Solar Energy Materials & Solar Cells 99 (2012) 88–94

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Effect of counter electrode reaction on coloration properties ofphthalate-based electrochromic cell

Yuichi Watanabe, Kinji Imaizumi, Kazuki Nakamura, Norihisa Kobayashi n

Department of Image and Materials Science, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

a r t i c l e i n f o

Article history:

Received 30 November 2010

Received in revised form

21 April 2011

Accepted 17 June 2011Available online 12 July 2011

Keywords:

Electrochromism

Phthalate derivative

Counter electrode reaction

NiO

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.06.020

esponding author. Tel.: þ81 43 290 3458; fax

ail address: koban@faculty.chiba-u.jp (N. Kob

a b s t r a c t

We investigated the effect of counter electrode reaction on the coloration properties of phthalate-based

electrochromic (EC) cells using various counter materials. Although the coloration characteristics of

dimethyl terephthalate (DMT) cells were improved using a counter material of ferrocene, an organic-

based counter material could not endure the overload because of the high reduction potential of DMT.

On the other hand, a DMT cell using an inorganic counter material showed good long-term switching

stability. As a result, we achieved good switching stability over 5000 cycles for the phthalate-based EC

cell with NiO as the counter material.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Recently, electrochromic display (ECD) has attracted muchattention as a strong candidate for application in novel paper-likedisplay devices. Flexibility is an essential requirement for realizingpaper-like features and texture in the construction of electronicpaper-like display. From this point of view, ECD has the advantage ofbeing a flexible plastic substrate that can be applied easily withconductive indium tin oxide (ITO) [1]. Furthermore, ECD has a highreflective contrast ratio and satisfies high visibility requirements.

Recently, many researchers have focused extensively on violo-gen [2–6] or conductive polymers such as poly(3,4-ethylenediox-ythiophene) (PEDOT) [7–10], as typical organic EC materials.However, their electrochemically induced colors were still notsatisfactory for display applications. EC materials indicating colorchanges from colorless clear to the three primary colors (such asred, green, and blue, or cyan, magenta, and yellow) are requiredfor full-color electronic paper-like imaging devices. On this basis,we studied the electrochemical properties of phthalate deriva-tives [11]. In the literature, the three primary colors (cyan,magenta, and yellow) were electrochemically obtained withdiacetyl benzene, dimethyl terephthalate, and biphenyl dicar-boxylic acid diethyl ester, respectively, in the ITO sandwich cell.Each color obtained is regarded as one of the three primary colorsby the CIE 1931 colorimetric measurement. It was revealed thatthe anion radical of dimethyl terephthalate generated at the

ll rights reserved.

: þ81 43 290 3490.

ayashi).

cathode showed magenta color and that the coloration wasaffected by the supporting electrolyte and solvent. In addition,we have demonstrated red, green, and blue colors by stackingtwo or three primary color EC cells [12]. Flexible EC cells havebeen demonstrated with gel electrolyte containing N-methyl-2-pyrrolidinone (NMP) as the solvent [13]. The coloring andbleaching properties of flexible EC cells were comparable toliquid electrolyte cells. However, the memory properties didnot show much improvement, even in the gel electrolyte cell.After further discussion, it was revealed that the coloration andmemory properties of phthalate derivative-based EC cells wereobviously improved by employing dimethyl sulfoxide (DMSO) asthe solvent [14].

For application in display devices, it is necessary to achievelong-term switching stability. Until now, we have studied thecoloration properties of phthalate derivatives. However, the long-term switching stability and the effect of counter material inphthalate derivative-based EC cells have not been studied indetail. In this paper, in order to discuss the effect of countermaterial in a phthalate derivative-based EC cell, the colorationcharacteristics and long-term switching stability of EC cells usingvarious counter materials were investigated.

2. Experimental

2.1. Materials

DMT (Tokyo Chemical Industry Co., Ltd.) as the EC material andferrocene (Fc; Tokyo Chemical Industry Co., Ltd.) as a counter

Table 1Configurations of the all EC cells.

Liquid electrolyte-based

DMT–Fc cell

Gel electrolyte-based

DMT–Fc cell

DMT–carbon cell DMT–NiO cell

Working electrode ITO electrode ITO electrode ITO electrode ITO electrode

EC solution

Electrochemically active material DMT Fc DMT Fc DMT DMT

Electrolyte TBAP DMSO TBAP DMSO TBAP DMSO THAP NMP

Gelator/additives PVB PVB TiO2 PVB

Inter-electrode distance (mm) 70 300 500 300

Counter electrode ITO electrode ITO electrode Carbon-modified ITO

electrode

NiO-modified ITO

electrode

Y. Watanabe et al. / Solar Energy Materials & Solar Cells 99 (2012) 88–94 89

material were used as received. We used LiOH (Kanto ChemicalCo., Inc.), tetra-n-butylammonium perchlorate (TBAP; Kanto Che-mical Co., Inc.) and tetra-n-hexylammonium perchlorate (THAP;Wako Pure Chemical Industries, Ltd.) as a supporting electrolytewithout further purification. DMSO (Sigma-Aldrich Co.) and NMP(Kanto Chemical Co., Inc.) were used as solvents after removingthe water by molecular sieves. The host polymer for the gelelectrolyte was poly(vinyl butyral) (PVB; BX-1) from SekisuiChemical Co. Ltd. Carbon paste (Asahi Chemical Research Labora-tory Co., Ltd.) and Ni(NO3)2 (Kanto Chemical Co., Inc.) were usedfor the fabrication of the counter material modified electrode. Weused ITO particles (Mitsubishi Materials Electronic Chemicals Co.,Ltd.) to improve the conductivity of the carbon electrode. TiO2

(Tayca Corporation) was used as white scattering material.

2.2. Fabrication of DMT–Fc cell

The EC solution with a counter material of Fc was prepared bydissolving 50 mM of DMT, 100 mM of TBAP and 25 mM of Fc inDMSO. One gram of the EC solution was mixed with the appro-priate amount of PVB, and the resulting mixture was allowed tostand for a week to obtain the PVB-based gel electrolyte. The ECsolution was placed between a pair of ITO electrodes, keeping theinter-electrode distance of 70 mm with a spacer to fabricate theliquid electrolyte-based DMT–Fc cell. The gel electrolyte-basedDMT–Fc cell was fabricated by sandwiching the gel electrolytewith ITO electrodes, keeping the inter-electrode distance of300 mm using a Teflon spacer.

2.3. Fabrication of DMT–carbon cell

Carbon paste containing 15 wt% of ITO particle was coated onthe ITO electrode using a blade-coating method. The carbonelectrode was obtained by baking the carbon paste-coated ITOelectrode for 1 h at 350 1C. The liquid electrolyte was prepared bydissolving 50 mM of TBAP in DMSO for the 3-electrode cyclicvoltammetry (CV) measurement. A repeat of the gel preparationmethod as in the DMT–Fc cell using a DMSO solution containing50 mM of DMT, 100 mM of TBAP, and 10 wt% of TiO2 was used toprepare the gel electrolyte for the DMT–carbon cell. The gelelectrolyte-based DMT–carbon cell was fabricated by sandwich-ing the gel electrolyte with the carbon electrode and the bare ITOelectrode, keeping the inter-electrode distance of 500 mm using aTeflon spacer.

2.4. Fabrication of DMT–NiO cell

The Ni(OH)2 film was electrodeposited by applying �3 mA tothe ITO electrode in a 200 mM Ni(NO3)2 aqueous solution for400 s using a 3-electrode electrochemical cell. We obtained theNiO electrode by baking the electrodeposited Ni(OH)2 film for30 min at 300 1C. The electrochemical activation of the NiOelectrode was performed in 100 mM LiOH aqueous solution by

potential scanning between þ1.0 and �0.2 V at a scan rate of300 mV/s for 100 cycles. The liquid electrolyte was prepared bydissolving 50 mM of THAP in NMP for the 3-electrode CVmeasurement. A repeat of the gel preparation method as in theDMT–Fc cell using an NMP solution containing 25 mM of DMTand 50 mM of THAP was used to prepare the gel electrolyte forthe DMT–NiO cell. The gel electrolyte-based DMT–NiO cell wasfabricated by sandwiching the gel electrolyte with the NiOelectrode and the bare ITO electrode, keeping the inter-electrodedistance of 300 mm using a Teflon spacer. The configurations of allthe EC cells are listed in Table 1.

2.5. EC measurements

The CV measurements were recorded on an ALS model 440 Apotentiostat/galvanostat equipped with a DOS/V computer. Thescan rate was 50 mV/s. The absorption spectra and reflectancespectra were recorded in situ using an Ocean Optics USB2000diode array detection system during the potential sweep. The3-electrode cell was equipped with a platinum wire as a counterelectrode, and an Ag/AgCl electrode as the reference electrode.Long-term switching stability tests were performed by applying�2.5 V (2 s)/2.5 V (2 s)/0 V (6 s) for the DMT–Fc cell, �2.1 V (2 s)/1.5 V (2 s)/0 V (6 s) for the DMT–carbon cell and �2.0 V (3 s)/1.5 V (3 s)/0 V (4 s) for the DMT–NiO cell, respectively. All the ECmeasurements were conducted in a glove box filled with dryargon gas at the ambient laboratory temperature (20–25 1C).

3. Results and discussions

3.1. The counter effect of Fc in a DMT cell

Fig. 1 shows the absorption change and CV of the DMT cellwith (red line) and without Fc (black line). In the case of the DMTcell without Fc, it was necessary to apply over �3.4 V to reducethe DMT. On the other hand, the electrochemical reductionand coloration of the DMT were observed over �1.9 V in theFc-containing cell. The operating voltage was clearly reduced byadding Fc. This decrease of the operating voltage would be due tothe effect of the counter electrode reaction of Fc. Electrochemicalreaction requires the existence of both reductive material andoxidative material. When electrochemical reduction of the reduc-tive material occurs on one electrode, electrochemical oxidationof the oxidative material should occur on the other electrode atthe same time and same charge amount. In the DMT cell withoutFc, an oxidative reaction of the supporting electrolyte, solvent,and/or side reaction would occur at the counter electrode againstthe reductive reaction of the DMT. This unfavorable oxidation wasconsidered to be the cause of the degradation of the DMT cell. Inthe DMT cell with Fc, the oxidative reaction of Fc occurred againstthe reductive reaction of the DMT. Moreover, the oxidation poten-tial of Fc is 0.45 V, which is lower than that of the supporting

-2.5

-0.4

-0.2

0.0

0.20.0

0.5

1.0

1.5

2.0

Cur

rent

/ m

A

Voltage / V

before

after 1000 cycles

530 nm

Abs

orba

nce

-2.0 -1.5 -1.0 -0.5 0.0

Fig. 2. Change in absorbance (top) and CV response (bottom) of gel electrolyte-

based DMT–Fc cell before (black line) and after (red line) switching stability test.

(For interpretation of the references to color in this figure legend, the reader is

referred to the web version of this article.)

00.0

0.1

0.2

0.3

0.4

Cycle number

DMT-Fc cell (530 nm)

Abs

orba

nce

250 500 750 1000

Fig. 3. Change in absorbance of gel electrolyte-based DMT–Fc cell during switch-

ing stability test. The voltage was applied by a potential-step technique of �2.5 V

(2 s)/2.5 V (2 s)/0 V (6 s).

-4-16.0

-12.0

-8.0

-4.0

0.00.0

0.5

1.0

1.5

Cur

rent

/ m

A

Voltage / V

with Fc

without Fc

Abs

orba

nce

530 nm

-3 -2 -1 0

Fig. 1. Change in absorbance (top) and CV response (bottom) of liquid electrolyte-

based DMT cell with (red line) and without Fc (black line). (For interpretation of

the references to color in this figure legend, the reader is referred to the web

version of this article.)

Y. Watanabe et al. / Solar Energy Materials & Solar Cells 99 (2012) 88–9490

electrolyte and solvent in the solution. The operating voltage ofthe EC cell depends on the difference of redox potential betweenthe reduction and oxidation materials. Therefore, the operationvoltage of the DMT–Fc cell was reduced. Low-voltage operation isimportant to EC cells to achieve long-term switching stabilitywithout degradation.

Fig. 2 shows the CV of the gel electrolyte-based DMT–Fc cellbefore (black line) and after (red line) the long-term switchingstability test. The change in the absorbance of the gel electrolyte-based DMT–Fc cell during the switching stability test is shown inFig. 3. Before the switching stability test, the gel electrolyte-basedDMT–Fc cell showed good absorbance change, and colorationoccurred at �1.9 V due to the effect of the counter material of Fc,as explained above. However, as can be seen in Fig. 3, theabsorbance of the DMT–Fc cell gradually decreased during thefirst 500 cycles. In addition, an increase of absorbance at thebleached state of DMT was observed after 500 cycles. After 1000cycles, the color of the DMT–Fc cell at the neutral state becameyellowish-brown compared with the color of the cell before the

switching stability test. An increase of absorbance at the bleachedstate of the DMT cell due to the yellowish brown color wasobserved. The degradation of the cell was also confirmed in Fig. 2.Clearly, the reduction current of the DMT and the absorbancechange were decreased after 1000 cycles. These decreases incoloration characteristics can be attributed to the degradation ofFc. In fact, we confirmed the degradation by 1H NMR spectrum inwhich the five-membered ring of Fc disappeared after the switch-ing stability test while the DMT signal remained. In the DMT cell,the oxidative reaction of Fc induced the reductive reaction of DMTbecause the absolute value of the oxidation potential of Fc waslower than that of the reduction potential of DMT. Therefore, theFc was under heavy load in the DMT–Fc cell. As a result, over-oxidation of Fc occurred during the coloration–bleaching switch-ing cycles. The Fc lost the electrochemical activity and no longershowed its function as a counter reaction material, and the colorchanged to yellowish brown due to overoxidation. From theseresults, we may deduce that an organic-based counter reactionmaterial like Fc could not endure an overload as a counter materialused in DMT cells. From a view point of switching stability,inorganic materials were expected to improve the long-termswitching stability of DMT cells.

3.2. The counter effect of a carbon electrode in the DMT cell

Next, we examined a carbon compound as a counter reactionmaterial to improve the switching stability. A carbon electrode hasmany features such as an electric double layer capacitor, the chargestorage effect by ion adsorption and desorption on the surface,longer operating life, and so on [15–17]. The CV of a carbonelectrode measured in a 3-electrode cell is shown in Fig. 4. Thecarbon electrode showed charge storage capacity. We expected theelectric double layer capacitor property to work as a counterreaction against the reductive reaction of the DMT.

We fabricated a gel electrolyte-based DMT cell using carbon asa counter electrode. TiO2 was added to the gel electrolyte as awhite reflector to detect the color change of the DMT because thecarbon electrode was black in color and nontransparent. To obtainthe pure white color in the cell, the inter-electrode distance wasset as 500 mm. Fig. 5 shows the change in reflectance and CV ofthe DMT–carbon cell before (black line) and after (red line) thelong-term switching stability test. The changes in reflectance andcurrent of the DMT–carbon cell during the long-term switchingstability test are shown in Fig. 6. Before the test, the reflectance

-0.5

-1.0

-0.5

0.0

0.5

1.0

Cur

rent

/ m

A

Potential / V

carbon

0.0 0.5 1.0 1.5

Fig. 4. CV response of carbon electrode in DMSO solution containing 50 mM

of TBAP.

-2.5

-1.2

-0.8

-0.4

0.0

0.40

20

40

60

80

100

Cur

rent

/ m

A

Voltage / V

before after 1000 cycles

530 nmRef

lect

ance

/ %

-2.0 -1.5 -1.0 -0.5 0.0

Fig. 5. Change in reflectance (top) and CV response (bottom) of gel electrolyte-

based DMT–carbon cell before (black line) and after (red line) switching stability

test. (For interpretation of the references to color in this figure legend, the reader

is referred to the web version of this article.)

0

-4.0

0.0

4.0

0

20

40

60

80

100

DMT-carbon cell

Cur

rent

/ m

A

Cycle number

530 nm

Ref

lect

ance

/ %

250 500 750 1000

Fig. 6. Change in reflectance (top) and time dependence of current (bottom) of gel

electrolyte-based DMT-carbon cell during switching stability test. The voltage was

applied by potential-step technique of �2.1 V (2 s)/1.5 V (2 s)/0 V (6 s).

Y. Watanabe et al. / Solar Energy Materials & Solar Cells 99 (2012) 88–94 91

change of the DMT–carbon cell occurred over �2.0 V, which islower than the reduction voltage of DMT without the countermaterial. This result clearly indicates that the carbon electrodeworked as a counter material of DMT. After the stability test,although the oxidation peak current of the DMT–carbon cellslightly decreased as shown in Fig. 5, the cell maintained goodelectrochemical activity over 1000 cycles.

As shown in Fig. 6, the current in the electrochemical reactionand the reflectance change between 90% and about 30% werestably obtained over 1000 cycles. This was due to the goodswitching stability of the carbon electrode.

Photographs of the bleached states and colored states of theDMT–carbon cell before and after the switching stability test areshown in Fig. 7. On examining the above results, we note that thefeatures of the bleached states and colored states do not showmuch change before and after the cycling test. Additionally, thephotographs show no degradation of the DMT–carbon cell.

These results clearly indicate that carbon worked well as acounter electrode to achieve over 1000 cycle stability. However,the carbon electrode is black in color whereas a white reflector isrequired to achieve good color change. Furthermore, it is difficultto stack the EC cell to prepare a full color display. Therefore, atransparent counter material is required to develop full colordisplay with a phthalate derivative EC system, as described in thenext stage.

3.3. The counter effect of NiO electrode in DMT cell

In order to attain a transparent state in the EC cell, weexamined an EC cell containing a NiO-modified electrode. Afterthe preparation of the NiO electrode, the deposited NiO film waselectrochemically activated in a 100 mM LiOH aqueous solutionusing a consecutive potential sweep method. Generally, the ECproperties of NiO were evaluated in an alkaline aqueous solution[18–21]. It is known that no significant EC properties are observedin the organic solvent because the redox and EC reaction of NiOfilm occurs by intercalation and deintercalation of OH� ions in acrystal lattice. However, water is not suitable as a solvent tophthalate derivative-based EC cells. Hence, we performed theelectrochemical activation of the NiO film by a consecutivepotential sweep method over 100 cycles in a LiOH aqueoussolution. During the consecutive potential step, OH� ions wereintroduced into the crystal lattice of the NiO film. Fig. 8 shows theabsorption change and CV of the electrochemically activated NiOfilm in the NMP solution. Even after the electrochemical activa-tion, the absorption change of the NiO film was not significant inthe organic solvent. In contrast, the electrochemical activity wasclearly increased after the introduction of OH� ions to the NiOfilm. The improvement of the electrochemical activity would bedue to the OH� ions intercalated in the crystal lattice. The redoxprocess of electrochemically activated NiO film in the organicsolvent can be attributed to the following electrochemicalreaction:

NiO (transparent)þOH�2NiOOH (brownish)þe�

Moreover, the electrochemically activated NiO film showedgood electrochemical stability even in the organic solvent over1000 switching cycles. This result clearly indicated that the OH�

ions intercalated in the NiO film would not diffuse into the liquidelectrolyte. We studied the electrochemical activities of the NiOfilm in various organic solvents. Our results indicated that theNMP solution showed better electrochemical activity in the NiOfilm than other conventional organic solvents. We have alreadyreported that the coloration characteristics of DMT alone inNMP are not better than those in DMSO [14]. As a result of the

Fig. 7. Photographs of gel electrolyte-based DMT–carbon cell (a) before (1st cycle) and (b) after (1000 cycles) switching stability test. (For interpretation of the references

to color in this figure legend, the reader is referred to the web version of this article.)

-1.0-1.0

-0.5

0.0

0.5

1.0

1.50.0

0.1

0.2

0.3

Cur

rent

/ m

A

Potential / V

NiO electrode

Abs

orba

nce

700 nm

-0.5 0.0 0.5 1.0

Fig. 8. Change in absorbance (top) and CV response (bottom) of electrochemically

activated NiO electrode in NMP solution containing 50 mM of THAP.

4000.0

0.5

1.0

1.5

Abs

orba

nce

Wavelength / nm

Bleached

Colored

500 600 700

Fig. 9. Absorption spectra of DMT–NiO cell in a bleached (black line) and colored

state (red line). (For interpretation of the references to color in this figure legend,

the reader is referred to the web version of this article.)

Y. Watanabe et al. / Solar Energy Materials & Solar Cells 99 (2012) 88–9492

optimization of the electrolyte in the NMP solution, we concludedthat THAP was a suitable electrolyte for the NMP solution. Then,we prepared a DMT–NiO cell using an NMP solution containingTHAP as the electrolyte, and studied the coloring and bleachingbehavior.

Fig. 9 shows the absorption spectra of the DMT–NiO cell in thecolored and bleached state. The inset shows photographs of theDMT–NiO cell at a colored and bleached state before the switch-ing stability test. The cell showed a nice spectral change based onDMT, and the color in the bleached state was transparent enoughto the naked eye. The absorbance of the colored state of theDMT–NiO cell increased uniformly throughout the whole range ofthe visible region. This was due to the color change of the NiO filmat the oxidized state. However, the absorbance change of theNiO film is smaller than that of DMT. Therefore, we could ignorethe coloration of the NiO film at the colored state of theDMT–NiO cell.

The absorption change and CV of the DMT–NiO cell are shownin Fig. 10 (black line). The DMT–NiO cell showed quite goodcoloring and bleaching behaviors. Fortunately, the coloration ofthe DMT–NiO cell was found to occur at around �1.5 V as shownin Fig. 10. This coloring voltage was significantly lowered incomparison to the DMT–Fc and DMT–carbon cell of �1.9 V. Theseresults clearly indicated that the NiO-modified electrode wasquite effective as a counter electrode material. According to thelow coloring voltage, the DMT–NiO cell was expected to achievegood long-term switching stability.

We then continued the switching stability measurement of theDMT–NiO cell to evaluate the cell lifetime. Fig. 11 shows theresult of the switching stability measurement in the DMT–NiOcell. Reflecting on the low coloring voltage, a stable switchingproperty was obtained over 1000 cycles in the DMT–NiO cell. Theabsorbance change at 530 nm was approximately 0.3. The absor-bance change and current of the electrochemical reaction were

-3

-0.4

-0.2

0.0

0.20.0

0.4

0.8

1.2

Cur

rent

/ m

A

Voltage / V

before

after 2000

after 5000

Abs

orba

nce

530 nm

-2 -1 0 1 2

Fig. 10. Change in absorbance (top) and CV (bottom) of gel electrolyte-based

DMT–NiO cell before (black line), after 2000 cycle (red line), and after 5000 cycle

switching stability test (blue line). (For interpretation of the references to color in

this figure legend, the reader is referred to the web version of this article.)

0

-4.0

-2.0

0.0

2.0

4.0

0.0

0.2

0.4

0.6

Cur

rent

/ m

A

Cycle number

DMT-NiO cell

530 nm

Abs

orba

nce

200 400 600 800 1000

Fig. 11. Change in absorbance (top) and time dependence of current (bottom) of

gel electrolyte-based DMT–NiO cell during switching stability test. The voltage

was applied by potential-step technique of �2.0 V (3 s)/1.5 V (3 s)/0 V (4 s).

Fig. 12. Photographs of gel electrolyte-based DMT–NiO cell (a) before and (b) after

switching stability test.

Y. Watanabe et al. / Solar Energy Materials & Solar Cells 99 (2012) 88–94 93

quite stable over 1000 cycles. The results of the CV measurementafter additional switching stability measurements are also shownin Fig. 10. No significant change was observed in the CV after 2000cycles and 5000 cycles compared with the initial state. As can beseen in the photographs of the DMT–NiO cell in the bleached andcolored state after 5000 cycles of switching stability test, shownin Fig. 12(b), some solvent was evaporated due to a less effectivesealing method. However, after 5000 cycles of switching in thisDMT–NiO cell, no degradation was apparent to the naked eye.This clearly indicated that the decrease of coloring voltage waseffective in improving the switching stability even in phthalate-based EC cells. These results showed that NiO film was suitable asa counter material for DMT-based EC cells, resulting in theimprovement of the electrochemical stability of the cell.

4. Conclusion

In this paper, we investigated the effect of counter electrodereaction on the coloration properties for phthalate-based EC cells

using various counter materials. Although the coloration char-acteristics of the DMT cell were improved using counter materi-als, the organic-based counter material could not endure theoverload because of the high reduction potential of DMT. On theother hand, DMT cells using carbon or NiO as counter materialshowed good long-term switching stability. NiO film workedespecially well as a counter electrode material in a phthalate-based EC cell. We achieved good switching stability over 5000cycles in a DMT–NiO cell. The phthalate-based EC cell using NiOfilm as a transparent counter reaction material is expected to beapplied successfully in stacked full-color EC display systems.

Acknowledgments

This work was partly supported by the Support Program forImproving Graduate School Education ‘‘Nano Imaging ExpertProgram’’ from Chiba University and JSPS, Japan, and by theFutaba Electronics Memorial Foundation.

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