All-solid-state electrochromic device based on poly(butyl viologen), Prussian blue, and succinonitrile

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 1755–1760

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All-solid-state electrochromic device based on poly(butyl viologen), Prussian blue, and succinonitrile Tsung-Hsien Kuo a, Chih-Yu Hsu a, Kun-Mu Lee b, Kuo-Chuan Ho a,b, a b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

a r t i c l e in fo

abstract

Article history: Received 15 November 2008 Accepted 2 June 2009 Available online 14 July 2009

A new complementary electrochromic device (ECD) is described; it is based on poly(butyl viologen) (PBV) and Prussian blue (PB) confined to the electrode surfaces. PBV is a cathodically colored organic polymer, while PB is an anodically colored inorganic material. The two electrochromic materials were individually characterized in a 0.5 M KCl aqueous solution. On the basis of their properties, a PBV–PB ECD containing a solid-state electrolyte prepared by adding lithium tetrafluoroborate (LiBF4) as a salt to succinonitrile (SN) was investigated. This all-solid-state ECD system showed good optical contrast with a coloration efficiency of ca. 163 cm2/C at 650 nm and good stability during 4000 cycles. The transmittance of the ECD at 650 nm changed from 73% (bleached) to 8% (darkened), with an applied potential of 1.7 V (1.0 to 0.7 V) across the two electrodes. After 4000 cycles, the transmittance attenuation (DT) of the device was still at 86% of its original value, i.e. the DT value had decreased from 65% to 56%. & 2009 Elsevier B.V. All rights reserved.

Keywords: Electrochromic device Poly(butyl viologen) Plastic crystal Prussian blue Succinonitrile

1. Introduction Upon electron-transfer or redox reactions, certain electroactive materials undergo visible and reversible color changes with significant variations in their optical absorbance spectra. This phenomenon is called electrochromism [1–3]. One of the applications for electrochromic technology is smart windows which are operated by means of optical characteristics under an electrical field which is used to adjust the amount of incident sunlight which is transmitted [4–6]. A desirable electrochromic device (ECD) should be complementary [7], which means that a device composed of one anodic and one cathodic coloring material undergoes a simultaneous coloring or bleaching process. A complementary device offers the benefit of increasing the coloration efficiency to enhance the colored/bleached contrast and also to promote cycling stability by preventing undesirable side reactions between the electrolyte and counter electrode. In this study, poly(butyl viologen) (PBV) and Prussian blue (PB) thin films were, respectively, used as the cathodic and anodic coloring materials. Viologens were first reported to possess electrochromic behavior by Michaelis and Hill in 1933 [8]. They named the  Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040. E-mail address: [email protected] (K.-C. Ho).

0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.06.003

1,10 -disubstituted-4,40 -bipyridinium salt viologen since they found that 1,10 -dimethyl-4,40 -bipyridinium, or methyl viologen, was violet in a reduced state. Viologens exhibit three redox states, namely, di-cation, radical-cation, and di-reduced units, and each state shows electrochromism. Early application of viologen was in aqueous electrolyte display systems [9]. However, leakage of solution type devices was a potential stability problem. Thus, viologens were immobilized on electrode surfaces to form thin films through formation of polymer complexes [10], functional cross-linked polymers [11], or electropolymerization [12]. Higher switching speeds and memory effects are also advantages of thin film-type electrodes [7]. In this work, poly(butyl viologen) was electropolymerized from the viologen monomer [13]. PBV is an organic material with three redox states. The most stable form is the di-cation, PBV2+, which is purple. The radical-cation, PBV+  , and the di-reduced species, PBV0, are colorless and yellowishbrown, respectively. Prussian blue (iron(III) hexacyanoferrate) is an inorganic material well-known for its electrochromic properties [15–17]. PB is also multi-electrochromic and has four redox states. PB itself is in blue, and can be reduced to colorless Everitt’s salt (ES) or oxidized to Berlin green (BG) or an even more-highly oxidized state, Prussian yellow (PY). As a result of its particular spectroelectrochemical characteristics, applications of PB analogs include displays [15], electrocatalysts [18], batteries [19,20], sensors [21,22], and photochargable devices [23]. ECD was first applied by Miyamoto et al. [24], and later PB-based ECDs including

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aqueous [25] and non-aqueous [26,27] electrolytes were both extensively discussed. In this study, an all-solid-state ECD was fabricated using PBV and PB with a solid-state succinonitrile (SN)-based electrolyte (ITO/PBV/LiBF4 in SN/PB/ITO). Succinonitrile is a plastic crystal which is in a solid state at room temperature due to its melting temperature of 58 1C. It has been reported that in the plastic crystalline phase of the highly polar succinonitrile, various salts can be dissolved to give solid electrolytes with high ionic conductivities over a wide temperature range [28]. The use of solid-state electrolytes instead of conventional liquid electrolytes in electrochemical systems improves the safety through reducing vapor pressure and eliminating electrolyte leakage [29]. We investigated the remarkable color contrast of this ECD because of the complementary colors of PBV and PB. 2. Experimental procedures 2.1. Chemicals All chemicals were ACS reagent grade and were used without further purification. Potassium chloride (KCl), potassium ferricyanide (K3Fe(CN)6), iron(III) chloride hexahydrate (FeCl3), potassium dihydrogen phosphate (KH2PO4), lithium tetrafluoroborate (LiBF4), and potassium hydrogen phosphate (K2HPO4) were all purchased from Aldrich. Succinonitrile, potassium ferrocyanide (K4Fe(CN)6), and hydrochloric acid (HCl) were purchased from TCI, Fisher, and Fluka, respectively. All aqueous solutions were made with deionized water (DIW) with a resistivity of about 18.2 MO cm. All experiments were performed at room temperature in air. 2.2. Preparation of PBV and PB films The PBV thin films were potentiostatically electropolymerized from bis(4-cyano-1-pyridino)butane dibromide (BVBr2) [13]. The bath solution was composed of 20 mM BVBr2, 10 mM KH2PO4, 90 mM K2HPO4, and 100 mM K4Fe(CN)6. When preparing the PBV thin films, a constant potential of 0.75 V (vs. Ag/AgCl/sat’d KCl) was applied to the ITO glass (Ritdisplay Corporation, Hsinchu, Taiwan, Rsh ¼ 15 O/&) with an electroactive area of 2.0  2.0 cm2 until a predetermined charge capacity of 100 mC/cm2 was reached. Note that this amount of total charge capacity is needed, since the deposition efficiency of the PBV film is less than 10%. In our previous study [14], we have confirmed that the viologen monomers would form charge transfer complex with ferrocyanide anions in the bath solution. Thus, the viologen monomers would become radical-cation state at 0.75 V. The electropolymerization then proceeds by the attack of those radical cations on the CN group of the BVBr2. The deposition bath of PB contained 10 mM K3Fe(CN)6, 10 mM FeCl3, 0.1 M KCl, and 1 M HCl. The PB thin films were prepared by applying a constant cathodic current density of 20 mA/cm2 for 540 s. Normally, the deposition efficiency of PB reaches 60–80%, depending on the deposition conditions, and is much higher than that of PBV. During the electrochemical polymerization process, the ITO glass served as the working electrode, a platinum sheet (4.0  1.0 cm) was the counter electrode, and a homemade Ag/ AgCl/sat’d KCl solution was used as the reference electrode. All applied potentials on the PBV and PB electrodes reported below are relative to the Ag/AgCl/sat’d KCl electrode.

The heated succinonitrile was sandwiched between the colorless PBV- and PB-modified electrodes separated by a 130-mm epoxy spacer (3M Company). The device had to slowly be cooled to room temperature so that succinonitrile could return to a solid state. Finally the device was sealed using Torr Seals (#9530001, Varian). The active area for each ECD was 4.0 cm2. 2.4. Characterization of films and ECD by electrochemical and spectroscopic analyses An Autolab model PGSTAT30 potentiostat/galvanostat was used for electrodeposition of both the PBV and PB thin films, and also all other electrochemical experiments. In situ optoelectrochemical measurements were carried out with a Shimadzu model UV-1601PC UV–Vis spectrophotometer. The PBV and PB electrodes were characterized by the three-electrode system in the presence of a 0.5 M KCl aqueous solution. The equilibrium absorbance spectra of the ECD at different applied potentials (PBV vs. PB) were obtained by in situ measurements using the above potentiostat/galvanostat and spectrophotometer. For the in situ transmittance response, the ECD was bleached at 0.7 V and colored at 1.0 V for each cycle at a wavelength of 650 nm. The voltages presented for the ECD are all relative between its two electrodes, i.e. PBV vs. PB.

3. Results and discussion 3.1. Studies of PBV and PB thin-film electrodes 3.1.1. Electrochemical properties Complete cyclic voltammograms (CVs) for PBV and PB are shown in Fig. 1a and b. In Fig. 1a, there are three coupled redox peaks for PBV. The first redox couple at ca. 0.27 V (AIPBV) and ca. 0.70 V (CIPBV) was correlated with the redox reaction between the colorless di-cationic species, PBV2+, and purple radical-cation, PBV+  , respectively. The second redox couple at ca. 0.64 V (AIIPBV) and ca. 1.17 V (CIIPBV) was assigned to the purple PBV+  and the yellowish-brown di-reduced species, PBV0, respectively. The third peaks were not related to a reversible electrochromic reaction as previously reported [13]. For PB, there were two coupled redox peaks (Fig. 1b), which were the anodic peaks at ca. 0.36 V (AIPB) and ca. 1.03 V (AIIPB), and the cathodic peaks at ca. 0.09 V (CIPB) and ca. 0.82 V (CIIPB). The first and second redox couples were related to the BG/PB and PB/ES reactions, respectively. Because of the multi-electrochromic feature of both the PBVand PB-modified electrodes, it was necessary to choose an appropriate operating range for each electrochromic material from the viewpoint of complementary color-matching considerations. Thus, to avoid unnecessary formation of other redox couples in PBV and PB, the redox couple for PBV was limited at PBV2+/ PBV+  and that for PB at PB/ES. PBV in this system acts as a cathodically colored electrochromic material, i.e., PBV is colorless at 0 V and purple at 0.8 V. On the other hand, PB in this system acts as an anodically colored material. When PB is in the ES state at 0.2 V it is colorless, and in the PB state at 0.6 V, it is blue. Since charge-matching is necessary to obtain a maximum optical contrast for the device [30], we have taken steps to ensure that the charge capacity is balanced on the two electrodes. This can be justified by integrating the current density in the CV curve under an appropriate potential range in Fig. 1.

2.3. Assembly of the PBV–PB ECD Before assembling the device, succinonitrile was heated to 80 1C to become liquid, and then 0.04 M LiBF4 was added as a salt.

3.1.2. Absorbance spectra and transmittance responses The equilibrium absorbance spectra in the visible region for the PBV thin-film electrode obtained with different applied

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potentials from 0 to 0.8 V in a 0.5 M KCl solution are shown in Fig. 2a. The absorbance peak became more obvious when the potential was cathodically stepped from 0 to 0.8 V at an interval of 0.1 V. At 0.8 V, the maximum absorbance peak was characterized at ca. 550 nm which corresponds to the region where violet appears. The wavelength at ca. 550 nm is especially sensitive to the human eye. At that wavelength, the largest optical attenuation for the PBV-modified electrode was also obtained. Moreover, the absorbance spectra showed no difference between 0 and 0.4 V, which means that no electrochromic reaction should have occurred. This phenomenon can be proven from the CV of PBV in Fig. 1a, i.e., a plateau (PLPBV) with a near-zero current exists within the applied potentials on the cathodic pathway. The inset in Fig. 2a shows that when the applied potential was more negative than 0.8 V, the intensity of the absorbance peak around 550 nm began to decrease and had almost vanished at 1.0 V. This indicated that the second redox process had occurred, the yellowish-brown PBV0 was generated, and as a result, the operating potential was limited to the first redox couple for complementary color-matching considerations as mentioned above. Similarly, Fig. 2b shows the equilibrium absorbance spectra in the visible region for PB at different applied potentials from 0.6 to 0.2 V. The absorbance increased upon oxidation (coloring

process) and decreased upon reduction (bleaching process). The maximum characteristic peak was located at ca. 690 nm. The PB spectra also showed no significant difference from 0.6 to 0.4 V, which was consistent with the result obtained by recording the CV of PB in Fig. 1b. On the cathodic pathway, the current was still small until a potential of 0.4 V was reached, which means that the reductive reaction started around 0.4 V and thus the bleaching process of PB began to occur. In addition, BG also formed when the applied potential was more positive than 0.6 V as reported in the literature [31]. The absorbance spectra was found to have shifted, and the color turned to green as shown in Fig. 2b (dashed line), so it was also necessary to control the applied potential on the PB side. From an analysis of the absorbance spectra, we realized that the best optical performance for each thin film was located at a wavelength of maximum absorbance for changing from a bleached to a darkened state. Changes in the wavelengths of the maximum absorbance in this case corresponded to those of the maximum absorbance peaks, which were 550 nm for PBV and 690 nm for PB. Fig. 3 shows the in situ transmittance responses for PBV and PB thin-film electrodes at their characteristic wavelengths. For PBV, it was bleached at 0.0 V and darkened at 0.8 V at 550 nm with switching intervals of 10 s, and the maximum transmittance change was about 70%. For PB, the

4.5

4.5 II

I

3.0 III

APBV

1.5 0.0 PLPBV -1.5

III CPBV

-3.0 II

-4.5 -1.6

II

3.0

APB

1.5 0.0 -1.5 II

CPB

-3.0 I

I CPBV scan rate = 100 mV/s

CPBV

I

APB

PB

APBV APBV Current density (mA/cm2)

Current density (mA/cm2)

PBV

1757

CPB

scan rate = 100 mV/s

-4.5 -0.4 0.0 0.4 0.8 1.2 1.6 Potential (V) vs. Ag/AgCl/Sat'd KCl

-1.2 -0.8 -0.4 0.0 0.4 0.8 Potential (V) vs. Ag/AgCl/Sat'd KCl

Fig. 1. CVs for (a) PBV scanned from 0.5 to 1.3 V and (b) PB from 0.2 to 1.4 V in 0.5 M KCl aqueous solution at a scan rate of 100 mV/s.

1.0

Absorbance

-0.8 V 0.8

Absorbance

PBV

-0.6 V 0.6

-0.5 V

2.8 2.4 2.0 1.6 1.2 -0.8 V 0.8 -0.9 V 0.4 -1.0 V -1.3 V 0.0 400 500 600 700 800 Wavelength (nm)

0.4

PB 0.8 0.6

0.0 V 500

-0.4 V 600 700 Wavelength (nm)

800

0.6 V 0.3 V 0.9 V

0.4 0.2

0.2 0.0 400

1.0

Absorbance

1.2

0.0 400

0.2 V 0.1 V 0.0 V -0.2 V 500 600 700 Wavelength (nm)

800

Fig. 2. Equilibrium absorbance spectra of (a) PBV and (b) PB thin-film electrodes at different potential stepping from 0 to 0.8 V and 0.2 to 0.6 V with 0.1 V decrement, respectively, measured in 0.5 M KCl aqueous solution. The inset in (a) shows the spectra of continuing the applied potential from 0.8 to 1.3 V and the dashed line in (b) is the spectrum measured at 0.9 V.

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100

100 0.0 V

PBV/0.04 M LiBF4 in SN/PB ECD scan rate = 100 mV/s

1.5

PBV 80

PB 60

60

40

40

-0.8 V

20

1.0

20

Current density (mA/cm2)

-0.2 V

Transmmitance at 690 nm (%)

Transmmitance at 550 nm (%)

80

0.5

0.0

-0.5

-1.0 0.6 V 0

0 0

5

10

15

-1.5

20

Time (s)

-1.5 Fig. 3. In-situ transmittance responses measured in 0.5 M KCl aqueous solution. The PBV film (solid line) was bleached at 0 V and darkened at 0.8 V at 550 nm. The PB film (dashed line) was bleached at 0.2 V and darkened at 0.6 V at 690 nm.

 PBV2 þ ðBF4 Þ2 þ e Ð PBVþ d BF 4 þ BF4 and

ðcolorlessÞ

ðpurpleÞ

ðPB; blueÞ

(2)

þ  PBV2þ ðBF4 Þ2 þ ES Ð PBVþ d BF 4 þPB þ Li þBF4 .

ðcolored stateÞ

PBV/0.04 M LiBF4 in SN/PB ECD 1.2

1.0 -1.3 V -0.7 V 0.8 -0.2 V 0.6

0.0 V

0.4

0.2 V 1.0 V

0.2

Thus, the overall reaction of the complementary ECD can be represented by combining Eqs. (1) and (2):

ðbleached stateÞ

1.0

(1)

Li2 FeII ½FeII ðCNÞ6  Ð LiFeIII ½FeII ðCNÞ6  þ Liþ þ e . ðES; colorlessÞ

0.5

1.4

Absorbance

3.2.1. Cyclic voltammograms The reactions of PBV and PB in the solid-state electrolyte, SN, with LiBF4 are illustrated by Eqs. (1) and (2) according to their redox nature:

-0.5 0.0 Potential (V) (PBV vs. PB)

Fig. 4. CV of all-solid-state ECD at a scan rate of 100 mV/s.

maximum transmittance change was also about 70% at 690 nm when being bleached at 0.2 V and darkened at 0.6 V with the same switching intervals as PBV. Moreover, the response times for PBV were 4.7 s when bleaching and 1.3 s when darkening, and for PB they were 2.5 s when bleaching and 0.8 s when darkening. The definition of the response time here is the time required to reach a 95% transmittance change from a bleached to a darkened state and vice versa. Both PBV and PB had the same result of the response of the darkening process being faster than the bleaching process; however, the responses of PBV were all slower than those of PB.

3.2. Performance of PBV–PB ECD

-1.0

(3)

With the above electrochromic reaction in ECD, the CV, performed at a scan rate of 100 mV/s, was obtained and is shown in Fig. 4. A two-stage reaction of the ECD was observed from the CV: the first stage was between 1.0 and 0.2 V, and the second stage was between 0.2 and 0.7 V. PBV contributed the most to the reaction in the first stage, while PB did so for the second stage, which was verified by the absorbance spectra as discussed below. Furthermore, the PBV–PB ECD showed a reversible CV operation from 1.0 to 0.7 V due to the complementary reaction.

0.0 400

500

600 700 Wavelength (nm)

800

900

Fig. 5. Equilibrium absorbance spectra of PBV–PB ECD at different applied potential stepping from 1.0 to 1.3 V with 0.1 V decrement.

3.2.2. Absorbance spectra and coloration efficiency A two-stage reaction of the PBV–PB ECD was also found in the equilibrium absorbance spectra as shown in Fig. 5. The spectra were equilibrated at each applied potential from 1.0 to 1.3 V (PBV vs. PB). Starting from 1.0 V (the bleached state), with each 0.1 V decrement, the intensity of absorption increased due to coloration. When the voltage reached ca. 0.2 V, PB-like absorbance spectra were found as shown in Fig. 5, with a peak appearing at ca. 690 nm. Therefore, PB contributed to this coloring stage which corresponded to the second stage of the CV in Fig. 4. When the applied voltage went increasingly negative from 0.2 to

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1759

100 PBV/0.04 M LiBF4 in SN/PB ECD

1.0

PBV/0.04 M LiBF4 in SN/PB ECD 80

0.7 V

Transmittance at 650 nm (%)

ΔOD at 650 nm

0.8

0.6 ηECD = 183 cm2/C 0.7 V ~ -1.1 V

0.4

60

T 40

b

Td ΔT

20

0.2

-1.0 V 0.0

0 0

1

2 3 4 Charge Density (mC/cm2)

5

6

Fig. 6. Optical density change (DOD) of PBV–PB ECD at 650 nm as a function of reacted charge density equilibrated at different applied potential. The reference state is set at 0.8 V. The solid line is obtained by linear fitting from the data point of 0.7 and 1.1 V.

PBV/0.04 M LiBF4 in SN/PB ECD tb : 9.4 s

Transmittance at 650 nm (%)

1000 Cycle Number (N)

Fig. 8. The transmittance at bleached state (Tb), at darkened state (Td), and the transmittance change (DT) at 650 nm as a function of cycle number shows the dynamic cycling stability of PBV–PB ECD during potential step between 1.0 and 0.7 V with 10 s step time for each.

The relationship between the optical density change (DOD) at 650 nm and the reaction charge density for the PBV–PB ECD is shown in Fig. 6. The reference state was set to 0.8 V and then switched to different potentials from 0.7 to 1.1 V at an interval of 0.1 V, with each potential step lasting 60 s to allow the reaction to reach equilibrium. The reaction charge density was obtained by integrating the it curve. The coloration efficiency was calculated to be ca. 183 cm2/C from the slope of the curve from 0.7 to 1.1 V.

100

td : 2.0 s

80

100

0.7 V

3.2.3. Transmittance response and cycling stability Fig. 7 shows the transmittance response of the ECD for the first two cycles. The switching times for the bleaching and darkening processes were calculated to be 9.4 and 2.0 s, respectively. Therefore, with the high conductivity of SN [28], the all-solidstate ECD showed a fast response. Fig. 8 shows the transmittance and transmittance change of the PBV–PB ECD in response to changing the potential between 0.7 and 1.0 V for bleaching and darkening, respectively. After consecutive operation for 4000 cycles, the optical performance of the device was still stable. The transmittance attenuation was about 65% (from 73% to 8%) of the first cycle, and after 4000 cycles, it had decreased to ca. 56% (from 65% to 9%). This device can be considered to have good cycling stability.

60

40

20 -1.0 V 0 0

100

200 Time (s)

300

Fig. 7. In-situ transmittance response of the PBV–PB ECD at 650 nm switched between 0.7 and 1.0 V.

1.3 V, another peak at ca. 550 nm appeared which PBV likely contributed to. Therefore, this implies that the first-stage reaction in CV corresponds to the PBV reaction. Moreover, in the mostdarkened state, a broad band of absorbance spectra appeared at ca. 650 nm due to contributions of both PBV and PB. Therefore, 650 nm was chosen as the wavelength to characterize the ECD.

4. Conclusions In this work, an all-solid-state complementary ECD composed of PBV, PB, and SN was examined. The electrochromic characteristics of cathodically coloring PBV and anodically coloring PB thin films were first studied. According to their properties, the PBV–PB ECD was then assembled and tested. The measured transmittances of the device at 650 nm changed from 73% (0.7 V) to 8% (1.0 V), and the coloration efficiency was calculated to be ca. 183 cm2/C. The device can reversibly be switched from colorless to purplish-blue. After 4000 cycles, the transmittance attenuation at 650 nm slightly decreased from 65% to 56%.

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