Bull. Soc. Photogr. Imag. Japan. (2023) Vol. 33 No. 1: 7–9Letter

Phthalate-based Electrochromic Device Using a Cellulose Substrate Enabling aForm of Reflective Representation As DesignedRyuha MIYAKOa, Kaito TODAa, Takayuki UCHIDAb, Toshifumi SATOHb, Shota TSUNEYASUa*

Abstract:

 e fabricated an electrochromic device with a cellulose substrate impregnated with a dimethyl terephthalate-based electrochromic solution.
W
By cutting the cellulose substrate into a shape of an alphabetical letter, we succeeded in displaying an electrochromic letter. The simple fabrication and the biodegradable cellulose substrate potentially contribute to the sustainability-oriented development.

Key words:

Electrochromism, Electrochromic display, Cellulose substrate, Dimethyl terephthalate, Display device

Introduction
Electrochromism is a reversible color change phenomenon triggered by electrochemical redox. Reflective display devices based onan electrochromic (EC) reaction are in commercial use as rearview
mirrors in cars and window glass in airplanes. This is because of their
low driving voltage and continuous variability in transmission andmemory. In recent years, new functionalities of EC devices havebeen demonstrated, such as energy storage,1) tactile sensing,2) andreversible handwriting functions.3) In addition to conventional EC
devices that change color when voltage is applied to them, promising candidates for next-generation devices are in development.4,5)
For routine use, electrolyte leakage must be prevented; this is generally done by mixing a polymer material into the electrolyte to forma gel electrolyte. For example, EC devices have been realized by using poly (methyl methacrylate) (PMMA),6-8) polyvinyl butyral(PVB),9-11) and poly (vinylidene fluoride) (PVDF).12,13) However,these polymers are non-biodegradable, warranting biodegradable
polymers to achieve sustainability.
Cellulose, which is a natural polymer derived from plants, is abiodegradable material but with excellent coating properties andprocessability. It is used as a substrate for various electronic devices
such as organic transistors,14) light-emitting electrochemical cells,15)
and powder electroluminescent devices.16-18) Dioxythiophene-based
EC devices can be realized by forming poly (3,4-ethylenedioxythiophene) :poly (styrenesulfonate) (PEDOT:PSS) electrodes on acellulose substrate.19) Recently, Su et al. reported the fabrication of ananocomposite film combined with biodegradable cellulose nanocrystals and molecular viologens (as EC materials), leading to theconstruction of a flexible and environmentally friendly EC device.20)
However, the procedure of fabricating cellulose nanocrystals is complicated by the requirement of a hydrolysis with strong acids such as
sulfuric acid.

We report the fabrication of a biodegradable composite EC film
via a simple process. We constructed a new EC device by sandwiching a commercialized cellulose substrate impregnated with the EC
electrolyte solution between a pair of indium tin oxide (ITO) transparent electrodes. As the EC material, a phthalate derivative21) was
employed because it clearly obtains the peak of the redox wave in a
two-electrode device22) and has a good switching stability.23) Finally,we evaluated its photoelectrochemical properties.

Experimental Method
We prepared the electrolyte solution by dissolving 200 mmol/L oftetrabutylammonium perchlorate (T0836, Tokyo Chemical Industry Co., Ltd.) in dimethyl sulfoxide (10378-00, Kanto chemical Co.,
Inc.). We fabricated the two-electrode device by placing a cellulose
substrate (Kimwipe S-200, Nippon paper Crecia Co., Ltd.) with dimensions of approximately 1 cm × 1 cm between a pair of ITO electrodes (1005, Geomatec Co., Ltd.) and adding 0.05 g of the electrolyte solution. We used double- sided tape to maintain aninter-electrode distance of 0.5 mm. We prepared the EC electrolyte
solution by dissolving 10 mmol/L of dimethyl terephthalate (DMT)(T0015, Tokyo Chemical Industry Co., Ltd.) and 20 mmol/L of ferrocene (D0444, Tokyo Chemical Industry Co., Ltd.) in the electrolyte solution and similarly fabricated the EC device. We performed
cyclic voltammetry (CV) measurements on both devices with a potentiostat (ECstat-301WL, EC Frontier Co., Ltd.) at a sweep rateof 100 mV/s. We measured the transmittance spectra of the EC device using a spectrometer (SEC2020, ALS Co., Ltd.) before andafter the application of a direct current voltage. With the EC device
placed between the spectrometer and the light source, we set thetransmittance to 100% when the light source was turned on and 0%
when the light source was turned off. Using the same instruments,
we measured the changes in the current and transmittance at a

Received 18th January, 2023; Accepted 3rd February, 2023; Published 2nd March, 2023.
* Corresponding authoraNational Institute of Technology, Oita College, 1666 Maki, Oita 870-0152, Japanb
Tokyo Polytechnic University, 5-45-1 Minami Iiyama, Atsugi, Kanagawa 243-0297, Japan

8Bull. Soc. Photogr. Imag. Japan. Vol. 33 No. 1 (2023)

Results And Discussion
To reveal the electrochemical behavior of DMT in cellulose substrates, we measured the CV of the two-electrode device at a sweep
rate of 100 mV/s (Fig. 1). In the device without EC materials, weobserved no clear electrochemical response over the applied voltage
range of 0–2.65 V. This suggests that the cellulose substrates are notelectrochemically active. On the other hand, in the device with EC
materials, we observed a current peak at approximately 2.3 V. Thissignal is consistent with the report of Y. Watanabe22) et al., suggesting that EC reaction of DMT occurred even in the cellulose substrates.
Next, to demonstrate the EC reaction of DMT, we measured thetransmission spectra of the EC device before and after voltage application (Fig. 2 (a)). Prior to voltage application, the visible light transmittance of the device was approximately 100%. In contrast, under
the application of +2.3 V, the transmittance of the device decreased
to 10–20% at wavelengths of approximately 500–550 nm, and theappearance of the device changed from translucent to magenta dueto the redox reaction of DMT (Fig. 2 (b)). The magenta coloration
was uneven because of the rough texture of the cellulose substrate.
When we applied −1.0 V to the device, the appearance changed backto translucent. This change in transmittance is consistent with a previous report23) that described the effect of counter electrode reaction
materials in DMT-based EC devices. From these results, we foundthat the EC reaction of DMT can be accomplished in cellulose substrates.
Next, we measured the changes in current and transmittance at awavelength of 500 nm when the applied voltage was switched be-tween +2.65 V for 25 s and −1.0 V for 5 s (Fig. 3). We did this toevaluate the response time and switching stability of the EC device.
When we applied the coloring voltage of +2.65 V to the device, weobtained the current response because of the Faraday reaction. Applying a bleaching voltage of −1.0 V to the device, we observed thecurrent response corresponding to the reversal of polarity. With thiscurrent behavior, the visible light transmittance decreased from approximately 97% to 25% upon application of the coloring voltage.
When applying a bleaching voltage, the transmittance returned tothe initial state of approximately 97%. The coloring and bleachingtimes, defined as the time required for a 90% change in transmittance, were approximately 3.8 and 14 s, respectively. The cycle was(a)100Transmittance / %wavelength of 500 nm when a coloring voltage (+2.65 V for 25 s)
and a bleaching voltage (−1.0 V for 5 s) were alternately and repeatedly applied to the device. In addition, before fabricating the EC
device, we cut cellulose substrates to form the letters O, I, T, and A.
We then fabricated the devices as above, observing the appearanceof the devices before and after the application of coloring andbleaching voltages.

806040200(b)Coloring state
Bleaching state

450500550600650
Wavelength / nm

700(c)

Fig. 2. (a) Change in transmittance spectra of the EC device using thecellulose substrate in the bleaching (black line) and coloringstates (red line). Photographs of the device in the (b) bleachingand (c) coloring states.

1.0

Current / mA

With DMT
Without DMT
0.5

0.0

-0.5
0.0

0.5

1.0

1.5

2.0

2.5

Voltage / V
Fig. 1. CV of the two-electrode device with a sandwiched cellulose substrate containing electrolyte solution with and without DMT.

Fig. 3. Changes in current (top) and transmittance (bottom) of the device at a wavelength of 500 nm using the cellulose substrateduring the application of +2.65 V for 5 s and −1.0 V for 25 s.

Ryuha MIYAKO, Kaito TODA, Takayuki UCHIDA, Toshifumi SATOH, Shota TSUNEYASU Phthalate-based electrochromic device using a cellulose substrate enabling a form of reflective representation as designed

(a)

(b)

Fig. 4. Photographs of the device using the cellulose substrate, which
was cut into letters prior to fabrication of the device, in the (a)bleaching and (b) coloring states.

repeated approximately 30 times. The performance of EC devices isdetermined by various factors such as material concentration, device
structure, and driving voltage conditions, and thus it is difficult tocompare with previous reports. It is considered that the performance
is not significantly decreased by the introduction of cellulose substrate because this response time is the same order of magnitude in
the three-electrode system.24)
Lastly, before fabricating the device, we cut the cellulose substrates into the letters O, I, T, and A, and each letter containing theEC solution was sandwiched between a pair of ITO electrodes, asseen in Fig. 4. Prior to application of the coloring voltage, the letterswere translucent; they changed color to magenta after the coloring
voltage was applied. When we applied a bleaching voltage to thedevice, the original translucent state returned. This indicates that bycutting the substrate prior to device fabrication, it is possible to realize a display device showing intended forms of color change. Thiseliminated the need to etch the electrode, reducing the complexityof device fabrication.

Conclusion
In this letter, we fabricated composite EC films consisting of abiodegradable cellulose substrate and an EC material via a simple
process. The device demonstrated the EC reaction of phthalate derivatives. By cutting the cellulose substrate before device fabrication,
we successfully demonstrated reflective representation in a designed
form without etching the ITO electrode. We expect this new biodegradable display device, which was constructed via a simple process,to be suitable for use in sustainable display devices.

Acknowledgments
This research was partially supported by grants from the Japan
Society for the Promotion of Science ( JSPS) KAKENHI grant(Grant Number 21K14215), the Yutaka Kojima Research Fund of9

the Japan Society of Photography and Imaging Science, theGEAR5.0 project for education and advanced resources at the National Institute of Technology, KOSEN to S.T., and a grant from the
JSPS Core-to- Core Program (Grant Number JPJSCCB20220006)
to S.T. and T.S.

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