Electrochromic devices employing methacrylate-based polymer electrolytes

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 249–255

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Electrochromic devices employing methacrylate-based polymer electrolytes Jakub Reiter a,, Ondrˇej Krejza b, Marie Sedlarˇı´kova´ b a b

Institute of Inorganic Chemistry of the ASCR, v.v.i., 250 68 Rˇezˇ near Prague, Czech Republic Department of Electrotechnology, Brno University of Technology, 602 00 Brno, Czech Republic

a r t i c l e in fo

abstract

Article history: Received 7 April 2008 Accepted 13 October 2008 Available online 22 November 2008

Poly(ethyl methacrylate) (PEMA)- and poly(2-ethoxyethyl methacrylate) (PEOEMA)-based polymer gel electrolytes with entrapped solutions of lithium perchlorate in propylene carbonate (PC) were prepared by direct, UV-initiated polymerization. The electrolytes were studied using electrochemical methods and they exhibit good ionic conductivity (up to 0.7 mS cm1 at 20 1C) as well as electrochemical stability up to 2.5 V vs. Cd/Cd2+ (5.1 V vs. Li/Li+) on gold electrode. The electrolytes have thermal stability up to 125 1C. The electrolytes were successfully tested as ionic conductors in the electrochromic device FTO/ WO3/Li+-electrolyte/V2O5/FTO using coupled optoelectrochemical methods to discuss the relationship between the electrolyte composition and parameters such as change of transmittance, response time and stability. The transmittance change Dt was found to be 30–45% at 634 nm. & 2008 Elsevier B.V. All rights reserved.

Keywords: Polymer electrolyte Methacrylate Electrochromic device Lithium Propylene carbonate Transmittance change

1. Introduction Gel polymer electrolytes (GPEs) have attracted attention since M. Armand’s introduction of the electrolytes based on poly(ethylene oxide) [1,2] for their suitable electrochemical and mechanical properties, including high electrochemical and dimensional stability, good ionic conductivity, high design flexibility and long-term durability. Various systems have been investigated during the last three decades and applied in highenergy lithium-ion batteries [3–5], double-layer supercapacitors [6], fuel cells [7] and chemical sensors [8]. For the electrolyte application in electrochromic devices (ECDs), the properties mentioned above have to include also the excellent optical transparency, high photostability of the electrolyte and low volume contraction during polymerization as was proposed by Heckner and Kraft [9]. Aprotic systems based on methacrylates and propylene carbonate (PC) meet these requirements. Moreover, they are environmentally friendly and exhibit only a low toxicity and poly(methyl methacrylate) (PMMA) is used in dental praxis. Since 1985, when Iijima et al. [10] and later Bohnke et al. [11] introduced PMMA–PC and PMMA–g-butyrolactone polymer gel electrolytes as representatives of a new generation of polymer electrolytes, various modifications have been reported combining different methacrylates and aprotic solvents. Due to its good electrochemical stability and high ionic conductivity, the PMMA–PC system, with suitable inorganic lithium salts such as

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E-mail address: [email protected] (J. Reiter). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.10.010

perchlorate, tetrafluoroborate or hexafluorophosphate, was considered as a desirable material for the ECDs from the early investigation by Bohnke and other authors [11–17]. Other authors used for the ECD assembling either electrolytes based on pure PMMA [13,15], poly(2-hydroxyethyl methacrylate) [18] or methacrylate copolymers with poly(ethylene glycol), poly(propylene glycol), poly(ethylene oxide) [12,14,16,19,20] or other polymers like polyepoxides [21,22]. Besides the ECDs working on an intercalation principle, hybrid ECDs based on redox systems such as iodide–iodine [23,24], Prussian blue–Prussian white [9] or viologene [25] were also reported. Ionic liquids were also employed in ECDs by Lu et al. [26] and Marcilla et al. [27]. Methacrylate-based polymer electrolytes were also investigated in our laboratory, where direct methods of preparation were developed to obtain membranes of an exactly defined and uniform composition [28], also with room temperature ionic liquids for an improvement of the electrochemical parameters, mainly ionic conductivity and electrochemical stability [29]. Either thermal or UV-initiated direct polymerization was used, employing different initiators [28,29]. Recently prepared electrolytes based on poly(ethyl methacrylate) (PEMA) and poly(2-ethoxyethyl methacrylate) (PEOEMA) are combined with PC and lithium perchlorate. Our first work showed reasonable ionic conductivity (up to 0.7 mS cm1 at room temperature), but quite low electrochemical stability from 2.5 to 1.5 V vs. Cd/Cd2+ on glassy carbon, when the electrolytes were prepared and studied in air [28,30]. Our work also showed better mechanical properties of PEMA and PEOEMA polymer electrolytes as compared to PMMA-based membranes. This paper presents the results of their further electrochemical investigation focused on the influence of composition on electrochemical and mechanical

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properties and their application in an ECD having the following configuration: glass jFTOjWO3 jGPEjV2 O5 jFTOjglass, where FTO is fluoride tin oxide. Contrary to the solvent casting method of preparation, this method makes it possible to use a cross-linking agent and precise preparation of the initial mixture along with elimination of the time-consuming dissolving of the polymer and evaporation of the co-solvent. Also, contamination with water or oxygen traces was eliminated by modifying the preparation procedure and ECD assembling. Besides the electrochemical properties of prepared electrolytes and utilized method of preparation, low cost and good availability of used chemicals were also considered when designing the electrolyte.

2. Experimental 2.1. Materials PC (Sigma-Aldrich, 499.7%, water content o0.005%) was stored under molecular sieves (3A pellets, Sigma-Aldrich). Lithium perchlorate (Merck) was dehydrated in vacuum at 110 1C for 48 h and then stored in argon-filled glove box (MBraun, USA). Monomers, ethyl (EMA) and 2-ethoxyethyl (EOEMA) methacrylate, were obtained from Sigma-Aldrich and twice distilled under reduced pressure. Cross-linking agents, ethylene dimethacrylate (EDMA) and 1,6-hexanediol dimethacrylate (HexadiMA; both Sigma-Aldrich) were used as received. The polymerization initiator benzoine ethylether (BEE; Fluka) was recrystallised from chloroform. All monomers and the initiator were stored at 4 1C. Prior to storage in the glove box, the monomers were bubbled by argon for 30 min, the initiator was pulverised and dried in vacuum at 25 1C for 2 h. 2.2. Polymer electrolyte preparation and ECD assembling Polymer electrolytes were prepared using direct, radical polymerization initiated by UV light [28,30]. The procedure was done at room temperature, which, contrary to the previously used thermal initiation [29], prevents evaporation of the volatile monomer. Also for the electrochromic unit assembling, direct in-situ polymerization gives better results and the ECD samples exhibit a better contact between the electrolyte and the oxide layer. The initial mixture was prepared in the glove box by mixing the monomer (EMA or EOEMA), the cross-linking agent (EDMA for EMA, HexadiMA for EOEMA; 0.3 mol% of monomer) and the polymerization initiator BEE (1 mol% of monomers). Then, certain amounts of lithium perchlorate and PC were added. The electrolyte composition was expressed in molar percentage. The cell consisting of FTO/WO3 and FTO/V2O5 glasses (5 cm  5 cm) separated with a 1 mm acrylic foam tape VHB 3M (3M, UK) was filled with argon and then with the initial mixture. The Pilkington K-GlassTM (fluorine-doped tin oxide) was obtained from Flabeg (Germany) and the WO3 and V2O5 layers (thickness of 300 nm) were deposited by vacuum evaporation in oxygencontrolled atmosphere. After careful filling, the polymerization was initiated by exposing the cell to UV light for a period of 5 h at room temperature and using a pair of 15 W ReptiGlo 8.0 lamps emitting wide UV-A and UV-B light (Hagen, Czech Republic). Developed technology of the ECD assembling assured that, owing to the newly developed technology of the ECD assembling, no gas

bubbles appeared during the polymerization and the whole cell volume was completely filled with the electrolyte. For the electrochemical investigation of prepared polymer electrolytes, the membranes were prepared in a cell described previously [28] and were formed of a polypropylene plate, a packing distance frame (silicone rubber) and a glass plate. Here the UV light was for 2 h. 2.3. Methods and measurements The potentiogalvanostats PGSTAT 10 and 30 (both Eco Chemie, The Netherlands) were used for electrochemical measurements, including the frequency response analyser (FRA-2) module for impedance measurements. The UV–visible transmission spectra (300–900 nm) were recorded on the Perkin Elmer Lambda 35 UV/vis spectrophotometer coupled with the potentiogalvanostat PGSTAT 30. The ECD devices were cycled in a two-electrode arrangement (working electrode—FTO/WO3 glass, reference and counter electrode—FTO/V2O5 glass) from 0 to 2.5 and +1.5 V with scan rates 10, 20, 40 and 60 mV s1. All optoelectrochemical measurements were done at room temperature. The voltammetrical measurements of prepared polymer gel electrolytes were performed in a glove box (MBraun, USA; argon atmosphere, O2 and H2O below 2 ppm) with a gold (BASi, 1.6 mm in diameter) or glassy carbon (BASi, 3 mm in diameter) working electrode and a glassy carbon counter electrode. The solid-state PMMA–Cd–Cd2+ system was used as a reference electrode and all potentials in this paper are related to this Cd/Cd2+ system, which was developed in our laboratory for electrochemical investigation of liquid and polymer aprotic systems with E(PMMA–Cd– Cd2+) ¼ 0.44 V vs. SCE in PC [31]. The surfaces of the working and the counter electrodes were polished by abrasives (0.3 alumina, Metroohm) and soft cloth after each measurement. Conductivity measurements were performed by using impedance spectroscopy, whereby the influence of temperature was studied in the range 70 to 70 1C. Here, a slice of gel (2 cm  2 cm) was sandwiched between two parallel stainless-steel electrodes and a single potential impedance spectrum was measured in the frequency range 200 kHz–1 Hz. The obtained spectrum was analysed by the EcoChemie Autolab software, producing the values of the equivalent circuit elements. The resulting ohmic resistance value was converted to specific resistivity or conductivity values. The DSC analysis was performed in the temperature range 160 to 100 1C at the heating rate of 10 1C min1. Thermogravimetric analysis (TGA) was done in argon at the heating rate of 5 1C min1 with a simultaneous thermal analysis Netzsch STA 409 (Germany).

3. Results and discussion 3.1. Material properties The prepared electrolytes are elastic and homogeneous membranes of a thickness corresponding to the thickness of the silicon spacer in the preparation cell, thus 0.5 or 1.0 mm. All used chemicals are colourless; therefore the prepared membranes are highly transparent (the optical transparency is over 93% in the visible part of spectra). The membranes are stable stored either in dessicator or exposed to air. The long-term storage of PEMA–PC–LiClO4 in air (over 6 months) did not cause phase separation of the sample or any visible changes of homogeneity or mechanical properties. The required foils can be easily cut out and the membranes are well

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sticky, but do not leave any traces on glass or electrode after removal. However, the in-situ polymerization inside the ECD enhances the contact between electrodes and the electrolyte. Following our previous results [28,30,32], we optimised the composition of used polymer electrolytes to fulfil the requirements for their application in the ECDs (high optical transparency, good ionic conductivity, suitable mechanical properties). The present polymer is responsible mainly for the mechanical properties (elasticity and good adhesion to the substrate). For improvement of the mechanical properties, a partial cross-linkage of the network was employed. Cross-linking agents EDMA and HexadiMA were already successfully copolymerised with PEMA and PEOEMA electrolytes, where besides the positive effect on the mechanical properties a remarkable improvement of the ionic conductivity was also observed [28]. The suitable cross-linking agent concentration was found to be 0.3–0.5 mol% of the monomer. Increasing content of the solvent and salt improves the material conductivity. On the other hand, when an excess amount of an organic solvent is added to the polymer matrix, the material does not have enough strength to keep a solid film. Here, owing to the partial cross-linkage of the network the sample keeps its toughness and elasticity even with a lower content of the polymer. For manipulation, polymer electrolyte samples should contain 40 mol% of polymer at least, but for in-situ preparation of ECDs, initial mixtures with a lower content of polymer can be used because the electrolyte is formed directly in the ECD and no further mechanical operations are required. Here, the advantage of direct polymerization of electrolyte inside the device should be noted in the proposed practical application of the method of preparation. TGA of PEMA–PC–LiClO4 and PEOEMA–PC–LiClO4 showed the thermal stability up to 125 1C. The weight loss was found to be less than 4 wt% up to 125 1C and 10 wt% under 150 1C. These changes can be explained as a partial evaporation of the immobilised solvent (PC). At temperatures above 150 1C both samples start to decompose (joined processes of the polymer decomposition with evaporation of the solvent).

3.2. Impedance and conductivity measurements Our measurements showed that the PEMA–PC and the PEOEMA–PC binary systems without added inorganic salt behave as insulators with conductivity lower than 107 S cm1 at 20 1C. Therefore, the membranes prepared with LiClO4 can be considered as pure ionic conductors. Experiments with various methacrylates (methyl, ethyl, butyl and hexyl methacrylate) demonstrated that methyl methacrylate and EMA form the highest-conductivity electrolytes among the polymers with non-polar carbon chain [28]. EOEMA is a representative of aprotic monomers of higher polarity due to the presence of an ethoxyethyl group in the molecule. This property affects the solubility of lithium perchlorate in EOEMA itself and allows preparation of a homogeneous binary electrolyte PEOEMA–LiClO4 without aprotic solvents

251

(composition PEOEMA/LiClO4 ¼ 90.5/9.5 mol%). Similar to PEO– LiClO4 electrolytes [1], here also the ionic conductivity at room temperature was low (4.2  107 S cm1), not very far from the conductivity of the non-salt system mentioned above. This is mainly caused by the absence of an immobilised phase suitable for ion dissociation (aprotic solvent) and by strong ion–ion association. These features result in very low mobility of all ion species in the membrane. Addition of an aprotic liquid phase strongly increases the mobility and therefore conductivity of Li+ and ClO 4 ions due to a decreased microscopic viscosity. The conductivity increases by about four orders of magnitude from 4.2  107 to 103 S cm1 at room temperature. Thus, the present aprotic phase works as a conductive phase, where the ions are mainly located. Within the optimisation of the sample composition not only a high absolute content of salt must be considered, but also the ratio salt–solvent, which strongly affects the resulting conductivity. This conclusion is particularly important in the case of lithium salt solution in aprotic solvents, where a strong tendency for ion–ion association was found and described in literature [13,14,17,21, 22,33,34] and also in our previous work [28–30]. Table 1 shows the conductivity and EA values for the electrolytes used for ECD assembling. Comparing PEMA electrolytes of different composition (samples 1 and 2), an increase of the conductivity by more than one order of magnitude was obtained by the composition optimisation (reduction of the polymer content). Also the activation energy decreased due to a higher content of aprotic solvent. This effect plays its role mainly in practical applications, when a substantially higher conductivity (1.3  104 S cm1) is retained even at 20 1C. A similar improvement of conductivity was reached for the PEOEMA electrolytes. An attempt was made to prepare PEOEMA electrolytes with a higher content of LiClO4 (c. 25 mol%), but at temperatures under 50 1C, sample 4 exhibited a lower conductivity than expected (see Table 1). Also a lower optical transparency of this sample was observed when the electrolyte was slightly opaque. Both results imply that LiClO4 is partially segregated from the electrolyte. Fig. 1 presents the relationship between the ionic conductivity of prepared polymer electrolytes and temperature; the data are plotted in Arrhenius coordinates (specific conductivity is plotted as a decadic logarithm). Impedance measurements showed that at temperatures under 25 1C, bulk conductivity is influenced by the polymer structure transformation (from elastomeric to more crystalline), which is accompanied by conductivity decrease. At low temperatures, a wide semicircle representing the parallel connection of a capacitor and a resistor was observed corresponding to the dielectric behaviour of the polymer and the bulk resistivity of the sample, while with increasing temperature the semicircle was depressed and a simple serial connection of resistor and pseudocapacitor (a constant phase element) was found. In both cases, similar impedance spectra were obtained in our previous research [28,29]. Similar conclusions were obtained in the case of styrene–butadiene–acrylonitrile-based electrolytes [35], poly(vinyl acetate)–DMF–LiClO4 polymer electrolytes [36], PEO–LiTFSI electrolytes [37] and other binary or ternary polymer electrolytes [38–40].

Table 1 Specific conductivities (at 20, 20 and 60 1C) of prepared PEMA and PEOEMA polymer electrolytes containing immobilised LiClO4–PC solution together with estimated apparent activation energy values EA, glass transition temperatures (Tg) and ideal glass transition temperatures (T0). Sample 1 2 3 4

Polymer electrolyte PEMA–PC–LiClO4 PEMA–PC–LiClO4 PEOEMA–PC–LiClO4 PEOEMA–PC–LiClO4

Composition (mol%) 39: 56: 5 58: 39: 3 48: 46: 6 47: 29: 25

s at 20 1C (S cm1) 4

1.3  10 3.2  107 9.6  106 1.9  106

s at 20 1C (S cm1) 4

6.9  10 1.2  105 1.8  104 7.9  105

Samples 1 and 3 were used in studied ECDs, while samples 2 and 4 were described previously [28].

s at 60 1C (S cm1) 3

1.5  10 7.7  105 7.0  104 1.1 103

EA (kJ mol1)

Tg (K)

T0 (K)

4.9 10.0 9.0 11.8

175 179 195 190

150 145 140 145

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-3

log σ / S.cm-1

-4

-5

-6 PEMA PEOEMA

-7

-8 3.0

3.5

4.0

4.5

5.0

1000 T-1 / K -1 Fig. 1. Arrhenius plot of PEMA–PC–LiClO4 (39:56:5) and PEOEMA–PC–LiClO4 (48:46:6 mol%) electrolyte (temperature range 70 to 70 1C).

The values for the temperature region from 20 to 70 1C can be fitted with the Vogel–Tamman–Fulcher (VTF) equation in the logarithmic form:

sT 1=2 ¼ A exp½EA =RðT  T 0 Þ

(1)

In this particular relationship, A is the parameter related to the number of charge carriers, EA the activation energy for conduction, R the universal gas constant and T0 the ideal glass transition temperature, indicating the temperature at which the free volume extrapolates to zero. The analysis of the experimental conductivity data in terms of the VTF relationship leads to the determination of three empirical parameters: A, EA and T0, where T0 is determined by fitting the experimental data with relationship (1). The conductivity activation energy corresponds to the slope in Arrhenius coordinates (see Fig. 1) and explains how conductivity is influenced by temperature. The activation energies for liquid systems are summarised in Table 1, where we can see that the EA value is strongly dependant on the sample composition. Not only the polymer content, but also the solvent–salt ratio influences both EA and bulk conductivity. The samples recently used in the ECDs are more resistant considering the influence of temperature on conductivity. Bulk conductivity remains high even at low temperatures around 20 1C (above 104 S cm1), which is an important condition for practical application in smart windows or smart displays. The achieved values of ionic conductivity are highly above the limit value of 106 S cm1 declared by MacFarlane as the minimum for electrolytes in electrochromic window applications [21].

the accessible electrochemical window lies between 4 and 5 V in the case of LiClO4–PC solutions [41,42]. Also the methacrylatebased electrolytes exhibit high electrochemical stability, which allows their application also in lithium-ion batteries [43–45]. Our previously described PMMA–PC–LiClO4 polymer electrolytes prepared from the polymer resin Superacryls (Spofa, Czech Republic) suffered from contamination with oxygen, which limited their electrochemical stability to 2 V window. The development of a new method of preparation using a direct, UV-initiated polymerization improved the electrochemical window up to 3.6 V (cit. [29]). Figs. 2 and 3 show the cyclic voltammograms of the prepared PEOEMA and PEMA–PC–LiClO4 electrolytes on the gold electrode done in argon atmosphere. Comparing with the voltammetrical measurements of liquid PC-based electrolytes done in argonfilled glove box, we found that the electrochemical stability of the polymer electrolytes is limited by the properties of PC. 1At potentials around 1.2 V we found an irreversible cathodic wave accordant with formation of a protective layer similar to the solid electrolyte interface (SEI) formation on graphite electrodes and followed by reversible reduction and oxidation of lithium at 2.6 V. Contrary to the real SEI layer described in lithium-ion batteries, the cathodic wave of the protective layer content is visible on each scan, from which one can conclude that the layer is recovered during every cycle. Both the PEMA and PEOEMA electrolytes exhibit a similar anodic behaviour when PC is oxidised above 2.5 V vs. Cd/Cd2+. The linear sweep voltammograms on glassy carbon (Fig. 2) showed an anodic wave at 2.3 V, which does not appear on gold electrode under the same conditions. This peak can be possibly attributed to pre-oxidation of PC [46]. If the cyclic voltammogram is measured up to 3 V, a cathodic wave around 2.4 V appears both on gold and glassy carbon electrodes. From comparison with literature and also from our voltammetrical measurements of liquid PC electrolytes we can conclude that the present polymer lowers the reactivity of the electrolyte

0.20

0.15

0.10

0.05 E vs. Cd/Cd2+ / V 0.00

-0.05 1st scan 10th scan

-0.10

-0.15 -3

3.2.1. Voltammetrical measurements The electrochemical stability of PC-based electrolytes was already described in literature when the PC was successfully tested in various electrochemical applications, mainly chemical power sources. Depending on the electrode material,

Au GC

i / mA.cm-2

-2

i / mA.cm-2

252

-2

-1

0

1

2

3

E vs. Cd/Cd2+ / V Fig. 2. Cyclic voltammograms (1st and 10th scan) of PEOEMA–PC–LiClO4 (48:46:6 mol%) electrolyte on the gold electrode at 1 mV s1. Inserted: linear sweep voltammograms of the same electrolyte on gold and glassy carbon electrode at 1 mV s1.

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from +1.5 to 2.5 V with a scan rate of 10 mV s1 for about 10 cycles. After this procedure, the optoelectrochemical experiments were performed. However, this formatting can be eliminated by using V2O5 layers with intercalated Li+ ions, similar to that in Refs. [54–56].

1.0

0.5

i / mA.cm-2

253

0.0

10 -0.5

th

scan

st 1 scan

-1.0

-1.5 -3

-2

-1 E vs.

0 Cd/Cd2+

1

2

3

/V

Fig. 3. Cyclic voltammograms of PEMA–PC–LiClO4 (39:56:5 mol%; solid line; 1st and 10th scan) on the gold electrode and FTO/WO3 (dash line) and FTO/V2O5 (dash–dot line) electrochromic glass in 1 M LiClO4 in PC (all measurements done at 10 mV s1 scan rate, counter electrode platinum, reference PMMA–Cd–Cd2+ electrode).

perhaps by influencing the reaction rate [47] and possibly enlarges the accessible potential window, which is important for recent testing in lithium-ion batteries [45]. As tungsten oxide is a widely used electrochromic material for the battery-type (rocking-chair type) ECDs [48], we investigated the electrochemical behaviour of FTO/WO3 glasses in aprotic, both liquid and polymer electrolytes, to ensure electrochemical compatibility with the developed polymer electrolyte. The electrochemical insertion of lithium cations into WO3 is performed in the potential range +1.5 to 0.3 V vs. Cd/Cd2+ in agreement with literature [9,18,49,50], expressed as

Fig. 4. Optical transmittance in the coloured (2.5 V) and bleached (+1.5 V) electrochromic device with PEMA–PC–LiClO4 electrolyte (39:56:5 mol%; solid line) and PEOEMA–PC–LiClO4 electrolyte (48:46:6 mol%; dash line), device size 50  50 mm2.

60

50

WO3 þ x Liþ þ xe 2Lix WO3 ðblueÞ

A similar reaction occurs on the counter FTO/V2O5 electrode, where the electrochemical process can be described by the reaction V2 O5 þ x Liþ þ xe 2Lix V2 O5 and takes place at potentials from +2 to 0.5 V vs. Cd/Cd2+. Park et al. [51], Livage [52], and Wang and Cao [53] described a similar behaviour of V2O5. As can be seen from Fig. 3, where the voltammetrical curves of WO3 and V2O5 are compared with a voltammogram of the PEOEMA–PC–LiClO4 electrolyte, both electrochemical processes take place within the electrochemical potential window of the used polymer electrolytes (see also Fig. 2 for a detailed voltammogram of PEOEMA–PC–LiClO4).

40

τ/%

ðcolourlessÞ

30

20

10

0 3.3. Optoelectrochemical measurements of EC devices employing PEMA and PEOEMA electrolytes As the V2O5 layer acts as a counter electrode and at the beginning of the experiment does not contain any Li+ cations in its structure, the ECD had to be formatted by voltammetrical cycling

0

50

100 t/s

150

200

Fig. 5. Colouration-bleaching characteristics of the electrochromic device with PEMA–PC–LiClO4 electrolyte (39:56:5 mol%; solid line) and PEOEMA–PC–LiClO4 electrolyte (48:46:6 mol%; dash line) using a square wave potential of 2.5 V (100 s) and +1.5 V (100 s) measured at 634 nm.

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60

50

50

40

40

τ/%

τ/%

60

30

30

10 mV/s 20 mV/s 40 mV/s 60 mV/s

20

10

-3

-2

-1

0

1

10 mV/s 20 mV/s 40 mV/s 60 mV/s

20

2

10

-3

-2

-1

0

1

2

E/V

E/V

Fig. 6. In-situ UV–visible spectroelectrochemical measurements of the electrochromic device with PEMA–PC–LiClO4 electrolyte (39:56:5 mol%; A) and PEOEMA–PC–LiClO4 electrolyte (48:46:6 mol%; B) under different scan rates measured at 634 nm.

The transmission characteristics of the ECD were recorded in a bleached (+1.5 V) and a coloured state (2.5 V) in the 300–900 nm range and are plotted in Fig. 4. The optical modulation was reversible with an appropriate potential being applied on the assembled device. As is visible from the spectra, the device with the PEOEMA electrolyte offers a higher transmittance change (Dt) in the whole spectrum range as compared to the ECD with the PEMA electrolyte. The potentiostatic tests performed on both types of ECDs showed that during the negative pulse the ECD becomes dark blue due to the intercalation of Li+ cations into WO3, while reversing the polarity causes a rapid decolouration (deintercalation of lithium). The potentiostatic tests showed that the main transparency change occurs within the first 20–25 s (see Fig. 5). However, the PEMA-based ECD showed a faster recovery after polarizing from 2.5 to +1.5 V. On the other hand, the PEOEMAbased ECDs provide a deeper colouration change Dt40–45% contrary to 35% for the PEMA-based device. Cyclic voltammograms of the EC device performed at various scan rates (10, 20, 40 and 60 mV s1) showed corresponding cathodic (colouring) and anodic (bleaching) waves, which reflected the insertion/extraction of Li+ ions into the WO3 film (Figs. 6A and B). From the in-situ UV–visible optoelectrochemical measurements performed on this EC device, we can see, that even at a high scan rate of 60 mV s1, the device responds rapidly. In the case of PEOEMA-based ECD we included tests with a lower cathodic potential (down to 3 V), but no difference in the ECD colouration was found (see Fig. 6B). With regard to the chosen battery-type ECD, the long-term memory effect (keeping of transmittance in the coloured state without applied voltage) was preserved and self-erasing was practically not observed. The assembled ECDs were functional for more than 220 cycles with no observed phase separation or visible decomposition of either the electrolyte or the electrodes (potential range 2.5 V to +1.5 V).

4. Conclusion In this paper, we have demonstrated the development of polymer electrolytes based on EMA and EOEMA, achieving both

high ionic conductivity and acceptable mechanical properties. The electrochemical stability, ionic conductivity as well as high optical transparency and good adhesion to the substrate (WO3 or V2O5) enabled us to assemble an ECD with directly prepared polymer electrolyte. Also the method of direct in-situ preparation was found suitable for possible industrial application. The ionic conductivities for the used electrolytes are 6.9  104 for PEMA and 1.8  104 S cm1 for PEOEMA–PC–LiClO4. Voltammetrical measurements showed good electrochemical stability of both of the used electrolytes when the accessible potential window was found up to 5.1 V vs. Li/Li+ on gold electrode. The electrolytes were thermally stable up to 125 1C. The optoelectrochemical tests showed that the electrolytes are suitable media for lithium-ion transportation towards the WO3 and V2O5 thin layer electrodes and for subsequent reversible lithium intercalation. The transmittance change Dt was found to be 30–45% at 634 nm and the potentiostatic tests showed that the main transparency change was performed within the first 20–25 s.

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