Electrochromism of a bipolar reversible redox-active ferrocene-viologen linked ionic liquid

Article abstract of DOI:10.1039/c6cc09412k

A ferrocene-viologen linked “bipolar” type redox-active ionic liquid ([FcC₁₁VC₁][TFSI]₂) was synthesized as an electrochromic (EC) material that functions without any other additives: solvents, supporting electrolytes and sacrificial agents. The efficiency of a prototype symmetrical EC cell was 70 cm² C^?1 at 1.0 V. The EC process was stable even after over 10?000 potential cycles.

Full text of DOI:10.1039/c6cc09412k

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Electrochromism of a Bipolar Reversible Redox-Active Ferrocene–
Viologen Linked Ionic Liquid
Hironobu Tahara*a, Rei Baba a, Kodai Iwanaga a, Takamasa Sagara a and Hiroto Murakami*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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A ferrocene–viologen linked “bipolar” type redox-active ionic solid-state devices, we confront a difficulty in attaining
liquid ([FcC11VC1][TFSI]2) was synthesized as an electrochromic effective electronic contacts between the solid electrolyte, EC
(EC) material that functions without any other additives: solvent, material, and electrodes. To overcome such problems, we
supporting electrolyte and sacrificial agent. The efficiency of a develop an ionic liquid (IL), a salt of a V-Fc-linked bipolar IL, as
prototype symmetrical EC cell was 70 cm2C-1 at 1.0 V. The EC a single-component EC material.
process was stable even after over 10000 potential cycles.
ILs composed of cations and anions behave as not only a
non-volatile solvent but also an electrolyte.5 Functionalization
of a constituent ion of an IL with a redox-active unit provides
us with a redox-active ionic liquids (RAILs), a type of task-
specific IL6, as a solvent-free and supporting electrolyte-free
electrochemically active material.7–9 RAILs based on viologens7
and ferrocenes8 have been reported. For instance, we
synthesized a viologen RAIL with two bis(trifluoromethane-
sulfonyl)imide (TFSI) counter anions and explored its ionicity
Electrochromic (EC) materials change their colour markedly
and rapidly upon applying electrode potential or adding a
redox agent.1 The innovation of EC materials is of profound
importance in the development of electronic devices such as
electronic displays and optical sensors. Usually, liquid EC
materials are multi-component solutions that fill the separated
anodic and cathodic compartments of an EC cell. Realization of
a single-component compartment-free liquid EC device is
eagerly awaited. In such a device, the novel EC material should
play the three roles simultaneously as reductant, oxidant, and
electrolyte (ionic or electronic conductor).
characteristics.7j,k Kavanagh and coworkers prepared
a
a trioctylphosphonium cation and
viologen IL bearing
investigated its EC behavior.7h However, this RAIL required
dimethoxyacetophenone as an additive sacrificial agent.
Viologen derivatives are strongly blue (λ ≈ 600 nm, ε ≈
1.6×104 M−1cm−1)2 because of one electron reduction and are
among the most frequently used cathodic EC materials. When
the cathodic compartment of an EC cell is filled with a viologen
solution, the anodic compartment should be filled with an
electron donor to compensate the reaction charge. Viologens
are often combined with ferrocene as an anodic material to
compensate electrochemically reduction of viologens.3
Nishikitani and coworkers used a ferrocene–viologen (Fc-V)-
linked bipolar redox-active molecule to realize stable EC
performance and they used the material in a compartment-
free EC cell.4 The V-Fc-linked molecule has been used in both
liquid-phase and quasi-solid-state EC devices. However, these
EC devices still consist of multi-component, because a liquid
solvent or solid electrolyte is required. In liquid devices, the
volatilization of liquid materials is an inevitable problem. In
The Fc-V-linked molecule remains cation unless the
viologen unit becomes neutral (FcV0 in Fig. 1a) by two-electron
reduction. A bis(fluoroalkanesulfonyl)imide salt of the Fc-V-
linked molecule should behave as a bipolar additive-free EC
material. In the molecule, the V+•/V++ redox undergoes intense
colour change and the Fc/Fc+ redox couple drives counter
electrode process (Fig. 1a). An electrolyte-free EC material has
already been constructed that consists of a graphene quantum
dot–viologen composite system.10 However, bipolar RAILs in
which electron acceptor and donor units are combined in one
molecule have not yet been reported. In this communication,
we describe the synthesis of a Fc-V-linked bipolar RAIL, its
electrochemistry, and additive-free EC performance in a simple
two-electrode EC cell without a compartment separation.
The synthesis of Fc-V-linked RAIL was described in ESI†. X-
ray fluorescence and mass spectrometry analyses revealed
that the Fc-V linked RAIL ([FcC11VC1][TFSI]2) did not contain Br−
or I−. The melting point of [FcC11VC1][TFSI]2 determined by
differential scanning calorimetry was 52 °C (see Fig. S1, ESI†).
Once melted [FcC11VC1][TFSI]2 remained in a viscous liquid
state even at temperatures below its melting point. Long-lived
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the electrode area defined using Himilan®
thermal fusion bonding film (DuPont™).
The separation of the two parallel ITO
substrates was 25 μm (Fig. 1c). Fig. 1d
shows the CV of neat [FcC11VC1][TFSI]2
without any additives. Four peaks were
observed at ±0.860 and ±0.474 V. The
midpoint potential between peak 1 and 2
was 0.667 V and that between peak 3 and
4
was
−0.667
V,
corresponding
approximately to the potential difference
between the oxidation of the Fc unit (0 V)
and the first reduction potential of the V++
unit (−0.752 V), as illustrated in Fig. 1b. The
peak separation for both couple was 0.386
V at 20 mV･
s-1, which is much greater than
the theoretical separation for reversible
one-electron redox electrochemistry,
presumably because of the resistance
between the two ITO electrodes
originating from the high viscosity of
[FcC11VC1][TFSI]2.
A difference absorption spectrum of
the EC cell containing only neat
[FcC11VC1][TFSI]2 electrolyzed at 0.7 V is
depicted in Fig. 2a, and that obtained
following electrolysis at 1.0 V is displayed
in Fig 2d. The voltage of 0.7 V is under the
midpoint potential difference (|Em,B - Em,A
|
in Fig. 1b), whereas the voltage of 1.0 V is
over the difference (see Fig. 1d). Note that
supercooled liquid states have been found for many ILs11
including previously reported viologen type RAIL
([C4VC7][TFSI]2) below its melting point (52 °C).7j. Because the
viscous liquid state of [FcC11VC1][TFSI]2 remained for several
months even in a refrigerator at 4 °C. We can handle
[FcC11VC1][TFSI]2 as an IL at room temperature lower than the
melting point.
the reference absorption spectrum, which was subtracted
from the absorption spectra at constant potentials after the
potential steps from 0 V, was obtained at 0 V. In both figures,
two bands were observed at 550 and 880 nm, and the
absorbance difference (ΔAbs) of the bands increased with the
electrolysis time. These two bands are assigned to the V+•
dimer.2 The peak maxima of the bands were at longer
wavelength compared with those of the V+• dimers reported
previously.3a,7h,10,12 This difference would be caused by the
different concentrations of viologen and dielectric
surroundings in the EC cells. A shoulder was also observed at
600 nm, corresponding to the V+• monomer.2 Fig. 2b shows the
time course of ΔAbs at 550, 600, and 880 nm for neat
[FcC11VC1][TFSI]2 electrolyzed at 0.7 V. In Fig. 2b, the increases
in ΔAbs550 and ΔAbs600 were almost the same until over 10 s,
whereas a longer electrolysis time resulted in a faster increase
of ΔAbs550 than that of ΔAbs600, suggesting the acceleration of
the dimerization of the V+• species. This acceleration is
attributable to the high concentration of [FcC11VC1][TFSI]2 (1–2
Cyclic voltammograms (CVs) of 1.0 mM [FcC11VC1][TFSI]2
in acetonitrile containing 0.1 M potassium TFSI as the
supporting electrolyte were measured with a three-electrode
configuraꢀon (see ESI† for detail condiꢀon). Fig. 1b shows the
CV obtained at a sweep rate of 0.10 V･
s−1. Three consecutive
one-electron transfer reactions of [FcC11VC1][TFSI]2 were
observed as three sets of quasi-reversible redox waves with
midpoint potentials at Em,A = 0 V (standard of the potential),
Em,B = –0.752 V, and Em,C = –1.200 V, corresponding to Fc+∙/0
(peak A in Fig. 1a), V++/+∙ (peak B), and V+∙/0 (peak C) in this
order. The peak separations of all the three redox couples
were approximately 0.090 V at 0.10 V･
s−1. The position of Em,B
was approximately 0.080 V more positive than the formal
potential of a reported Fc-V-linked compound;4 the difference
is attributed to the different counter anions of the two
materials.
M for a viologen IL7j,k) and its high viscosity (several Pa
･s for a
viologen IL7j around room temperature). The time courses of
ΔAbs and electric charge (Q) for the electrolysis of neat
[FcC11VC1][TFSI]2 at 0.7 V both took 100 s to reach diffusion-
controlled linear regions with respect to t1/2 (Fig. 3b and c,
respectively), suggesting that a time period of 100 s is needed
to reach local redox equilibria at both electrode/solution
The CV measurement of neat [FcC11VC1][TFSI]2 melt was
carried out using
a symmetrical two-electrode EC cell
consisting of two indium tin oxide (ITO) glass substrates with
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interfaces, which is represented by [V++]c[Fc]a/[V+•]c[Fc+•]a
=
V
+• dimer species. Subsequent holding of the potential at 0.0 V
constant, where the subscript “c” and “a” indicates the resulted in a rapid decrease of the colour contrast. The gradual
concentration at the cathode and anode surface. After the decrease in the recovery of the contrast was attributed to the
potential has been applied for 100 s, infinite planar diffusion residual coloured species (V+•) away from the ITO electrode
processes of the redox species control the redox reaction at surface. Colour contrast between 0 V and 1 V decreased with
both electrodes. In contrast, both ΔAbs and Q of neat potential step cycles. The complete decolouration of this EC
[FcC11VC1][TFSI]2 electrolyzed at 1.0 V reached the region cell can be observed by a long enough hold time at 0 V. This
linear to t1/2 in 1 s (Fig. 2e and 2f, respectively), indicating that issue also should be overcome by decreasing the thickness of
totally diffusion-controlled situations were established. In the Fc-V liquid phase to achieve more rapid decolouration. The
other words, the maximum rate of EC coloration controlled by time course of the initial potential cycles is shown in Fig. S2
diffusion is attained in a very short time when a voltage ca. 0.3 (ESI†). However, the linear relationship between Q and ΔAbs
V greater than the midpoint voltage (0.7 V) is applied.
Here we should mention two intrinsic drawbacks of the
Fc-V-linked IL EC device. First, the EC device is weakly coloured
even before the potential application, because the ferrocene
unit is only a weak brown (λ ≈ 440 nm, ε ≈ 1×102 M−1cm−1)12.
Second, theoretically only a quarter of the total Fc-V-linked
molecules are coloured, and their colour fades through the
charge recombination between V+• and Fc+• unless a potential
is continuously applied. In other words, continuous electrolysis
is needed to keep the coloured state of this device.
Nevertheless, the former drawback can be overcome by
decreasing the thickness of the Fc-V liquid phase. Meanwhile,
the latter drawback may be avoided by restricting the
electrolysis time to shorter than several minutes to prevent
the mutual contact of the edges of the diffusion layers of V+•
and Fc+•. In fact, the charge recombination reaction, which
should result in the saturation of absorbance, was not
observed experimentally in the first 100 s (see Fig. 2b and 2e).
Fig. 3 shows the response of the EC cell to a repeated
potential-step electrolysis between 0 and 1.0 V every 30 s.
Before electrolysis, the colour of the EC cell was light brown,
originating from [FcC11VC1][TFSI]2 (Fig. 3a). The colour contrast
of the device responded clearly to the potential cycles. At 1.0
V, the device turned dark purple, mainly originating from the
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Conclusions
In conclusion, we proposed an electrochromic RAIL, ferrocene-
viologen linked ionic liquid ([FcC11VC1][TFSI]2), for a simple EC
cell without any other additives: solvent, supporting
electrolyte, and sacrificial agent. We synthesized
[FcC11VC1][TFSI]2 and demonstrated that it functions as a
single-component liquid EC material in a compartment-free
two-electrode EC cell. As the EC device, we built a prototype
symmetrical, optically-transparent cell. Remarkable properties
of [FcC11VC1][TFSI]2 as an EC material include: (i) its non-
volatile properties allowed construction of a solvent- and
electrolyte-free electrochemical system; (ii) the EC cell
containing [FcC11VC1][TFSI]2 was durable for at least 10000
potential cycles without any degradation. These results
suggest that V-Fc-linked RAILs are competent candidate EC
materials. At the present stage, however, the EC cell did not
show the good contrast after several dozen potential step
cycles because of the slow alternate response of the EC cell.
Recently, viologens have exerted the potential of energy
storage because of their high energy density in aqueous
solution systems.16 The energy density should depend on the
concentration of viologens. Viologen-based RAILs contains a
high concentration reaching ca. 1.5 M of viologen. Therefore,
Fc-V-linked RAILs can be also used for energy storage in
electrolyte- and solvent-free systems. To achieve a more rapid
response and clear contrast, the viscosity of Fc-V-linked RAILs
needs to be lowered and the EC cell system should be
optimized. These investigations are currently in progress in our
laboratory.
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‡This work was supported by a Japan Society for the Promotion
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