Polymer electrolyte with large temperature-dependent conductivity for novel electrochromic imaging

Electrochimica Acta 50 (2005) 3886–3890

Polymer electrolyte with large temperature-dependent conductivity for novel electrochromic imaging Norihisa Kobayashi ∗ , Mami Nishimura, Hirosada Ohtomo Department of information and Image Sciences, and Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan Received 13 September 2004; received in revised form 24 January 2005; accepted 10 February 2005 Available online 23 May 2005

Abstract Novel polymer electrolyte aimed for space-selective electrochromic imaging was prepared with poly(vinylbutyral) (PVB), poly(ethyleneglycole) (PEG2000) and tetrabutylammonium perchlorate (TBAP). A spreading of the electrochromic image is a shortcoming when the electrochromic image is space-selectively formed by electrochemical reaction. This is due to a cell formation between colored and uncolored parts through 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. On this basis, the present electrolyte was designed to have large difference in ionic conductivity between high and low temperatures. Namely, this polymer electrolyte enables writing and erasing at high temperature due to high ionic conductivity, and the image is expected to be preserved without change at ambient temperature due to very low ionic conductivity. The thermal and conductive properties of the polymer electrolyte were analyzed. Further, space-selective electrochromic image was formed on the device with the present polymer electrolyte at 100 ◦ C, and was revealed to be stable without change for more than a week when the device was kept at 20 ◦ C. © 2005 Elsevier Ltd. All rights reserved. Keywords: Polymer electrolyte; Ionic conductivity; Melting temperature; Thermally responsive conductivity; Electrochromic imaging

1. Introduction Polymer electrolytes have been collecting keen interests because of its potential applications to solid-state battery and other electrochemical devices. Particularly, polymer electrolytes for rechargeable battery are widely studied to satisfy the trend of increasing portable electric devices [1]. Conductive properties of solvent-free polymer electrolytes are not enough at present for this application because of its larger temperature dependence and low ionic conductivity at low temperature (<0 ◦ C). Therefore, gel polymer electrolytes with the ionic conductivity of higher than 10−3 S/cm and smaller temperature dependence are extensively studied for battery application. In any event, higher ionic conductivity should be required to drive a device with lower energy consumption. ∗

Corresponding author. Tel.: +81 43 290 3458; fax: +81 43 290 3490. E-mail address: [email protected] (N. Kobayashi).

0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.059

Electrochromism is defined as reversible color change induced by electrochemical reaction requiring ionic conduction. Therefore, electrochromic devices have advantages such as color variation, large viewing angle and memory effect in a viewpoint of passive display [2]. 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 are gradually improved. However, organic materials have the advantage in that color variation due to easier design can be produced over a wide range. In fact, many organic electrochromic materials including dye, pigments, conducting polymers and functional molecules-pendant polymers have been studied to analyze its electrochemical and spectroscopical properties and to extend color variation [3–5]. Electrochromism with organic material is a successful way to obtain multi-color images on an electrode [6].

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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 potential 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, solidstate 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 cell formation between the colored and uncolored part 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 improve this contradiction is to prepare a polymer electrolyte with high ionic conductivity in 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. Namely, the polymer electrolyte should require excellent ionic conductivity at higher temperature but very low conductivity at ambient temperature. This polymer electrolyte enables the electrochromic reaction leading to image formation at higher temperature, but any electrochemical reaction should not occur at ambient temperature. However, the question is how we can prepare such a thermally responsive polymer electrolyte? Conductivity is the product of the number of ionic carriers and their mobilities. The control of either or both factors by thermal stimuli should be required to realize such a polymer electrolyte. This is a similar idea that we have prepared polymer electrolytes with photocontrollable conductivity [7]. Since salt dissociates to be ions under high dielectric environment and ions migrate through low viscosity media, one can notice 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 can melt in the polymer matrix, and molten additive is expected to form viscous and high dielectric domain resulting in high ionic conduction. Below 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, poly(vinylbutyral) (PVB), tetrabutylammonium perchlorate (TBAP) and PEG2000 were employed as host polymer, salt and additive for the polymer electrolyte in the present paper. PEG2000 is a 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. The ionic conductive properties of the polymer electrolyte were

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studied, and space-selective electrochromic imaging was also demonstrated.

2. Experimental PVB, suitable amount of TBAP and PEG2000 were dissolved into chloroform to prepare the polymer electrolyte. TBAP and PEG2000 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 ◦ C under N2 flow and further dried at 70 ◦ C 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 by Mac-Science DSC system. The melting point was determined as the top of the endothermic peak. Scanning rate was 3 ◦ C min−1 . The sample for the ionic conductivity measurement had a disk shape with 1 cm diameter and 200–500 ␮m thickness. The sample was sandwiched between stainless steel electrodes and was kept at 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 temperaturecontrolled ac measurement over the range 20–100 ◦ C. 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 consisted of polymer, additives and salt 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 TBAP, PEG2000 and the polymer electrolyte. DSC thermograms of these chemicals were shown in Fig. 1. PEG2000 and TBAP showed endothermic behavior at around 53 and 62 ◦ C, respectively. Surprisingly, TBAP showed quasi-melting behavior at 62 ◦ C, which was lower than the melting temperature reported (214 ◦ C). The melting point at 210 ◦ C was actually observed for the present TBAP. Timmermans [8] reported plastic crystals that some organic and inorganic compound was soft and plastic when they were observed below their melting points. They found one or more solid–solid transitions at temperatures below melting. 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

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Fig. 1. DSC thermograms of the polymer electrolyte, TBAP and PEG2000.

62 ◦ C is attributed to the plastic-crystalline transition possibly due to the fragmental motion of the alkyl unit. The DSC thermogram of the polymer electrolyte also showed the endothermic peak at 53 ◦ C. This is due to the melting behavior of PEG2000. The melting temperature of PEG2000 was not changed even when PEG2000 was mixed with TBAP in PVB matrix. This indicates that TBAP does not form a complex with PEG2000. Therefore, TBAP hardly dissociates into ions in PEG2000 below PEG2000 melting temperature, assuming low ionic conductivity at room temperature. These results support our strategy to obtain very low ionic conductivity below the melting temperature of additives. On this basis, the conductivity measurement was carried out for polymer electrolytes with different TBAP/PEG2000 content over the temperature range 20–100 ◦ C, and the ionic conductivity at each temperature was summarized in Fig. 2. The polymer electrolyte containing PVB of 20 wt.% provided

Fig. 2. Temperature dependence of the ac ionic conductivity for PVB/PEG2000/TBAP polymer electrolytes.

free-standing film. Above 60 ◦ C, we can calculate the ionic conductivity from the complex impedance plots with the applied voltage of 10 mV. However, at 50 ◦ C, we could not obtain semi-circle complex impedance plots with 10 mV. Applied voltage of 1000 mV was employed to obtain the ionic conductivity from semi-circle impedance plots. Further, the complex impedance plane plots were scattered below 40 ◦ C, and we could not carry out curve fitting to estimate the bulk ionic conductivity. Similar complex impedance plots were found in our previous study [9]. These results indicate that the melting behavior at around 40–50 ◦ C affects the ionic conduction of the polymer electrolyte and that the melting of PEG2000 makes the polymer electrolyte ionically conductive. In order to analyze the ionic conductivity at around room temperature, dc conductivity measurement was carried out with dc polarization method. The dc transient current at each temperature was shown in Fig. 3. The initial conducting current as high as 10−4 A was observed for the dc transient current above 60 ◦ C. However, it decreased to 10−7 A at 50 ◦ C and 10−9 A at 30 ◦ C. Further, the initial slope of the dc transient current was gentle at room temperature in comparison with that at higher temperature. These clearly indicate that the ionic conduction is restricted at around room temperature. The dc ionic conductivity at each temperature was summarized in Fig. 4 in addition to the ac ionic conductivity. The dc ionic conductivity showed good agreement with the ac ionic conductivity, indicating that the ionic conductivity at around room temperature should be lower than 10−10 S/cm. This suggests that the polymer electrolyte works as designed. The increase in the mobility of ionic carriers is without doubt from the observation of the melting behavior that the rubber-like polymer electrolyte turned to gellike one 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 mobilities, resulting in the increase of 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 ◦ C. From these results, we assume that the effective electrochemical reaction does not occur at room temperature. 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 ◦ C, and the memory effect was evaluated at room temperature. Good reversibility between coloring and bleaching was obtained by applying +2 and −2 V to PEDOT electrode at 100 ◦ C. Further, no electrochromic reaction was observed for this cell at room temperature. In order to analyze the memory effect in detail at room temperature, we employed partially etched ITO electrode to prepare the EC cell. As

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Fig. 3. The dc transient current (Vdc = 1000 mV) for PVB/PEG2000/TBAP polymer electrolytes at each temperature.

schematically shown in Fig. 5, half part of ITO was removed from the glass surface, resulting in that the part is electrically inactive. Only the electrically active part was reduced at 100 ◦ C to prepare the boundary between neutral and oxidized parts of PEDOT as shown in Fig. 5(a). The EC cell was kept at 20 or 100 ◦ C for a day to study the boundary. As can be seen in Fig. 5(b), the boundary completely disappeared for the EC cell kept at 100 ◦ C for a day. However, for the cell kept at 20 ◦ C (Fig. 5(c)), the boundary clearly

remained, suggesting that the cell shows good memory effect. These clearly 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. 6. The focused light was absorbed by PEDOT layer, leading to the temperature increase of the illuminated part. Therefore, the electrochromic reaction in the EC cell with the present polymer electrolyte could be induced even at room temperature. As can be seen in Fig. 6, the alphabet “T” was formed for the cell. Optical density and contrast was not enough at present since the study is still in early stage. This image was remaining without any change for more than 1 week when the cell

Fig. 4. Temperature dependence of the dc and ac ionic conductivity for PVB/ PEG2000/TBAP polymer electrolytes.

Fig. 5. Electrochromic characteristics of the PEDOT based EC cell with PVB/PEG2000/TBAP polymer electrolyte.

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Fig. 6. Optical imaging with the present EC cell.

was kept at 20 ◦ C. This indicates that the polymer electrolyte has a possibility to realize space-selective imaging with electrochromic system. Acknowledgment This work is partly supported by New Energy and Industrial Technology Development Organization (NEDO)’s “Nanotechnology Materials Program × Full Color 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).

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