes. When -2.5 V was applied for 60 s, the cell transmittance decreased in the order: IL-C6-Im-C10 < IL-C6-Im-C7 < IL-C6-Im-C4 < ILC8 < IL-C6 < IL-C4 (Figure 2b), indicating the reduction of silver ions and the formation of Ag mirrors.

Figure 3. SEM images of the electrodeposited metallic Ag films with diverse IL-based electrolytes (-2.5 V for 60 s): a) IL-C4, b) IL-C6, c) IL-C8, d) IL-C6-Im-C4, e) IL-C6-ImC7, and f) IL-C6-Im-C10. 14


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The SEM images in Figure 3 show that the particle size and distribution of Ag nanoparticles deposited on ITO glass are strongly affected by both the alkyl chain length substituted at the N3 position and the charge density of imidazolium cations. The longer alkyl chain decreased the size and made the distribution of Ag nanoparticles more uniform (Figure 3a-c). This is because longer alkyl chains increased the hydrophobicity of the ILelectrolytes, which is confirmed by the contact angles of IL electrolytes on the ITO electrode (Figure S4). The alkyl chains substituted at the N3 position of imidazolium cations can modify the electrode surface53 and provide strong interactions between the electrode and deposited Ag nanoparticles. Hence, smaller and denser Ag nanoparticles were deposited on the ITO glass as the alkyl chain length increased. The Ag nanoparticle deposition also showed a correlation to the imidazolium cation charge density. In Figure 3, bis-imidazolium-type ILs produced smaller and homogeneously distributed Ag nanoparticles compared with mono-imidazolium based ILs. Thus, a higher charge density results in smaller size and more uniform distribution of deposited Ag nanoparticles. Compared with mono-imidazolium type ILs, the higher charge density of the bis-imidazolium type ILs contributed to the formation of a more stable electrical double layer on metal ions surface.54,55 This prevented aggregation and agglomeration during the voltage-on state, leading to a relatively uniform distribution of electrodeposited Ag nanoparticles. In addition, the transmittances changes (ΔT) of bisimidazolium type ILs were higher than those of analogous mono-imidazolium type ILs. 15



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For example, the ΔT of IL-C6-Im-C10 was 60.5%, which is higher than that of IL-C8 (38.9%) at 650 nm.

Figure 4. a) Chemical structures of imidazolium-type PILs. b) FT-IR spectra of the PILs. Optical transmittance spectra of PIL-REMs and IL-REMs containing 10 wt% PVA: c) voltage-off state, d) voltage-on state (-2.5 V for 60 s).

In practical use, however, liquid electrolytes often suffer from leakage and hydrostatic pressure issues, and the use of solid or semi-solid electrolytes in electrochromic devices may overcome these issues. The results of REMs based on IL-based liquid electrolytes motivated the synthesis of semi-solid PIL-based electrolytes for use in Ag REMs. The 16


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corresponding imidazolium-type PILs: poly(1-butyl-3-vinylimidazolium bromide) (PILC4),

poly(1-hexyl-3-vinylimidazolium

bromide)

(PIL-C6),

and

poly(1-octyl-3-

vinylimidazolium bromide) (PIL-C8) were successfully synthesized via free radical polymerization (Figure 4a and Figure S5). The synthesized PILs were characterized using FT-IR spectroscopy (Figure 4b). The characteristic peaks of an IL monomer (VmIL-C4) at 3134, 1652, and 1173 cm−1 were attributed to the stretching vibration of imidazolium cations, and the bands at 1554 and 953 cm−1 belong to the C=C stretching vibration. Compared with IL monomers, all corresponding PILs showed a stretching vibration of imidazolium cations, while the bands at 1554 and 953 cm−1 belonging to the C=C stretching vibration disappeared, indicating the successful polymerization of imidazolium-based PILs. Figures 4c and d respectively show the transmittance of PIL-type semi-solid electrolyte-based REMs in the voltage-off and voltage-on states. When a voltage (-2.5 V) was applied for 60 s, the light transmittance of the devices decreased to almost zero (Figure 4d), indicating the reduction of silver ions and the formation of dense Ag mirrors. The SEM images of the Ag nanoparticles deposited on the ITO electrode in Figure 5 show that PIL-C4-REM had the largest particles and particle size distribution, while PIL-C8-REM had the smallest and densest Ag film. Longer alkyl chains attached to imidazolium cations resulted in denser electrodeposited Ag films, which is consistent with the results of the ILbased liquid electrolytes. 17



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Figure 5. SEM images of the electrodeposited metallic Ag films of different PIL-REMs (2.5 V for 60 s): a) PIL-C4, b) PIL-C6, c) PIL-C8. The photographs of the PIL-C8-REM: d) voltage-off state, e) voltage-on state.

Compared with IL-based liquid electrolytes, all PIL-based gel electrolytes exhibited more compact and denser electrodeposited metallic Ag films. Other reports have similarly noted that the viscosity of the electrolyte strongly affects ion diffusion and the nucleation and growth of Ag nanoparticles.56-58 The viscosity and ionic conductivity of IL- and PILelectrolytes in Table 1 show that the viscosity of IL electrolytes is relatively low (200 300mPa ･ s). Such a low viscosity values caused relatively larger aggregation and agglomeration of Ag particles (Figure 3) because sliver ions were quickly electro-driven on the surface of ITO electrode. As for the PIL-based gel electrolytes, the viscosity (5710 18


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- 7160mPa･s) was much higher than that of the IL electrolytes (Table 1). Higher electrolyte viscosity decreased the ion mobility and slowed down the electrodeposition rate of Ag nanoparticles, which led to uniform and compact Ag mirrors (Figure 5e). Moreover, PILbased electrolytes display solid-like elasticity with a higher storage modulus (G’) than loss modulus (G’’), indicating that the gel state is achieved through physical crosslinking (Figure S6)

Table 1. Viscosity and conductivity of IL- and PIL-based electrolytes at 25oC Viscosity a

Conductivity b

(mPa･s)

(10-2S/m)

IL-C4

220.1±9.2

19.5±0.5

IL-C6

221.1±10.1

19.4±0.2

IL-C8

238.7±8.7

13.6±0.3

IL-C6-Im-C4

250.8±12.2

13.8±0.1

IL-C6-Im-C7

256.5±11.6

9.6±0.2

IL-C6-Im-C10

264.9±13.2

12.9±0.4

PIL-C4

5716.0±87.3

9.6±0.3

PIL-C6

6513.5±126.3

8.2±0.2

PIL-C8

7168.8±160.7

7.4±0.1

Electrolyte

a

AgNO3 (0.5 mmol), CuCl2 (0.5 mmol) and IL (or PIL, 2.5 mmol) dissolved in 1.8 mL DMSO.

b IL

(or PIL, 2.5 mmol) dissolved in 1.8 mL DMSO.

19



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Figure 6. SEM images of electrodeposited metallic Ag films based on electrolytes using 10 wt% PVA as the host polymer (-2.5 V for 60 s): a) IL-C8-PVA, b) IL-C6-Im-C10-PVA.

To further investigate how electrolyte viscosity affects the electrodeposition of Ag nanoparticles, a host polymer, PVA, was added in the IL-based liquid electrolytes to increase the viscosity. The viscosity and conductivity of IL-C8-PVA and IL-C6-Im-C10PVA are summarized in Table S1. Upon about 10 wt% PVA is added, the viscosity of ILC8-PVA and IL-C6-Im-C10-PVA increased to approximately 7000 and 7400 mPa ･ s, respectively. Figure 6 shows the SEM images of electrodeposited Ag films based on ILPVA electrolytes. When PVA is used, compact and dense electrodeposited Ag nanoparticles were observed in IL-C8-PVA (Figure 6a), which is contrast to IL-C8 which showed Ag nanoparticle agglomeration (Figure 3c). Similar results were observed for ILC6-Im-C10-PVA, and together, these results further confirmed that the viscosity is crucial for successful electrodeposition of silver mirrors. Unfortunately, the poor compatibility of 20


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IL/PVA gel electrolyte prevents practical applications since phase separation was observed after two weeks of room-temperature storage in the IL-C8-PVA gel electrolyte. In contrast, the PIL-C8 gel electrolyte remained stable even after three months under the same experimental conditions (Figure S7). It should be noted that the transmittance of PILelectrolyte based REM is lower than that of IL/PVA electrolyte. Without PVA, the IL-C6Im-C10 based REM shows more significant change in transmittance than IL-C8. Upon the addition of PVA, however, an opposite result was observed probably due to the higher viscosity of IL-C6-Im-C10-PVA which prevented the electrodeposition of Ag nanoparticles. Ultimately, the transmittance of IL-C6-Im-C10-PVA based REM is lower than that of IL-C8-PVA-based REM, indicating the formation of relatively homogeneous and denser electrodeposited Ag nanoparticles (Figure S8).

Figure 7. a) The transmittance of PIL-C8-REM when -2.5 V was applied for different times (0 s, 7 s, 15 s, and 30 s). b) Reflectance of PIL-C8-REM at voltage-off state for 60 s, 300 s, 600 s, and 1800 s after applying -2.5 V for 60 s. (c) Transmittance change at 600 nm

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during a cycling stability test of the 2-electrode REM device. Bias voltages were applied in the following sequence: -2.5 V (20 s), 2.5 V (8 s), and 0.5 V (50 s).

The device based on PIL-C8 electrolyte shows an 80.5% transmittance at 600 nm at voltage-off state. The transmittance decreased to around 10% within 7 s and then dropped to zero after 30 s, indicating the formation of a dense Ag film and a short response time between the transparent and the mirror states (Figure 7a). The high ionic conductivity of PIL electrolytes promotes rapid electrodeposition rates since ions can readily diffuse. In addition, a long open-circuit memory effect is also necessary for switchable mirrors. Figure 7b shows the reflectance of the Ag-REMs based on PIL-C8 characterized via in situ UV-vis spectroscopy which reached approximately 92.5% over the entire range of visible wavelengths at a voltage of -2.5 V for 60 s. A reflectance of 64% was maintained at 450 nm, even after 1800 s in the voltage-off state, indicating that the Ag mirror had a long memory effect. This long open-circuit memory effect may be due to the formation of a anion barrier in the high-viscosity PIL-based electrolyte, which protects the metal film from bromide ions in the voltage-off state.33, 56, 59 The cycling stability of the REM device based on PIL-C8 is shown in Figure 7c. The device showed an initial and maximum transmittance modulation of 81.2% and maintained a modulation of 70.6% after 1000 cycles, suggesting a good durability of electrochemical 22


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deposition and dissolution. When Ag nanoparticles were deposited or dissolved, auxiliary redox processes of Cu2+/Cu+ or Br3−/Br− occurred on the opposite electrode. The Cu2+/Cu+ played vital roles in maintaining the switching durability and possibly prevented the degradation of optical properties from an irreversible side reaction.31 The distribution of metallic nanoparticles was relatively homogeneous and denser in the presence of Cu2+ than without it (Figures 5c and S9). The transmittance of mirror was lower in the presence of Cu2+ than without it (Figure S10). Moreover, since the oxidation potential of Cu+ to Cu2+ is higher than that of Ag to Ag+, Cu2+ mediates the oxidation of the Ag nanoparticles. Through this mediation, Ag nanoparticles were fully oxidized to Ag+, resulting in an increase of the transmittance to its initial value after 10 times cycling (Figure S10). A supporting movie was created to demonstrate the reversible switching between the transparent and mirror states of PIL-C8-REM by an alternating potential cycle (Movie S1).

Figure 8. Photographs of the flexible PIL-C8-REM: a) transparent state, b) mirror states, and c) flexible display. 23



Page 24 of 33

Figure 8 shows the electrodeposition of silver over ITO-coated PET sheets (active area 3.0 x 3.0 cm2) to form a flexible electrochromic device using a red push pin placed in front to show the mirror state when a negative potential was applied. Figure 8a shows the transparent state of the device prior to potential application. When a negative potential of -3.0 V for 10 s is applied, the device displayed a mirror state (Figure 8b), and the flexibility of the display is shown in Figure 8c. This demonstrates that the flexible REM could be fabricated using a highly-viscous PIL electrolyte and provides a viable solution to the unresolved issues of flexible displays, such as electrolyte leakage.

Conclusions In this work, semi-solid (gel) electrolytes based on imidazolium type PIL were developed for silver-based REMs. The electrolytes based on imidazolium type IL monomers and their corresponding PIL homopolymers were prepared to ensure uniform deposition and high optical contrast. The length of the substituted carbon chain length at the N3 position and charge density of imidazolium cation was shown to influence the electrodeposition and spectral properties. Compared with IL-based liquid electrolytes, PILbased gel electrolytes with higher viscosity formed smaller and denser electrodeposited metallic Ag nanoparticles. From these findings, simple two-electrode semi-solid (gel) electrolyte based dynamic window and flexible electrochromic device were fabricated. The PIL-based devices showed a fast switching speed, superb cycling durability, and small 24


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particle sizes and a uniform electrodeposited Ag nanoparticle film. These excellent metrics suggest that semi-solid (gel) imidazolium type PIL electrolyte-based REMs offer a promising and competitive alternative to traditional smart windows.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic route for ILs and PILs, photographs, cyclic voltammogram, contact angle, viscosity and conductivity data of the electrolytes.

Acknowledgements This work was supported by the National Nature Science Foundation for Distinguished Young Scholars (21425417), the National Natural Science Foundation of China (21835005, U1862109, 21704071), and by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Notes The authors declare no competing financial interest

25



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Table of Contents

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Poly(ionic liquid) Electrolytes for Switchable Silver Mirror

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