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Electrochromic imaging with polymer electrolyte having high-temperature-dependent conductivity
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 538–545 www.elsevier.com/locate/solmat
Electrochromic imaging with polymer electrolyte having high-temperature-dependent conductivity Norihisa Kobayashi, Mami Nishimura Department of information and Image Sciences, and Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan Received 30 November 2004; accepted 13 April 2005 Available online 9 June 2005
Abstract Space-selective electrochromic imaging was successfully performed with a novel polymer electrolyte composed of poly(vinylbutyral) (PVB), poly(ethyleneglycole) (PEG) and tetrabutylammonium perchlorate (TBAP). The spreading of the electrochromic image is a shortcoming when the image is space-selectively formed by electrochemical reaction. This is due to a cell formation between colored and uncolored parts through an ionic conductor. In order to inhibit the spreading, it would be effective to apply a polymer electrolyte with very low ionic conductivity at room temperature to the imaging system. Based on this, we designed the polymer electrolyte to have a large difference in ionic conductivity between high and low temperatures. This polymer electrolyte enables writing and erasing at a high temperature due to high ionic conductivity, and the image is to be preserved without change in ambient temperature due to very low ionic conductivity. In fact, space-selective image was successfully formed on the electrochromic device with the present polymer electrolyte at 100 1C, and was stable without change for more than a week when the device was kept at 20 1C. r 2005 Elsevier B.V. All rights reserved. Keywords: Electrochromic imaging; Polymer electrolyte; Melting temperature; Thermally responsive ionic conductivity
Corresponding author. Tel.: +81 43 290 3458; fax: +81 43 290 3490.
E-mail address: [email protected] (N. Kobayashi). 0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.04.034
ARTICLE IN PRESS N. Kobayashi, M. Nishimura / Solar Energy Materials & Solar Cells 90 (2006) 538–545
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1. Introduction Electrochromism is defined as reversible color change induced by electrochemical reaction, and has advantages such as color variation, large viewing angle and memory effect in a viewpoint of passive display [1]. The device is also applicable to optical shutter and smart window, and has been investigated mainly among inorganic materials. Inorganic materials show better electrochromic characteristics, reliability and durability although the organic electrochromic materials have gradually improved. However, organic materials have the advantage in that color variation due to easier design can be produced over a wide range. Many organic electrochromic materials including dye, pigments, conducting polymers and functional molecules-pendant polymers were studied to analyze electrochemical and spectroscopical properties and to extend color variation [2–4]. Electrochromism with organic material is a successful way to obtain multi-color images on an electrode [5]. Recently, the demand for global environmental conservation and decrease of CO2 evolution require the development of ‘‘new imaging media’’ instead of paper. This is called ‘‘electronic paper’’, and electrochromic display is also a potent candidate to realize this new media. Electrochromic device consists of not only electrochromic materials but also ionic conductor. As can be seen in polymer battery, solid-state system is much more advantageous in electrochromic display. However, the image space-selectively generated on a plane electrode by electrochromism has a disadvantage in that the image is spreading due to the cell formation between the colored and uncolored parts through an ionic conductor. Namely, high ionic conductivity in ionic conductor such as electrolyte solution should be required for quick response in electrochromic reaction, but it leads to spreading of the image resulting in low resolution. This is a contradiction in ionic conduction required for electrochromic display. One of answers to overcome this contradiction is to prepare a polymer electrolyte with high ionic conductivity for only writing and erasing processes. Thermally responsive polymer electrolyte is a candidate for this purpose to inhibit the spreading of electrochromic images at ambient temperature, that is, the polymer electrolyte should require excellent ionic conductivity at higher temperature but very low conductivity at ambient temperature. This polymer electrolyte would enable the electrochromic reaction leading to image formation at higher temperature, but any electrochemical reaction should not occur at ambient temperature. However, the next question is, how can we prepare such a thermally responsive polymer electrolyte? Conductivity is the product of the number of ionic carriers and their mobilities. Control of either or both numbers by thermal stimuli should be required to realize such a polymer electrolyte. This is a similar idea with which we have prepared polymer electrolytes with photocontrollable conductivity [6]. Since salt dissociates to be ions under high dielectric environment and ions migrate through low-viscosity media, one notices that the addition of crystals consisted of molecule with high dipole moment, into suitable polymer–salt composite is effective to induce ionic conduction only above the melting temperature of the additional crystals. In other words, above its melting temperature, additives
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can melt in the polymer matrix, and molten additive is expected to form viscous and high dielectric domain, resulting in high ionic conduction. Below the melting temperature, the additive would crystallize. Salt does not seem to dissociate and it would be difficult for ions if any, to migrate in the polymer matrix. On this basis, polyvinylbutyral (PVB), tetrabutylammonium perchlorate (TBAP) and poly(ethyleneglycole) (PEG) were employed as host polymers, salt and additive for the polymer electrolyte in this paper. PEG is a well-known molecule effective to form ionic conduction pathway above its melting temperature. TBAP also has a character of melting. These thermal properties were examined to confirm whether the resulting polymer electrolyte showed desired properties. Further, space-selective electrochromic imaging was also demonstrated with the polymer electrolyte.
2. Experimental PVB and suitable amount of TBAP and PEG were dissolved in chloroform to prepare the polymer electrolyte. TBAP and PEG were purchased from Kanto Chemical Co. Ltd. and were used as received. The chloroform solution was cast on a Teflon plate and was allowed to stand for 5 h at 56 1C under N2 flow and further dried at 70 1C for 12 h in vacuo. The resulting film was kept in vacuo before use. The thermal properties of the polymer electrolyte and chemicals were studied with MacScience DSC system. The melting point was determined as the top of the endothermic peak. Scanning rate was 3 1C min1. The sample for the ionic conductivity measurement had a disk shape with 1 cm diameter and 200500 mm thickness. The sample was sandwiched between stainless steel electrodes and was kept at a constant temperature. Ionic conductivity measurement (AC 10 or 1000 mV) were carried out with Solartron 1260 impedance gain phase analyzer (LCR meter) over the frequency range 10–106 Hz. The AC ionic conductivity was calculated from a complex impedance plot (Cole–Cole plot) with computer curve fitting. The temperature dependence of conductivity was determined by temperature-controlled AC measurement over the range 20–100 1C. The DC polarization was carried out with Advantest R6144 programmable DC voltage/current generator and R6552 digital multimeter.
3. Results and discussion A feature of the present polymer electrolyte is, thermally responsive conductivity. In order to satisfy this feature, additives should melt and form ion conduction pathway in the polymer electrolyte above the melting temperature. We first analyzed thermal properties of polymer electrolytes containing variable molecular weight of PEG. Composition of PVB/PEG/TBAP in the polymer electrolyte was 20/40/40 (wt%). DSC thermograms of these polymer electrolytes are shown in Fig. 1. Each polymer electrolyte showed endothermic behavior at around 54 1C for PEG400, 18 and 56 1C for PEG600, 30 and 57 1C for PEG1000, and 51 and 59 1C for PEG2000.
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PVB / PEG 400 / TBAP PVB / PEG 600 / TBAP PVB / PEG 1000 / TBAP PVB / PEG 2000 / TBAP
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40
60 Temperature (°C)
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Fig. 1. DSC thermograms of polymer electrolyte with variable molecular weight of PEG.
The endothermic peak at lower temperature was based on the melting of PEG in the polymer electrolyte. The peak temperature increased with molecular weight of PEG. The endothermic peak at higher temperature also increased with PEG molecular weight. However, this higher peak was also found for TBAP alone (62 1C). TBAP was revealed to show quasi-melting behavior at 62 1C, which was lower than the melting temperature (210 1C). Timmermans [7] reported on plastic crystals that some organic and inorganic compounds were soft and plastic when they were observed below their melting points. They found one or more solid-solid transitions at temperatures below the meltingpoint. Timmermans suggested that these solid-solid transitions are a result of the rotation of the whole molecular unit or some substantial fragment of the molecule in the plastic crystalline phase. TBAP has soft alkyl chain in its structure. The endothermic behavior at around 60 1C is attributed to the plastic-crystalline transition possibly due to the fragmental motion of the alkyl unit. The reason why the plastic-crystalline transition temperature of TBAP increased with PEG molecular weight is probably because of a plasticizing effect of low molecular weight of PEG. The melting temperature of PEG2000 did not change so much even when PEG2000 was mixed with TBAP in PVB matrix. This indicates that TBAP does not form a complex with PEG2000. TBAP hardly dissociates into ions in PEG2000, below PEG2000 melting temperature. Therefore, the ionic conductivity at room temperature should be low enough. These results support our strategy to obtain very low ionic conductivity below the melting temperature of the additives. On this basis, the conductivity measurement was carried out for polymer electrolytes with various molecular weight of PEG over the temperature range of 30–100 1C, and the ionic conductivity at each temperature was summarized, as in Fig. 2. The polymer electrolyte containing PVB of 20 wt% provided free-standing film. The polymer electrolyte with lower molecular weight showed higher ionic
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PVB / PEG 400 / TBAP PVB / PEG 600 / TBAP PVB / PEG1000 / TBAP PVB / PEG2000 / TBAP 2.7
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Fig. 2. Temperature dependence of the AC ionic conductivity for PVB/PEG/TBAP polymer electrolytes.
conductivity. However, a considerable conductivity change was not found for the polymer electrolyte with PEG400, 600 and 1000 over this temperature range. On reflection of Fig. 1, the polymer electrolyte with PEG2000 showed considerable conductivity increase above 40 1C. Above 60 1C, we can calculate the ionic conductivity from the complex impedance plots with an applied voltage of 10 mV. However, at 50 1C, we could not obtain the semi-circle complex impedance plots with 10 mV. Applied voltage of 1000 mV was employed to calculate the ionic conductivity from semi-circle complex impedance plots. Further, the complex impedance plane plots were scattered below 40 1C, and we could not carry out curve fitting to estimate the bulk ionic conductivity. Similar complex impedance plots were found in our previous study [8]. These results indicate that the melting behavior at around 40–50 1C affects the ionic conduction in the polymer electrolyte and that the melting of PEG2000 makes the polymer electrolyte ionically conductive. DC ionic conductivity measurement was carried out with DC polarization method in order to analyze the ionic conductivity at around room temperature. The DC ionic conductivity above 50 1C showed good agreement with AC conductivity, and the ionic conductivity at around room temperature was estimated to be lower than 1010 S cm1 The polymer electrolyte with PEG2000 works as designed. Temperature dependence of the ionic conductivity in heating and cooling processes was measured for the polymer electrolyte with the composition of PVB/ PEG2000/TBAP ¼ 20/50/30, and was shown in Fig. 3. The polymer electrolyte with this composition showed the highest conductivity in the polymer electrolyte containing 20 wt% of PVB. The increase in the mobility of ionic carriers above 40 1C is without doubt from the observation of the melting behavior of the polymer electrolyte, that the rubber-like polymer electrolyte turned gel-like polymer above the melting temperature of PEG2000. Therefore, it is believed that the melting behavior contributes to both the increase in the number of ionic carriers and their
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Fig. 3. Temperature dependence of the AC ionic conductivity in heating and cooling processes for PVB/ PEG2000/TBAP polymer electrolytes.
mobilities, resulting in aconsiderable increase in the ionic conductivity. As mentioned above, TBAP did not form a complex with PEG2000 to generate ions in the polymer electrolyte at room temperature. Therefore, some amount of TBAP possibly dissolves into PEG2000 melt above the melting temperature of PEG2000 to generate ions, resulting in the higher ionic conductivity above 60 1C. The ionic conductivity in heating process was different from that in cooling process, indicating that the polymer electrolyte had thermal history. However, the ionic conductivity at 20 1C was low enough. On this basis, we assume that effective electrochemical reaction does not take place at 20 1C. Poly(3,4-ethylenedioxythiophene) (PEDOT) based electrochromic (EC) cell (ITO/ PEDOT/Polymer electrolyte/ITO) was prepared to study electrochromic properties. The EC cell with the present polymer electrolyte was operated at 100 1C, and the memory effect was evaluated at 20 1C. Good reversibility between coloring and bleaching was obtained by applying 2 and +2 V to PEDOT electrode at 100 1C, respectively. Further, no electrochromic reaction was observed for this cell at room temperature. In order to analyze the memory effect at room temperature, we employed partially etched ITO electrode to prepare the EC cell. As schematically shown in Fig. 4, half part of ITO was removed from the glass surface, making that part electrically inactive. Only the electrically active part of PEDOT was reduced at 100 1C to prepare the interface between neutral and oxidized part of PEDOT as shown in Fig. 4 (left). The EC cell was kept at 20 or 100 1C for a day to study the interface. As can be seen in Fig. 4 (right), the interface completely disappeared for the EC cell kept at 100 1C for a day. However, for the cell kept at 20 1C, the interface clearly remained, suggesting that the cell shows good memory effect. These clearly
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Fig. 4. Electrochromic characteristics of the PEDOT based EC cell with PVB/PEG2000/TBAP polymer electrolyte.
Fig. 5. Optical imaging with the EC cell.
indicate that the polymer electrolyte works well as designed and this can be useful for electrochromic imaging. We tried optical imaging by focusing Hg light through a suitable filter on the EC cell as shown in Fig. 5. The focused light was absorbed by the PEDOT layer, leading
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to the increase in temperature of the illuminated part. Therefore, electrochromic reaction at the illuminated part could be induced, by applying bias voltage for oxidation even at room temperature. As can be seen in Fig. 5, the alphabet ‘‘T’’ was formed for the cell. Optical density and contrast are not enough at present since the study is still in the early stages. This image was remaining without any change for more than 1 week when the cell was kept at 20 1C. The image could be erased by means of the reduction of PEDOT at 100 1C as shown in Fig. 5. Further, we could repeat the processes of optical imaging and thermal erasing under application of suitable bias voltage. This indicates that the polymer electrolyte has a possibility to realize rewritable space-selective imaging with electrochromic system.
Acknowledgments This work is partly supported by New Energy and Industrial Technology Development Organization (NEDO)’s ‘‘Nanotechnology Materials Program & Rewritable Paper Using Functional Capsules Project’’ based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI) and administered by the Japan Chemical Innovation Institute (JCII). References [1] C.G. Granqvist, Solid State Ionics 53–56 (1992) 479. [2] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, in: Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. [3] R.J. Mortimer, Chem. Soc. Rev. 26 (1997) 147. [4] N. Kobayashi, K. Teshima, R. Hirohashi, J. Mater. Chem. 8 (1998) 497. [5] H. Urano, S. Sunohara, H. Ohtomo, N. Kobayashi, J. Mater. Chem. 14 (2004) 2366. [6] N. Kobayashi, S. Sato, K. Takazawa, K. Ikeda, R. Hirohashi, Electrochim. Acta 40 (1995) 2309 and references therein. [7] J. Timmermans, J. Phys. Chem. Solid 18 (1961) 1. [8] N. Kobayashi, H. Chinone, A. Miyazaki, Electrochim. Acta 48 (2003) 2323.
Solar Energy Materials & Solar Cells 90 (2006) 538–545 www.elsevier.com/locate/solmat
Electrochromic imaging with polymer electrolyte having high-temperature-dependent conductivity Norihisa Kobayashi, Mami Nishimura Department of information and Image Sciences, and Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan Received 30 November 2004; accepted 13 April 2005 Available online 9 June 2005
Abstract Space-selective electrochromic imaging was successfully performed with a novel polymer electrolyte composed of poly(vinylbutyral) (PVB), poly(ethyleneglycole) (PEG) and tetrabutylammonium perchlorate (TBAP). The spreading of the electrochromic image is a shortcoming when the image is space-selectively formed by electrochemical reaction. This is due to a cell formation between colored and uncolored parts through an ionic conductor. In order to inhibit the spreading, it would be effective to apply a polymer electrolyte with very low ionic conductivity at room temperature to the imaging system. Based on this, we designed the polymer electrolyte to have a large difference in ionic conductivity between high and low temperatures. This polymer electrolyte enables writing and erasing at a high temperature due to high ionic conductivity, and the image is to be preserved without change in ambient temperature due to very low ionic conductivity. In fact, space-selective image was successfully formed on the electrochromic device with the present polymer electrolyte at 100 1C, and was stable without change for more than a week when the device was kept at 20 1C. r 2005 Elsevier B.V. All rights reserved. Keywords: Electrochromic imaging; Polymer electrolyte; Melting temperature; Thermally responsive ionic conductivity
Corresponding author. Tel.: +81 43 290 3458; fax: +81 43 290 3490.
E-mail address: [email protected] (N. Kobayashi). 0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.04.034
ARTICLE IN PRESS N. Kobayashi, M. Nishimura / Solar Energy Materials & Solar Cells 90 (2006) 538–545
539
1. Introduction Electrochromism is defined as reversible color change induced by electrochemical reaction, and has advantages such as color variation, large viewing angle and memory effect in a viewpoint of passive display [1]. The device is also applicable to optical shutter and smart window, and has been investigated mainly among inorganic materials. Inorganic materials show better electrochromic characteristics, reliability and durability although the organic electrochromic materials have gradually improved. However, organic materials have the advantage in that color variation due to easier design can be produced over a wide range. Many organic electrochromic materials including dye, pigments, conducting polymers and functional molecules-pendant polymers were studied to analyze electrochemical and spectroscopical properties and to extend color variation [2–4]. Electrochromism with organic material is a successful way to obtain multi-color images on an electrode [5]. Recently, the demand for global environmental conservation and decrease of CO2 evolution require the development of ‘‘new imaging media’’ instead of paper. This is called ‘‘electronic paper’’, and electrochromic display is also a potent candidate to realize this new media. Electrochromic device consists of not only electrochromic materials but also ionic conductor. As can be seen in polymer battery, solid-state system is much more advantageous in electrochromic display. However, the image space-selectively generated on a plane electrode by electrochromism has a disadvantage in that the image is spreading due to the cell formation between the colored and uncolored parts through an ionic conductor. Namely, high ionic conductivity in ionic conductor such as electrolyte solution should be required for quick response in electrochromic reaction, but it leads to spreading of the image resulting in low resolution. This is a contradiction in ionic conduction required for electrochromic display. One of answers to overcome this contradiction is to prepare a polymer electrolyte with high ionic conductivity for only writing and erasing processes. Thermally responsive polymer electrolyte is a candidate for this purpose to inhibit the spreading of electrochromic images at ambient temperature, that is, the polymer electrolyte should require excellent ionic conductivity at higher temperature but very low conductivity at ambient temperature. This polymer electrolyte would enable the electrochromic reaction leading to image formation at higher temperature, but any electrochemical reaction should not occur at ambient temperature. However, the next question is, how can we prepare such a thermally responsive polymer electrolyte? Conductivity is the product of the number of ionic carriers and their mobilities. Control of either or both numbers by thermal stimuli should be required to realize such a polymer electrolyte. This is a similar idea with which we have prepared polymer electrolytes with photocontrollable conductivity [6]. Since salt dissociates to be ions under high dielectric environment and ions migrate through low-viscosity media, one notices that the addition of crystals consisted of molecule with high dipole moment, into suitable polymer–salt composite is effective to induce ionic conduction only above the melting temperature of the additional crystals. In other words, above its melting temperature, additives
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can melt in the polymer matrix, and molten additive is expected to form viscous and high dielectric domain, resulting in high ionic conduction. Below the melting temperature, the additive would crystallize. Salt does not seem to dissociate and it would be difficult for ions if any, to migrate in the polymer matrix. On this basis, polyvinylbutyral (PVB), tetrabutylammonium perchlorate (TBAP) and poly(ethyleneglycole) (PEG) were employed as host polymers, salt and additive for the polymer electrolyte in this paper. PEG is a well-known molecule effective to form ionic conduction pathway above its melting temperature. TBAP also has a character of melting. These thermal properties were examined to confirm whether the resulting polymer electrolyte showed desired properties. Further, space-selective electrochromic imaging was also demonstrated with the polymer electrolyte.
2. Experimental PVB and suitable amount of TBAP and PEG were dissolved in chloroform to prepare the polymer electrolyte. TBAP and PEG were purchased from Kanto Chemical Co. Ltd. and were used as received. The chloroform solution was cast on a Teflon plate and was allowed to stand for 5 h at 56 1C under N2 flow and further dried at 70 1C for 12 h in vacuo. The resulting film was kept in vacuo before use. The thermal properties of the polymer electrolyte and chemicals were studied with MacScience DSC system. The melting point was determined as the top of the endothermic peak. Scanning rate was 3 1C min1. The sample for the ionic conductivity measurement had a disk shape with 1 cm diameter and 200500 mm thickness. The sample was sandwiched between stainless steel electrodes and was kept at a constant temperature. Ionic conductivity measurement (AC 10 or 1000 mV) were carried out with Solartron 1260 impedance gain phase analyzer (LCR meter) over the frequency range 10–106 Hz. The AC ionic conductivity was calculated from a complex impedance plot (Cole–Cole plot) with computer curve fitting. The temperature dependence of conductivity was determined by temperature-controlled AC measurement over the range 20–100 1C. The DC polarization was carried out with Advantest R6144 programmable DC voltage/current generator and R6552 digital multimeter.
3. Results and discussion A feature of the present polymer electrolyte is, thermally responsive conductivity. In order to satisfy this feature, additives should melt and form ion conduction pathway in the polymer electrolyte above the melting temperature. We first analyzed thermal properties of polymer electrolytes containing variable molecular weight of PEG. Composition of PVB/PEG/TBAP in the polymer electrolyte was 20/40/40 (wt%). DSC thermograms of these polymer electrolytes are shown in Fig. 1. Each polymer electrolyte showed endothermic behavior at around 54 1C for PEG400, 18 and 56 1C for PEG600, 30 and 57 1C for PEG1000, and 51 and 59 1C for PEG2000.
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PVB / PEG 400 / TBAP PVB / PEG 600 / TBAP PVB / PEG 1000 / TBAP PVB / PEG 2000 / TBAP
20
40
60 Temperature (°C)
80
100
Fig. 1. DSC thermograms of polymer electrolyte with variable molecular weight of PEG.
The endothermic peak at lower temperature was based on the melting of PEG in the polymer electrolyte. The peak temperature increased with molecular weight of PEG. The endothermic peak at higher temperature also increased with PEG molecular weight. However, this higher peak was also found for TBAP alone (62 1C). TBAP was revealed to show quasi-melting behavior at 62 1C, which was lower than the melting temperature (210 1C). Timmermans [7] reported on plastic crystals that some organic and inorganic compounds were soft and plastic when they were observed below their melting points. They found one or more solid-solid transitions at temperatures below the meltingpoint. Timmermans suggested that these solid-solid transitions are a result of the rotation of the whole molecular unit or some substantial fragment of the molecule in the plastic crystalline phase. TBAP has soft alkyl chain in its structure. The endothermic behavior at around 60 1C is attributed to the plastic-crystalline transition possibly due to the fragmental motion of the alkyl unit. The reason why the plastic-crystalline transition temperature of TBAP increased with PEG molecular weight is probably because of a plasticizing effect of low molecular weight of PEG. The melting temperature of PEG2000 did not change so much even when PEG2000 was mixed with TBAP in PVB matrix. This indicates that TBAP does not form a complex with PEG2000. TBAP hardly dissociates into ions in PEG2000, below PEG2000 melting temperature. Therefore, the ionic conductivity at room temperature should be low enough. These results support our strategy to obtain very low ionic conductivity below the melting temperature of the additives. On this basis, the conductivity measurement was carried out for polymer electrolytes with various molecular weight of PEG over the temperature range of 30–100 1C, and the ionic conductivity at each temperature was summarized, as in Fig. 2. The polymer electrolyte containing PVB of 20 wt% provided free-standing film. The polymer electrolyte with lower molecular weight showed higher ionic
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Fig. 2. Temperature dependence of the AC ionic conductivity for PVB/PEG/TBAP polymer electrolytes.
conductivity. However, a considerable conductivity change was not found for the polymer electrolyte with PEG400, 600 and 1000 over this temperature range. On reflection of Fig. 1, the polymer electrolyte with PEG2000 showed considerable conductivity increase above 40 1C. Above 60 1C, we can calculate the ionic conductivity from the complex impedance plots with an applied voltage of 10 mV. However, at 50 1C, we could not obtain the semi-circle complex impedance plots with 10 mV. Applied voltage of 1000 mV was employed to calculate the ionic conductivity from semi-circle complex impedance plots. Further, the complex impedance plane plots were scattered below 40 1C, and we could not carry out curve fitting to estimate the bulk ionic conductivity. Similar complex impedance plots were found in our previous study [8]. These results indicate that the melting behavior at around 40–50 1C affects the ionic conduction in the polymer electrolyte and that the melting of PEG2000 makes the polymer electrolyte ionically conductive. DC ionic conductivity measurement was carried out with DC polarization method in order to analyze the ionic conductivity at around room temperature. The DC ionic conductivity above 50 1C showed good agreement with AC conductivity, and the ionic conductivity at around room temperature was estimated to be lower than 1010 S cm1 The polymer electrolyte with PEG2000 works as designed. Temperature dependence of the ionic conductivity in heating and cooling processes was measured for the polymer electrolyte with the composition of PVB/ PEG2000/TBAP ¼ 20/50/30, and was shown in Fig. 3. The polymer electrolyte with this composition showed the highest conductivity in the polymer electrolyte containing 20 wt% of PVB. The increase in the mobility of ionic carriers above 40 1C is without doubt from the observation of the melting behavior of the polymer electrolyte, that the rubber-like polymer electrolyte turned gel-like polymer above the melting temperature of PEG2000. Therefore, it is believed that the melting behavior contributes to both the increase in the number of ionic carriers and their
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Fig. 3. Temperature dependence of the AC ionic conductivity in heating and cooling processes for PVB/ PEG2000/TBAP polymer electrolytes.
mobilities, resulting in aconsiderable increase in the ionic conductivity. As mentioned above, TBAP did not form a complex with PEG2000 to generate ions in the polymer electrolyte at room temperature. Therefore, some amount of TBAP possibly dissolves into PEG2000 melt above the melting temperature of PEG2000 to generate ions, resulting in the higher ionic conductivity above 60 1C. The ionic conductivity in heating process was different from that in cooling process, indicating that the polymer electrolyte had thermal history. However, the ionic conductivity at 20 1C was low enough. On this basis, we assume that effective electrochemical reaction does not take place at 20 1C. Poly(3,4-ethylenedioxythiophene) (PEDOT) based electrochromic (EC) cell (ITO/ PEDOT/Polymer electrolyte/ITO) was prepared to study electrochromic properties. The EC cell with the present polymer electrolyte was operated at 100 1C, and the memory effect was evaluated at 20 1C. Good reversibility between coloring and bleaching was obtained by applying 2 and +2 V to PEDOT electrode at 100 1C, respectively. Further, no electrochromic reaction was observed for this cell at room temperature. In order to analyze the memory effect at room temperature, we employed partially etched ITO electrode to prepare the EC cell. As schematically shown in Fig. 4, half part of ITO was removed from the glass surface, making that part electrically inactive. Only the electrically active part of PEDOT was reduced at 100 1C to prepare the interface between neutral and oxidized part of PEDOT as shown in Fig. 4 (left). The EC cell was kept at 20 or 100 1C for a day to study the interface. As can be seen in Fig. 4 (right), the interface completely disappeared for the EC cell kept at 100 1C for a day. However, for the cell kept at 20 1C, the interface clearly remained, suggesting that the cell shows good memory effect. These clearly
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Fig. 4. Electrochromic characteristics of the PEDOT based EC cell with PVB/PEG2000/TBAP polymer electrolyte.
Fig. 5. Optical imaging with the EC cell.
indicate that the polymer electrolyte works well as designed and this can be useful for electrochromic imaging. We tried optical imaging by focusing Hg light through a suitable filter on the EC cell as shown in Fig. 5. The focused light was absorbed by the PEDOT layer, leading
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to the increase in temperature of the illuminated part. Therefore, electrochromic reaction at the illuminated part could be induced, by applying bias voltage for oxidation even at room temperature. As can be seen in Fig. 5, the alphabet ‘‘T’’ was formed for the cell. Optical density and contrast are not enough at present since the study is still in the early stages. This image was remaining without any change for more than 1 week when the cell was kept at 20 1C. The image could be erased by means of the reduction of PEDOT at 100 1C as shown in Fig. 5. Further, we could repeat the processes of optical imaging and thermal erasing under application of suitable bias voltage. This indicates that the polymer electrolyte has a possibility to realize rewritable space-selective imaging with electrochromic system.
Acknowledgments This work is partly supported by New Energy and Industrial Technology Development Organization (NEDO)’s ‘‘Nanotechnology Materials Program & Rewritable Paper Using Functional Capsules Project’’ based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI) and administered by the Japan Chemical Innovation Institute (JCII). References [1] C.G. Granqvist, Solid State Ionics 53–56 (1992) 479. [2] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, in: Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. [3] R.J. Mortimer, Chem. Soc. Rev. 26 (1997) 147. [4] N. Kobayashi, K. Teshima, R. Hirohashi, J. Mater. Chem. 8 (1998) 497. [5] H. Urano, S. Sunohara, H. Ohtomo, N. Kobayashi, J. Mater. Chem. 14 (2004) 2366. [6] N. Kobayashi, S. Sato, K. Takazawa, K. Ikeda, R. Hirohashi, Electrochim. Acta 40 (1995) 2309 and references therein. [7] J. Timmermans, J. Phys. Chem. Solid 18 (1961) 1. [8] N. Kobayashi, H. Chinone, A. Miyazaki, Electrochim. Acta 48 (2003) 2323.