Highly stable viologens-based electrochromic devices with low operational voltages utilizing polymeric ionic liquids

Article abstract of DOI:10.1016/j.cplett.2020.137434

In this work, we proposed an effective route of introducing poly(ionic liquid) (PIL) gel as electrolyte to suppress dimer formation of viologen radical cation during switching in electrochromic devices (ECDs). The ECDs were fabricated based on ion gels consisting of various viologens, PIL and ferrocene (Fc). We found that incorporation of PIL contributed to the suppression of dimer production. The suppression of dimer formation can provide ECDs with improved colouration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. The results showed that an excellent cyclic stability over 96% of initial transmittance change after 4000 cycles of switching.

Full text of DOI:10.1016/j.cplett.2020.137434

ChemicalPhysicsLetters749(2020)137434
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Research paper
Highly stable viologens-based electrochromic devices with low operational
voltages utilizing polymeric ionic liquids
Xiuxiu Wanga,b, Lijuan Guob, Shaokui Caoa,⁎, Weizhen Zhaob,⁎
a School of Materials Science and Engineering, Zhengzhou University, Henan 450001, PR China
b Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of
Sciences, Beijing 100190, PR China
H I G H L I G H T S
Asymmetrical 1-alkyl-1′-aryl substituted viologen and polyviologen were synthesized.
Viologens-based electrochromic devices with polymeric ionic liquids were assembled.
The ECDs exhibited well electrochromic behavior.
Cyclic stability over 96% of initial transmittance change after 4000 cycles of switching.
•
•
•
•
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, we proposed an effective route of introducing poly(ionic liquid) (PIL) gel as electrolyte to suppress
dimer formation of viologen radical cation during switching in electrochromic devices (ECDs). The ECDs were
fabricated based on ion gels consisting of various viologens, PIL and ferrocene (Fc). We found that incorporation
of PIL contributed to the suppression of dimer production. The suppression of dimer formation can provide ECDs
with improved colouration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs.
The results showed that an excellent cyclic stability over 96% of initial transmittance change after 4000 cycles of
switching.
Electrochromism
Electrochemical displays
Ion gel electrolytes
Viologens
Polymeric ionic liquids
1. Introduction
characteristics of viologens could be easily tuned with different sub-
stituent groups on bipyridinums [17]. However, the long-term stability
Electrochromic (EC) materials have the ability to reversibly alter
their optical characteristics via redox reactions driven by an application
voltage bias [1-3]. Therefore, EC materials have been widely used in
electrochemical applications such as anti-glare rear view mirrors, sen-
sors, military equipment camouflage, information displays, electro-
chemical device and so on [4,5]. For all applications, the device per-
formance highly depends on the electrochemical and optical properties
of EC materials.
of viologen-based solution-type ECDs suffers from the poor solubility of
V+% [18]. The V+% tends to form dimer via spin-pairing during the
operation and results in an irreversible bleaching process [19]. To avoid
the dimerization of viologen, asymmetric molecular modification of
viologen and gel-based ECDs have been considered as two feasible
method [20–22]. On the other hand, in general, ECDs containing vio-
logens in a liquid electrolyte suffer from not only the side reactions
mentioned above but also the possibility of electrolyte leakage [23].
Polymer gel electrolytes (PGEs), which are widely used in super-
capacitors [24], actuators [25] and dye-sensitized solar cells (DSSCs)
[26], are of great potential for ECDs. However, one important challenge
for display applications of ECDs based on electrolyte gels is their low
ionic conductivity (10−3–10−1 mS/cm) [27]. Therefore, many research
groups has focused on how to improve the conductivity [28,29]. Ti-
mothy P. Lodge reported an ion gel, a composite of ABA triblock co-
polymer with an IL-soluble B block and IL-insoluble A blocks and room
EC materials such as metal oxides [6,7], conducting polymers [8,9],
organic small molecules with redox activity [10–12], and metal com-
plexes [13] have been reported. Among them, viologens, N,N-sub-
stituted bipyridiniums, have received wide attention since their re-
markable and reversible color contrast from colorless to blue or violet
during their first redox process [14]. Viologens possess three redox
states, including neutral state (V0), deep colored radical cation state
(V+%
) ) [15,16]. The optical
and colorless dication state (V2+
⁎ Corresponding authors.
E-mail addresses: caoshaokui@zzu.edu.cn (S. Cao), wzzhao@ipe.ac.cn (W. Zhao).
https://doi.org/10.1016/j.cplett.2020.137434
Received 18 March 2020; Received in revised form 30 March 2020; Accepted 31 March 2020
Availableonline01April2020
0009-2614/©2020ElsevierB.V.Allrightsreserved.

X. Wang, et al.
ChemicalPhysicsLetters749(2020)137434
temperature ionic liquids (ILs), possessed higher ionic conductivity
(1–10 mS/cm) than that of conventional PGEs (10−3–10−1 mS/cm)
[30]. Additionally, ion gels consisting of room temperature ILs and
random copolymers [31,32] or star copolymers [33] have been devel-
oped. Here, the ion gels exhibited high capacitance, exceptional
thermal and chemical stability, insignificant vapor pressure and wide
electrochemical window [34–36]. Therefore, this is an effective
strategy for ECDs with high performance by introducing ionic liquid
into copolymers to obtain an ion gel. Ion gels could be considered as
promising candidates in a range of electrochemical electronics.
In this study, the strategy was extended that we introduced poly
(ionic liquid) as ion gel into ECDs to prevent leakage while enhancing
the cyclic stability by avoiding the formation of viologen dimer. Poly(1-
vinyl-3-butylimidazolium bromide) (poly(VBImBr)) was synthesized
and employed as electrolyte gel. We prepared diheptyl viologen bis-
(hexafluorophosphate) (DHV(PF6)2), heptyl vinyl benzyl viologen bis
(hexafluorophosphate) (HBV(PF6)2) and poly(heptyl vinyl benzyl vio-
logen bis(hexafluorophosphate)) (poly(HBV(PF6)2)) as electrochromic
chromophores for systematical investigation. All homogeneous EC ion
gels were prepared by blending poly(VBImBr), ferrocene (Fc) and var-
ious viologens. Then, the device were assembled by sandwiching the
obtained ion gel between two ITO conducting glasses. The influence of
viologen substituents on ECDs properties was studied and compared in
terms of kinetic stability, spectroelectrochemistry, and cyclic voltam-
metry as well.
aryl asymmetric viologen (HBV) and polyviologen (PHBV) were de-
picted in Scheme 1. The synthetic procedure included two-step section
wherein the aryl or alkyl substituent was first bonded through SN2
substitution reaction, whereas the relative hydrophobicity viologens
were second obtained through an anion exchange. Symmetric diheptyl
viologen bis-(hexafluorophosphate) (DHV(PF6)2) was synthesized fol-
lowing the literature [38].
2.3.1. Synthesis of heptyl vinyl benzyl viologen bis(hexafluorophosphate)
(HBV(PF6)2)
As reported in the literature [39], monoheptyl viologen bromide
(MHVBr) was synthesized as follows: a mixture of 4,4′-bipyridine
(1.825 g, 12 mmol) and heptyl bromide (2 g, 10 mmol) was dissolved in
ACN (20 ml) and stirred at 50 °C for 72 h. MHVBr was isolated from the
mixed solution by filtering, washed with diethyl ether and dried under
vacuum at 50 °C for 24 h. Then, the arylation of MHVBr utilizing DMF
as a solvent to prepare heptyl vinyl benzyl viologen bromide chloride
(HBVBrCl) was conducted in the similar fashion as MHVBr. HBV(PF6)2
was synthesized by an anion exchange with the HBVBrCl and excess
KPF6 dissolved in DIW. The white precipitated HBV(PF6)2 was ob-
tained, separated through filtration, and washed with DIW after stirring
for 24 h. Ultimately, HBV(PF6)2 was collected after drying under va-
cuum at 50 °C for 24 h. IR (cm−1, KBr): 3087 (m), 2924 (s), 2854 (s),
1635 (s), 1509 (m), 1454 (s), 920 (m), 828 (s), 726 (m). 1H NMR
(600 MHz, DMSO, ppm): δ 9.49 (d, J = 6.9 Hz, 4H), 9.37 (t, J = 7.9 Hz,
4H), 8.78 – 8.71 (m, 2H), 7.62 – 7.54 (m, 2H), 6.75 (dd, J = 17.6,
11.0 Hz, 1H), 5.90 (d, J = 16.3 Hz, 2H), 5.33 (d, J = 11.0 Hz, 2H), 4.67
(t, J = 7.4 Hz, 2H), 1.97 (s, 2H), 1.38 – 1.16 (m, 8H), 0.86 (t,
J = 7.0 Hz, 3H).
2. Experimental section
2.1. Materials
Ethanol absolute (≥99.8%), potassium bromide (KBr, ≥ 99.5%)
and 2,2′-azobis(isobutyronitrile) (AIBN, 99%) were purchased from
Aladdin. Vinyl benzyl chloride (VBC, 90%) was purchased from Sigma-
Aldrich. 4,4′-bipyridine (98%), heptyl bromide (98%), N,N-di-
methylformamide (DMF, 99.5%), ferrocene (Fc, 95%) potassium hex-
afluorophosphat (KPF6, 95%) and propylene carbonate (PC, 98%) were
provided by TCI. 1-vinyl-3-butylimidazolium bromide (VBImBr, 99%)
was obtained from Lanzhou Oricko Chemical Co., Ltd. Acetonitrile
(ACN, AR), acetone (AR) and diethyl ether (AR) were purchased from
Beijing Chemical Works. Deionized water (DIW) was used throughout
this study. The ITO substrate (sheet resistance is < 7 Ω/□) was ob-
tained from South China Science and Technology Co., Ltd, Shenzhen.
All the materials were used as received without any further purification
except the ITO conducting glass which was washed with deionized
water, acetone and ethanol for 10 min under sonication, respectively.
2.3.2. Synthesis
(hexafluorophosphate)) (poly(HBV(PF6)2))
of
poly(heptyl
vinyl
benzyl
viologen
bis
Poly(HBV(PF6)2) was synthesized following the literature [40]. In
detail, the radical polymerization reaction of HBVBrCl occurred in DMF
at 70 °C for 10 h with AIBN as an initiator. Poly(HBVBrCl) was isolated
from the mixed solution by filtering, washed with diethyl ether and
then dried under vacuum at 50 °C for 24 h. Poly(HBV(PF6)2) was syn-
thesized using the same process with HBV(PF6)2. IR (cm−1, KBr): 3086
(m), 2924 (s), 2854 (s), 1635 (s), 1557 (m), 1504 (m), 1448 (m), 809
(m), 728 (m). 1H NMR (600 MHz, DMSO, ppm): δ 9.42 (d, J = 71.2 Hz,
4H), 8.74 (s, 4H), 7.58 (s, 4H), 6.75 (dd, J = 17.4, 10.8 Hz, 1H), 5.90
(d, J = 17.7 Hz, 2H), 5.33 (dd, J = 11.7, 7.0 Hz, 2H), 4.67 (s, 2H), 2.07
– 1.79 (m, 2H), 1.47 – 1.06 (m, 8H), 0.91 – 0.71 (m, 3H).
2.4. Device fabrication
2.2. Synthesis of poly(1-vinyl-3-butylimidazolium bromide) (poly
(VBImBr))
All homogeneous EC gels consisting of poly(VBImBr) and PC at a
weight fraction of 1:4 were completely dissolved at 40 °C.
Subsequently, Fc and viologen compounds at a weight fraction of 3:20
were introduced into the above solution. The resulting EC gels were
then deposited on the ITO conducting glass by blade coating. Another
ITO conducting glass was placed in reverse on the gel quickly and two
ITO conducting glasses were laminated using a cell gap of 60 µm
double-sided tape firmly, as was demonstrated in Scheme 2.
Poly(VBImBr) was prepared following previous investigation
(Scheme 1) [37]. Firstly, the poly(VBImBr) homopolymer was synthe-
sized through radical polymerization of VBImBr in ethanol with AIBN
as an initiator. AIBN (5 wt‰ with respect to monomer) was injected to
the ethanol solution of VBImBr slowly, and the reaction mixture was
stired vigorously at 70 °C for 9 h under nitrogen. Subsequently, the raw
product was separated by precipitating into acetone, filtering the re-
sulting precipitate and washing it with acetone for several times. the
obtained yellowish poly(VBImBr) solid was dried in a vacuum oven at
40 °C for 24 h. IR (cm−1, KBr): 3442 (s), 3068 (m), 2961 (s), 2869 (s),
1660 (s), 1546 (m), 1456 (m), 1163 (m), 746 (m). 1H NMR (600 MHz,
DMSO, ppm): δ 8.45 – 7.45 (m, 4H), 4.15 (d, J = 24.2 Hz, 2H), 1.59 (d,
J = 302.2 Hz, 5H), 0.95 (s, 3H).
2.5. Characterization
The chemical structures of poly(VBImBr) and viologen compounds
synthesized in this work were confirmed with 1H NMR (Avance III 600)
and FTIR (Nicolet 380, Thermo Electron) instrument. The IR and 1H
NMR spectrum of them were shown in Figs. S1–S4. The applied square
wave voltage was supplied by a potentiostat (MS-605D, Maisheng). The
cyclic voltammogram (CV) curves of DHV, HBV and PHBV-containing
ECDs were recorded on an electrochemical workstation (CHI 660E,
Shanghai Chenhua CH Instruments, Inc.) under two-electrode system
configuration. The UV–vis absorption spectrum of the devices were
2.3. Synthesis of the symmetric, asymmetric viologen and polyviologen
The synthetic route of symmetric diheptyl viologen (DHV), alkyl-
2

X. Wang, et al.
ChemicalPhysicsLetters749(2020)137434
Scheme 1. Synthetic route for symmetric diheptyl viologen (DHV), asymmetric viologen (HBV) and polyviologen (PHBV).
recorded in transmission mode by using an UV–vis sprctrometer (UV-
2550) within wavelengths of the visible range at different applied
voltages. Dynamic transmittance change curves of the devices in spe-
potentiostat and UV–vis sprctrometer (UV-2550) under two-electrode
system configuration.
cific wavelength were measured by using
a combination of the
Scheme 2. The viologen-based ECDs architecture.
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X. Wang, et al.
ChemicalPhysicsLetters749(2020)137434
Fig. 1. Cyclic voltammograms of (a) DHV, (b) HBV, and (c) PHBV-containing ECDs under two-electrode system configuration at a rate of 20 mV/s. Fc served as both
anodic species and internal standard.
3. Results and discussion
Table 1
Summarized electrochemical properties of DHV, HBV and PHBV-containing
ECDs.
3.1. Electrochemical characteristics of viologen-based ECDs
Viologen
Peak potential, V vs Fc+/Fc
Peak current, mA
To understand the electrochemical characteristics of ion gel-based
ECDs with DHV, HBV and PHBV, the cyclic voltammograms (CV)
curves were conducted at scanning rate of 20 mV/s (Fig. 1). Fc was
introduced in viologen-based ECDs as both anodic species and internal
standard material, the redox potentials of these viologens were ob-
tained on the basis of that of the Fc+/Fc, which corresponded to the
1
2
1
2
Epa1/Epc
Epa2/Epc
Ipa1/Ipc
Ipa2/Ipc
DHV(PF6)2
HBV(PF6)2
Poly(HBV(PF6)2)
0.96/0.75
0.91/0.71
0.95/0.71
–
2.23/-2.19
1.27/-0.78
1.37/-1.00
–
1.36/1.15
1.34/1.15
0.75/0.82
0.36/0.55
Fig. 2. The UV–vis absorption spectrum in transmission mode at various applied potentials and the photographs of the colored and bleached (a) DHV, (b) HBV, and
(c) PHBV-containing ECDs states.
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X. Wang, et al.
ChemicalPhysicsLetters749(2020)137434
Fig. 3. Transient transmittance profiles of (a) DHV/Fc ECD at 606 nm and 1.1 V, (b) HBV/Fc ECD at 608 nm and 1.3 V, and (c) PHBV/Fc ECD at 606 nm and 1.3 V,
respectively.
results of cycle stability characterization (Fig. 4). However, the elec-
trochemical behavior of symmetric and asymmetric heptyl viologen in
ECDs with poly(ionic liquid) gel electrolyte was different from that
reported one in solution-type ECDs [41]. The reason may be that the
introduced PIL can effectively suppress the dimerization of viologen
radical cation.
Table 2
Electrochemical properties of DHV, HBV and PHBV-containing ECDs.
Viologen
λmax (nm)
ηa (cm2 /C)
△T (%)
Switching time (s)
b
c
Colored
Bleached
DHV(PF6)2
HBV(PF6)2
Poly(HBV(PF6)2)
606
608
606
109.8
106.2
105.7
47.7
45.4
41.1
11.1
10.8
9.2
19.9
28.4
20
3.2. Spectroelectrochemistry properties of viologen-based ECDs
The applied square wave voltage between 0 and 1.1 V for DHV-containing
ECDs, and between 0 and 1.3 V for HBV, and PHBV-containing ECDs.
Since the optical characteristics of viologen-based ECDs relied on
the applied voltage, the UV–vis absorption spectrum of the devices were
measured in transmission mode within wavelengths of the visible range
at different applied voltages. There was no obvious change in the in
UV–vis spectra of DHV-including ECDs (Fig. 2a) up to 0.7 V, whereas a
characteristic peak at 606 nm originated from the generated DHV+%
was observed at 0.9 V, which was in good agreement with coloration
voltage estimated by cyclic voltammograms shown in Fig. 1a. Because
the larger applied voltages accelerated the generation of DHV+%, giving
rise to a higher DHV+% concentration, more obvious and intense peak
was detected, but the characteristic peak was unchanged at 606 nm,
illustrating the major colored species was DHV+% in DHV-including
ECDs. Additionally, the ECD showing blue colored state at 1.1 V coin-
cided with the presence of DHV+% (Fig. 2d). Similar to the UV–vis
spectra of DHV-including ECDs, the transmittance peak of HBV and
PHBV-containing ECDs gradually increased with increasing applied
voltage as depicted in Fig. 2b and c, respectively, and stronger trans-
mmittance peaks were found at 608 and 606 nm. The photographs of
HBV and PHBV-containing ECDs in the colored and bleached states
were demonstrated in the image of Fig. 2e and f, respectively.
a
Coloration efficiency (η) was determined by utilizing the measured charge
density.
b
The requisite time to achieve 90% (△T90%) of the total transmittance
change for coloration.
c
The requisite time to achieve 90% (△T90%) of the total transmittance
change for bleaching.
requisite voltage for the coloration of the devices. The redox reaction of
DHV2+/DHV+% (Fig. 1a), HBV2+/HBV+% (Fig. 1b) and PHBV2+
/
PHBV+% (Fig. 1c) couples occurred at 0.86 V, 0.81 V and 0.83 V (vs.
Fc+/Fc), respectively. Here, the reduction of DHV2+ only to generate
the reduced state (DHV+%) along with one electron transferring. In
Fig. 1b and c, the first cathodic peak of the HBV and PHBV-containing
EC gels registered at 0.71 V (vs. Fc+/Fc) was correlated with the radical
cation (V+%), generated as a result of the reduction of the divalent ca-
tion (V2+), while the second cathodic peak of them observed at 1.15 V
(vs. Fc+/Fc) was related to the neutral state (V0) obtained through the
second reduction of the divalent cation. The electrochemical char-
acteristics obtained through the CV measurements were given in
Table 1. The redox peak of DHV2+/DHV+% was shown with high re-
versibility (ipa/ipc~1.02). In contrast, asymmetric HBV and PHBV ex-
hibited low reversibility of redox behaviors (Fig. 1b and c). The ipa/ipc
was only ~1.63 (HBV2+/HBV+%) and ~1.37 (PHBV2+/PHBV+%) re-
spectively. The high reversibility meant excellent cyclic stability. This
results indicated that symmetric DHV was more stable in ECDs with
poly(ionic liquid) as electrolyte. This was also consistent with the
3.3. Optical contrast and response time of viologen-based ECDs
The slow coloration rate was a major limitation to the commercia-
lization of ECDs as display materials, even though ECDs possessed many
advantages [42]. Therefore, the redox kinetics was a significant factor
to the development of ECDs. To study the coloration and bleaching
kinetics for DHV, HBV and PHBV-containing ECDs, the transient
Fig. 4. Cycling stabilities of (a) DHV/Fc ECD at 606 nm and 1.1 V, (b) HBV/Fc ECD at 608 nm and 1.3 V, and (c) PHBV/Fc ECD at 606 nm and 1.3 V, respectively.
5

X. Wang, et al.
ChemicalPhysicsLetters749(2020)137434
Table 3
Summarized the electrochemical properties of viologen-based ECDs utilizing liquid, gel or solid electrolyte.
ECD system
BzV/Fc
HV/TMPD
VRS/TMPD
HV/TMPD
HERFV/Fc
DHV/Fc
Type
liquid
liquid
liquid
solid
gel
gel
λmax (nm)
Vb/Vc (V)
tb/tc (s)
605
0/1.2
2.2/2.1
53.8
44.6 after 5000 cycles
[43]
615
0/1.0
2.5/8.0
75.5
100 after 100 cycles
[44]
620
0/0.4
2.7/2.7
62.6
72.4 after 1000 cycles
[45]
620
0/0.9
5.2/2.1
60.1
79.6 after 2000 cycles
[46]
604
0/-1.0
15/22
35.4
91.5 after 2000 cycles
[47]
606
0/1.1
a
11.1/19.9
46.8
96.0 after 4000 cycles
This work
△T (%)
Cyclic stabilityb (%)
Ref
a
The requisite time to achieve 90% (△T90%) of the total transmittance change for coloration/ bleaching.
Retained percentage of its initial ΔT.
b
transmittance curves were measured at either 606 nm or 608 nm, re-
both in colored and bleached state as seen in Fig. 4b. In contrast, the
PHBV/Fc ECD showing an initial ΔT of 41.1%, and 70.8% of the re-
tention after 4000 cycles of potential switching, exhibited poorer cyclic
stability. The electrochemical performance of viologen-based ECDs
utilizing solution, gel or solid electrolyte was summarized in Table 3.
Overall, the DHV/Fc ECD based on poly(ionic liquid) gel possessed
excellent operational stability under applied bias and a larger ΔT, in-
dicating another effective strategies to improve the cycle stability of
simple viologen in ECDs.
spectively. The requisite time to achieve 90% (△T90%) of the total
transmittance change was 11.1 s for coloration of the DHV/Fc ECD, as
shown in Fig. 3a. When coloration voltage (1.1 V for 30 s) was applied
to two ITO-coated substrates, DHV2+ and Fc were consumed on the
cathode and anode, respectively, which resulted in the generation of the
reduced state (DHV+%) and Fc+. The diffusion of the radical cation
(DHV+%) and Fc+ from the surfuce of two ITO-coated electrodes to the
center of the EC gel was performed owing to the concentration gradient.
Ultimately, these two species encountered and spontaneous electron-
transfer reaction occurred because of the negative △G for this reaction,
which was in agreement with the bleaching process of the devices
under open-circurt condition. Therefore, the requisite time to achieve
90% (ΔT90%) of the total transmittance change was 19.9 s for bleaching
of the DHV/Fc ECD under open-circurt condition. The HBV/Fc ECD and
PHBV/Fc ECD showed similar EC behavior (Fig. 3b and c): the times for
ΔT90% for coloration at 1.3 V were 10.8 s and 9.2 s, respectively, and
the times for ΔT90% for bleaching under open-circurt condition were
28.4 s and 20 s, respectively. According to the definition of optical
contrast, optical contrast of three kinds of ECDs was 47.7% (DHV/Fc
ECD), 45.4% (HBV/Fc ECD), 41.1% (PHBV/Fc ECD), respectively,
while the coloration efficiency (η) for them was estimated to be
109.8 cm2 /C, 106.2 cm2 /C, 105.7 cm2 /C respectively. The current-
time curves of DHV(PF6)2, HBV(PF6)2 and poly(HBV(PF6)2) were shown
in Fig. S5. The performance of ECDs was summarized in Table 2. It was
worth to mention that the sharp curves indicated fast switching in re-
spoonse to the applied potential bias. Additionally, excellent reversi-
bility of coloration/bleaching cycles was also seen in Fig. 3. Specifi-
cally, the bleached transmittance was nearly recovered under open-
circuit condition, illustrating that high transparency was obtained even
in the state of EC gels. With this consideration, devices with low-voltage
driven and high optical contrast indicated enormous potential in the
area of the electrochromic displays.
4. Conclusions
In this work, we proposed an effective strategy to improve the cyclic
stability of viologen with simple molecular structure. Three viologens,
including DHV, HBV and PHBV, were successfully synthesized, char-
acterized and incorporated into PIL ion gels containing Fc, giving ITO/
EC gel/ ITO configuration. The DHV, HBV and PHBV-containing ECDs
presented good electrochromic properties and revealed a better trans-
mittance change of 47.7%, 45.4% and 41.1% at both detected wave-
lengths and fast response time within 11.1 s, 10.8 s and 9.2 s, producing
a high coloration efficiency of 109.8 cm2 /C, 106.2 cm2 /C, 105.7 cm2
/C, respectively. Especially, the resulting DHV2+ exhibited an excellent
cyclic stability over 96% of initial transmittance change after 4000
cycles of switching. The results indicated that the introduced PIL can
effectively suppress the dimerization of viologen radical cation. It’s
expected that incorporation of poly(ionic liquid) gel with simple vio-
logen structure could be promising materials for energy-saving smart
window or electrochromic displays.
CRediT authorship contribution statement
Xiuxiu Wang: Writing - original draft. Lijuan Guo: Visualization.
Shaokui Cao: Writing - review & editing. Weizhen Zhao: Writing -
review & editing.
3.4. Cycling stability
Declaration of Competing Interest
For practical applications, it is important for ECDs to enhance the
stability of EC behavior. To estimate the cycling stability of the pre-
pared ECDs, two-electrode ECDs including DHV(PF6)2, HBV(PF6)2 and
poly(HBV(PF6)2) gels were exposed to the applied square wave voltage
between colored and bleached states, because faster coloration than
bleaching was achieved in these ECDs (see Fig. 3a–c). The variation of
the transmittance in the colored and bleached states for DHV/Fc ECD at
606 nm were given in Fig. 4a. The duration of coloration of ECDs was
30 s at 1.3 V except that of DHV/Fc ECD was 30 s at 1.1 V, and
bleaching was tested under open-circuit condition. According to the
definition of the △T retention, the DHV/Fc ECD showed an initial ΔT
of 47.7%, and 96% of the retention even after 4000 cycles of potential
switching, indicating outstanding repetitive operating properties. Si-
milarlly, the initial transmittance change for HBV/Fc ECD was observed
as 45.4% at 608 nm, with a 78.9% of the retention after 4000 sub-
sequent cycles, indicating no significant degradation of performance
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21808224).
Appendix A. Supplementary material
The IR spectrum of VBImBr, poly(VBImBr), DHV(PF6)2, HBV(PF6)2
and poly(HBV(PF6)2), the 1H NMR spectrum of poly(VBImBr), DHV
(PF6)2, HBV(PF6)2 and poly(HBV(PF6)2), and the current-time curves of
DHV(PF6)2, HBV(PF6)2 and poly(HBV(PF6)2). Supplementary data to
6

X. Wang, et al.
ChemicalPhysicsLetters749(2020)137434
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