Proceedings of 2nd International Symposium for Color Science and Art 2020-2021

Durability Improvement of Electrochromic Display withNano-Structured ElectrodeKatsumi Yamada, Shota Tsuneyasu, Fukuro Koshiji, Takayuki Uchida,Toshifumi SatohTokyo Polytechnic Univ., 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japanfor Au-ITO) formed by potential scanning polymerization of20 cycles on both electrodes showed a change in thetransmission spectrum as shown in Figure1.

ABSTRACT
Island-shaped Au nano-particles were immobilized onan ITO transparent conductive film by a pulse electrolysis
method capable of controlling the nucleation and thenuclear growth. Using this as an electrode, a polypyrrole
thin film was immobilized by electrochemicalpolymerization. It was confirmed that the electrochromic
durability of the polypyrrole film was greatly improved on
the ITO electrode modified with the Au nano-islands.

Transmittance / %80

1. INTRODUCTION
In the study of electrochromic (EC) displays using
conductive polymers, improving the responsiveness anddurability of the color changes associated with theelectrochemical redox reaction is the most important
subject for their practical application. Nevertheless, there
are not many direct studies for improving the durability ofthe EC changes.
In this study, using the island-shaped Au nano-particles
immobilized ITO electrode, the durability of the change intransmittance of the polypyrrole (PPy) film due to EC wasevaluated and compared to the normal ITO electrode.

Au-ITO:-400 mV
Au-ITO:-800 mV
ITO:-400 mV
ITO:-800 mV

6040
20500

550

600
650
Wavelength / nm

700750Fig. 1 Transmittance spectra of the PPy films (20cycles polym.) on the ITO and the Au-ITO in the redoxstates at the applied potentials of -400 mV and +800mV.

2. EXPERIMENTALS
2.1 Preparation of ITO Electrode Modified with Au
Nano-island (Au-ITO)
Au nano-island was immobilized on ITO electrode byelectrochemical deposition with pulse voltage (11 V) andconstant voltage (1.0 V). This reaction was carried out
using an aqueous solution of Au (I) trisodium disulphite
and potassium sulfite at room temperature.
2.2 Preparation of PPy Film
The PPy films were prepared on the electrode by anelectropolymerization with a cyclic potential application
between -500 mV and +1000 mV (vs. Ag-AgCl, scan rate:
50 mV/s). The polymerization solution was 10 mM pyrrole
and 0.1 M lithium perchlorate (LiClO4) dissolved in water.
2.3 Transmittannce measurement
The EC characteristics of the PPy on the electrodewere evaluated by the transmittance measurement in thetransparent glass cell.

In the case of the normal ITO electrode, thetransmittance decreased to about 80% at the wavelengthof 700 nm after 20 cycles of polymerization, and a 9.0%
electrochromic change in the transmittance was obtained
between both redox states. In the case of the Au-ITOelectrode, plasmon scattering of the Au nano-particles
immobilized on the ITO surface probably included to theabsorption of the ITO electrode.
Therefore, thetransmittance of the Au-ITO electrode decreased to about
50% at a wavelength of 700 nm after 20 cycles ofpolymerization, and a 25.0% electrochromic change intransmittance was obtained between both redox states.
Since the thickness of the PPy film on the ITO electrode
was around 100 nm, and on the Au-ITO electrode, it wasaround 150 nm, it is considered that the amount of ECchange in transmittance (ΔT) was mainly due to thedifference in the film thickness.
Using the same sample as in Figure 1, the timedependence of the transmittance at a wavelength of 700
nm was measured when cyclic scanning was performed
between the applied potentials of -400 mV and +800 mV at
the rate of 50 mV/s. The results of the comparison
between the normal ITO electrode and the Au-ITO

3. RESRESULTS AND DISCUSSION
When constant potentials of -400 mV and +800 mV
were applied to the PPy films on the electrode in an
aqueous solution containing lithium perchlorate, color
changes were observed due to their electrochromism.
The PPy films (film thickness: 100 nm for ITO and 150 nm21

Proceedings of 2nd International Symposium for Color Science and Art 2020-2021stage. During the subsequent stage, however, the two
decays seemed to have almost the same slope on bothelectrodes. It is considered that the decay in thesubsequent stage is due to the reason that an aqueous
solution is used as the electrolyte solution, so that thehydrolysis of PPy occurs due to peroxidation, and theelectrochemical activity is gradually deteriorated. The
rapid drop in the initial stage of the ITO electrodesuggested that the volume of the PPy film changes withthe redox reaction, and the PPy film is peeled off from theelectrode. In fact, in the ITO electrode, it was confirmedthat the PPy film was peeled off when the electrode wasremoved from the electrolytic cell after the experiment.
Regarding the difference in the PPy film thickness, theinitial decay occurred on both electrodes, but the steeper
decay was found from the thicker PPy film. The volumetricchange of the PPy film associated with the redox reaction
must be greater on the electrolyte solution side than on theelectrode substrate side.
Because the freedom ofmovement is limited on the electrode substrate side, themechanical stress of the PPy film is enhanced by thecontinuous redox reaction.
It is considered that thistendency becomes stronger as the film thickness
increases.
An aqueous electrolyte solution was used in the study
so that the decay of the EC change of PPy strongly took
place on both electrodes by hydrolysis due toover-oxidation.
However, the stacking between theelectrode and PPy film was strengthened by using theAu-ITO electrode, which had nano-structures consisting of
Au nano-particles. This can suppress peeling of the PPy
film due to the redox reaction, and the durability of the ECchanges would be finally improved.

Transmittance at 700 nm / %electrode are shown in Figure 2. With the ITO electrode,the change in transmittance, which was initially 9.0% (ΔT1),
decreased to 2.2% (ΔT20) after 20 redox cycles. On theother hand, with the Au-ITO electrode, the change intransmittance, which was initially 25.0%, decreased to13.4% after 20 redox cycles.

8070
Au-ITOITO6050403005101520

Redox cycle numbers

Fig. 2 Time dependences on the transmittance of PPyfilms (20 cycles polym.) on the ITO electrode and theAu-ITO electrode at the wavelength of 700 nm during
the applied potential cycling between -400 mV and+800 mV.
The decay of ΔT was evaluated as the ratio of ΔT20 toΔT1, [Decay of ΔT = ((ΔT20) / (ΔT1)) x 100] to be 25.0% for
the ITO electrode and 54% for the Au-ITO electrode. Thedecay was significantly suppressed by using the Au-ITOelectrodes, thus resulting in an improved durability.
The decay process of the ΔT was discussed for the PPy
film thickness in the latter half. Figure 3 shows the resultsof comparing the relationship between the decay and thenumber of redox cycles at each electrode by the number ofpolymerization cycles of PPy. On the ITO electrode, thefilm thickness was about 100 nm after 20 polymerization
cycles, and about 150 nm at the 40th cycle. On the otherhand, on the Au-ITO electrode, the PPy film thickness wasabout 150 nm after 20 polymerization cycles, and about
200 nm at the 40th cycle. For both electrodes, the PPy
film thickness tended to increase as the polymerization
cycle increased. The PPy film thickness on the Au-ITO
electrode was thicker than that on the ITO even for thesame polymerization cycles. This suggested that the Au
nano-particles can promote the deposition of the PPypolymer. The Au nano-particles obtained on the ITO
electrode were slightly flattened in the plane direction, so it
could not be simply determined, but it was one of thefactors affecting the increase in the film thickness of thePPy obtained on the Au-ITO electrode. As shown in
Figure 3, the decay of ΔT progressed as the number ofredox cycles increased on both electrodes. The large
difference between the two electrodes was that a rapid
drop was found on the ITO electrode during the initial

Decay of ∆T / %100
Au-ITO:40cycles polym.
Au-ITO:20cycles polym.
ITO:40cycles polym.
ITO:20cycles polym.

80

60

40

20051015Redox cycle numbers

20Fig. 3 Electrochromic durabilites of the PPy films on
the ITO electrode and the Au-ITO electrode for 22
applied potential cycles between -400 mV and +800mV at the scan rate of 50mV/s. The decay of ΔT wascalculated from [Decay of ΔT = ((ΔT20) / (ΔT1)) x 100].
4. REFERENCES[1] K. Yamada, E. Kanazawa, S. Tsuneyasu, F. Koshiji,
T.Uchida, T. Satoh, Bull. Soc. Photogr. Imag. Japan,30,19 (2020).

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