Imidazolium-based ionic liquid derivatives for application in electrochromic devices

ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 92 (2008) 126–135 www.elsevier.com/locate/solmat

Imidazolium-based ionic liquid derivatives for application in electrochromic devices A. Sˇurca Vuka, V. Jovanovskia, A. Pollet-Villardb, I. Jermana, B. Orela, a

National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia b Joseph Fourier University, BP 53,- 38041 Grenoble cedex 9, France Received 8 November 2006; accepted 25 January 2007 Available online 19 September 2007

Abstract Novel proton-conducting electrolytes were prepared from the sol–gel precursor 1-[3-(trimethoxy-l4-silyl)propyl]imidazole with the addition of either trifluoroacetic or acetic acid. The presence of trimethoxysilyl groups enabled the solvolysis and condensation reactions of silsesquioxane species. IR spectroscopy revealed that more cube-like species formed in the electrolyte prepared from trifluoroacetic acid, while cube- and ladder-like silsesquioxanes were present in the electrolyte with acetic acid. This assignation was independently confirmed by 29Si NMR analyses revealing the T3 signals of trisiloxane bonding. IR spectroscopy also pointed to the formation of hydrogen bonding in the latter electrolyte, since the frequencies of the observed bands at 1710, 1409, and 1272 cm1 approached those of acetic acid. In contrast, the IR bands at 1662, 1204, and 1130 cm1 confirmed the existence of trifluoroacetate anions in the case when the electrolyte was prepared from trifluoroacetic acid. The presence of free trifluoroacetate anions contributed to the moderately higher specific conductivity of this electrolyte (4.6  105 S/cm) compared to that of acetic acid (1.6  105 S/cm). The specific conductivity of the electrolytes could be further increased by the addition of a lithium salt. All electrolytes were employed in electrochromic devices with optically active WO3 and various inorganic counter-electrodes (CeVO4, V2O5, Ti/V-oxide). Photopic transmittance changes from 30% to 40% were achieved. r 2007 Elsevier B.V. All rights reserved. Keywords: Electrochromics; Ionic liquid; Protonic conductor; Sol–gel; Ir spectroscopy

1. Introduction Numerous studies of ionic liquids [1,2] conducted during the last decade have revealed that these liquids could successfully be used as electrolytes in various electrochemical devices. As a result of their low volatility, nonflammability, and thermal and chemical stability, ionic liquids were tested in dye-sensitised photoelectrochemical  cells of Gra¨tzel type [3–5] where the I redox pair 3 /I transports the charge. Imidazolium-based ionic liquids     with various anions (BF 4 , Br , PF6 , NO3 , CF3CO2 , Tf2N-trifluoromethanesulfonimide, y) were applied in batteries [6], electrochemical mechanical actuators [7], Corresponding author. Tel.: +386 (0)1 4760 276; fax: +386 (0)1 4760 300. E-mail address: [email protected] (B. Orel).

0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.01.023

numeric displays [7–9], and electrochromic (EC) windows [7–10]. For application in EC devices, attempts were directed towards the production of devices with PEDOT [7,8,10] and other conducting polymers with EC properties [7,8]. The results revealed fast switching times and excellent stability (up to a million cycles) of such EC devices. The compatibility of hydrophobic ionic liquids with various inorganic intercalation materials like WO3, Li0.5Ni0.5O, Li0.5Cr0.5O1.25 and Fe2O3 was tested by Delville et al. [11]. Inorganic oxides, it was claimed, should be amorphous or polycrystalline with a small grain size, to achieve the longterm chemical stability. The question of how to combine the excellent properties of ionic liquids with sol–gel precursors capable of network formation remained. Li et al. [12], for example, prepared a proton-conducting gelatinous electrolyte with the addition of 1-butyl-3-methyl-imidazolium-tetrafluoroborate ionic

ARTICLE IN PRESS A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135

liquid to a sol–gel precursor methyltrimethoxysilane (MTMOS) in ethanol. Water and strong mineral acid— H3PO4—were used as initiators of hydrolysis and condensation reactions. The addition of an ionic liquid influenced the morphology of the electrolyte and prolonged the gelation time. Unlike conventional silica, the final electrolyte did not shrink with time. The conductivity of this electrolyte reached 1.2  103 S/cm and was slightly lower than the conductivity of the MTMOS/H2O/H3PO4 controlling compound (9  103 S/cm). The change in the conductivity is a result of high viscosity and dilution effect of the ionic liquid and the hydrogen bonding established between the ionic liquid, H3PO4 acid, and water. Ionic liquids were also used as templates, for instance, in the preparation of highly ordered, super-microporous lamellar silica [13]. EC devices in which imidazolium-based ionic liquid derivatives were entrapped between an optically active nanocrystalline WO3 film [14] and counter-electrode films like CeVO4, V2O5 [15], and Ti/V-oxide [16] were studied. The electrolytes were based on a new sol–gel precursor—1[3-(trimethoxy-l4-silyl)propyl]imidazole (TMSPIm)—in which propyltrimethoxysilane groups were bound on the imidazolium ring, enabling condensation to a siloxane network. TMSPIm-based electrolytes were produced in a manner similar to our previous investigations of the ionic liquid 1-methyl-3-[3-(trimethoxy-l4-silyl)propyl]imidazolium iodide (MTMSPIm+I) [4,5], which was synthesised as a potential quasi-solid-state redox electrolyte for Gra¨tzel cells. Using IR and 29Si NMR spectroscopy, it was found that the trimethoxysilane groups of MTMSPIm+I simultaneously underwent hydrolysis and condensation during the 10 h immediately following the addition of the catalyst, acidified water (0.1 M HCl). In this stage, condensation to three- or four-membered siloxanes, and even to open cube-like products such as T7(OH)3 and T8(OH)4, was demonstrated by IR measurements and corroborated by the appearance of T2 and T3 bands in the 29 Si NMR spectra. Further ageing led to the formation of T8 cubes, although the open cube-like silsesquioxanes remained present. The specific conductivity of such a quasisolid-state stabilised at 0.11 mS/cm. A completely condensed structure was achieved after heating to 200 1C, whereupon only the T3 signal was observed in the 29Si NMR spectrum, pointing to the existence of siloxane linkages only. In combination with IR, it was deduced that ladder-like silsesquioxanes with terminating cube-like species developed [4,5].

OMe MeO

Si

+

OMe Cl

H N

The preparation of TMSPIm-based electrolytes tied in to the idea of synthesising quasi-solid-state electrolytes with specific conductivity values, appropriate for the functioning of EC devices. In distinction to MTMSPIm+I ionic liquid for Gra¨tzel cells, carboxylic acids (acetic and trifluoroacetic) were used as the initiators of (in this case) solvolysis and condensation reactions. The mechanisms of solvolysis had already been investigated on the simple methyltriethoxysilane (MTEOS) compound [17]. It was found that solvolysis proceeded through the formation of the silylester (Si-OAc) and its band was observed in the IR spectra, together with the corresponding alcohol, during the course of the formation of the silsesquioxane network. Protons served as the intercalation species. The remaining questions of interest concerning the application of TMSPIm-based electrolytes in EC devices involve the extent of their condensation, specific conductivity, stability, and the optical modulation of such devices.

2. Experimental 2.1. Electrolytes Three kinds of electrolytes were prepared from the sol–gel precursor TMSPIm. TMSPIm was synthesised from imidazole (Fig. 1), which was dissolved in methanol. NaOMe was added slowly, while stirring, to this solution. The solution was then refluxed for about half an hour to allow reaction between methoxide and imidazole. 3Chloropropyltrimethoxysilane was then added drop-wise. The mixture immediately turned white, as a result of NaCl precipitating from the solution. The reaction continued under reflux for 12 h. The mixture was finally filtered and the product obtained, by removal of methanol under reduced pressure and elevated temperature. The product— TMSPIm—was a yellowish non-viscous liquid. Electrolytes I–III (Fig. 2) were prepared from TMSPIm precursor by adding either: (I) trifluoroacetic acid (TFA) or (II, III) acetic acid (AcOH) in a molar ratio of 1:5.5. Four and a half of these equivalents were used for solvolysis, while one equivalent served for protonation (formation of ionic liquid). In electrolyte III, a mixture of acetic anhydride as dehydrating agent, and lithium acetate dihydrate as a source of lithium, ions were added. Solvolysis and condensation reactions of trimethoxysilanes were stimulated by heat treatment of the mixtures at 120 1C for some hours. Lastly, heating under reduced pressure was used to

Na OMe

MeO MeO

N

127

Si OMe

N

Fig. 1. Synthesis of the 1-[3-(trimethoxy-l4-silyl)propyl]imidazole (TMSPIm) precursor.

N

ARTICLE IN PRESS A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135

128

R OMe MeO

Si

R

O

+

OMe

5,5 F3C

N

OH

O Si

R

Si

OR

O O

Si

O

N

R=

+

O

O Si

Si O

CF 3

-

O

Si

O

H2C

R

O

R

O

N

O

O

N

Si

R

R

O

OMe MeO

H

Si

5,5 H3C

S

OH

OMe

R R

N O

N

R

Si

R

O O O

Si O

R

R

Si O Si

O Si O

O

O Si

O

R

Si

O

Si R O O Si

R

R

Si O

O

R

O

Si

O

O

Si

O

Si

Si

O

O Si

O

Si

R

R

R

O

Si

O

R R

R

O

Si

R

O

O Si O

OO

O

O Si

R

O Si

R

Si

R

R

R Si

Si O

O

O

R

O

Si

R

O

Si

R

O

5,5 H3C OMe MeO

OH

R= +

Si

AcOAc AcOLi 2H2O

OMe N

N

N

H O CH3

-

O H2 C

N

Fig. 2. Preparation and possible structures of TMSPIm-based condensed electrolytes: (I) TMSPIm+TFA, (II) TMSPIm+AcOH, and (III) TMSPIm+AcOH+Li+.

remove the remaining volatile components from the electrolytes. 2.2. Films and EC devices Thin films of WO3, V2O5, and Ti/V-oxide were prepared using the sol–gel procedures already reported in Refs. [14–16]. Thin films of CeVO4 were prepared from cerium (III) chloride heptahydrate and vanadium (V) oxoisopropoxide precursors, using the peroxo synthesis route. Modification of the synthesis by the addition of citric acid prevented the oxidation of the Ce-precursor to cerium (IV) oxide—a brown-coloured precipitate insoluble in water or alcohol. A transparent solution of peroxy acids–cerium complexes formed instead. This solution was thermally treated at 80 1C until a gel-like residue formed. It was then dissolved in ethanol and films were deposited using the dipcoating technique. Thermal treatment of CeVO4 films was performed at 500 1C for 30 min. The EC devices were assembled from an optically active WO3 electrode and from one of the three tested counter-electrodes (CeVO4, V2O5, and Ti/V-oxide). All films were deposited on SnO2:F glass substrates (Hartford Glass Co. Inc., USA). Prior to assembly, the thin films were briefly heated to remove

adsorbed water. A drop of an electrolyte was poured onto the WO3 film and covered with the chosen counterelectrode. The two electrodes were firmly pressed together to assure proper electrical contact. 2.3. Instruments IR measurements of electrolytes were performed on an FT-IR spectrometer (Perkin-Elmer System 2000). The electrolytes were applied onto CdTe plates. 29Si NMR spectra of sols were recorded on a Varian Unity Plus 300 MHz spectrometer using a Doty CPMAS probe head. Specific conductivity (s) measurements were carried out in an electrochemical cell using platinum electrodes (the cell constant was determined with 0.1 M KCl) by ac impedance measurements on an Autolab PGSTAT30 potentiostat– galvanostat equipped with an FRA module. The frequency range used was 106–102 Hz. In situ UV–vis spectroelectrochemical measurements of the EC devices were made using an HP 8453 diode-array spectrophotometer in combination with an Autolab PGSTAT30 potentiostat– galvanostat. Cyclovoltammetric (CV) measurements were made using a scan rate of 50 mV/s, and the UV–vis spectra were taken every 4 s. The photopic transmittance (Tvis) of

ARTICLE IN PRESS A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135

2853

3076 2962

3151 b

2889

2935

3115

a

B

C

B

C

c

A

4000

3000

2887

3123 2935

A

2000

1204 1180

Electrolyte

Description

s (S/cm)

I II III

TMSPIm+TFA TMSPIm+AcOH TMSPIm+AcOH+Li+

4.6  105 1.6  105 5.7  105

635

487

831 798 722 707

1500

1000

449 449

621 621

932 879 837

754 703 663

1064 1040

1272

1585 1515 1445 1416 1367

1127

1111 1067 1049 932 880 820 751 703 663

1272

1585 1512 1445 1409 1367

1710

1127 1111

b

2000

901

1287

1662

1591 1551 1456 1430 1412

1782 1740 a

1055

1037

c

Table 1 Specific conductivity (s) of electrolytes

1000

Wavenumber [cm-1]

1710

The specific conductivity measurements revealed that the s values of TMSPIm-based electrolytes I–III were of the order of magnitude of 105 S/cm (Table 1). The specific conductivity of TMSPIm+TFA (electrolyte I) surpasses that of TMSPIm+AcOH (electrolyte II) by almost threefold, and the reason could be found in the structural differences of these two electrolytes (Section 3.2.). Namely, using IR spectroscopy it was found that typical ionic liquid with an imidazolium cation and a TFA anion formed in electrolyte I, while hydrogen bonds established between protonated imidazolium cations and acetate anions in electrolyte II. Hydrogen bonding inevitably led to a partial decrease in conductivity, as was already shown for composite materials of the sol–gel precursor MTMOS and the ionic liquid 1-butyl-3-methyl-imidazolium-tetrafluoroborate [12]. Namely, conduction in imidazoliumbased compounds is of Grotthuss type, depending on the movement of protons between neighbouring imidazolium rings [19]. Kawada et al. [20] who studied protonic conductivity in imidazole single crystals confirmed the high anisotropic conductivity of this material. The ratio of conductivity in the c and a crystallographic directions was estimated to be 103, pointing to the importance of proton transfer in the direction of hydrogen bonding. These authors [20] also claim that this transfer did not occur step-wise to the next-nearest neighbour for the protonated imidazole, but that a one-step transfer of charge of several molecules down the chain may occur. Addition of lithium ions to electrolyte II produced with AcOH acid increased the number of conducting species and s values from 1.6  105 S/cm (electrolyte II) to 5.7  105 S/cm (electrolyte III: TMSPIm+AcOH+Li+). The conductivity values of TMSPIm-based electrolytes I–III did not reach the value of 0.11 mS/cm, i.e. the specific conductivity of the ionic liquid MTMSPIm+I in the

0.5

3.1. Conductivity of electrolytes

IR measurements confirmed that imidazolium cations in the TMSPIm-based electrolytes are arranged around the silsesquioxane cube- and ladder-like species in the course of solvolysis and condensation reactions (Figs. 2 and 3). According to our previous assignations of silsesquioxanes, the vibrations of MTEOS [17], and the ionic liquid MTMSPIm+I [4,5], we could deduce that more cube(T8) and open cube-like (T7(OH)3, T8(OH)4) species

0.5

3. Results and discussion

3.2. Structural properties of electrolytes

Absorbance

where l represents the wavelength, t(l) the spectral transmittance of the sample, Dl the spectral energy distribution, and V(l) the luminous efficiency of the observer [18].

quasi-solid state [4,5]. It should be stressed that the conducting species differ, while the former electrolytes I–III are protonic conductors, the conducting species in MTMSPIm+I are iodide ions.

Absorbance

EC devices was determined according to the equation: P780 nm l¼380 nm Dl tðlÞV ðlÞ Dl T vis ¼ P , (1) 780 nm l¼380 nm Dl V ðlÞ Dl

129

500

Wavenumber [cm-1] Fig. 3. IR spectra of TMSPIm-based electrolytes I–III: (a) TMSPIm+ TFA, (b) TMSPIm+AcOH, and (c) TMSPIm+AcOH+Li+ in two different spectral ranges: (A) 4000–400 cm1 and (B) 2000–400 cm1.

ARTICLE IN PRESS

806 731

858

1208 1151 1463

3440

0.5 3662

695 501

863 1058

2364

1734

1436

3158

1599

a: LiTFA 3335

formed in electrolyte I (TMSPIm+TFA) than in electrolyte II (TMSPIm+AcOH). This was inferred from the Si–O–Si bands at 1137 cm1 (T8) and its shoulder around 1092 cm1 (Fig. 3). The bands of TMSPIm+TFA at 1204, 1180, and 1130 cm1, however, were superimposed on both siloxane bands. The IR spectrum of a model salt compound 1-methylimidazolium trifluoroacetate (MIm+TFA) in Fig. 4a revealed that other bands of the model compound could find their counterparts in the electrolyte I bands at 1662, 1591, 1412, 1287, 831, 798, 722, and 635 cm1 (Fig. 3). Among these bands, the bands at 1662, 1204 cm1, and the band around 1130 cm1, the latter one superimposed on the siloxane bands, belonged to the trifluoroacetate anion. Namely, similar bands were observed in the IR spectrum of lithium trifluoroacetate (1688, 1208, and 1151 cm1 in Fig. 5). This proved that the

Absorbance

130

1688

A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135

b: LiOAc

4000

3000

2000 Wavenumber

1000

[cm-1]

Absorbance

3156 3124 3075

0.25

Fig. 5. IR spectra of salts: (a) lithium trifluoroacetate and (b) lithium acetate.

3123

a

B

C

A

b

4000

3000

2000

1000

2000

1500

1000

750

661 619

449

634

869 830 797

1020

721

1180 1130 1100 1085

933 880 831

b

1233

1518 1422 1367

1527

1270

1108 1087 1010

1206 1285

1419

1590 1557

1670

a 1711

Absorbance

0.25

Wavenumber [cm-1]

500

Wavenumber [cm-1] Fig. 4. IR spectra of model compounds: (a) 1-methylimidazolium trifluoroacetate (MIm+TFA) and (b) 1-methylimidazolium acetate (MIm+OAc) in the spectral range: (A) 4000–400 cm1 and (B) 2000–400 cm1.

TFA anion in electrolyte I did not form hydrogen bonds with N–H groups of imidazolium rings—i.e. electrolyte I exists in the form of an ionic liquid. The same route of deduction was used to draw conclusions about the electrolyte prepared from TMSPIm and AcOH acid (electrolyte II). In this electrolyte, ladderlike silsesquioxanes were present in addition to cube-like silsesquioxanes [4,5]. The presence of ladder-like silsesquioxanes could be deduced from the simultaneous presence of IR bands around 1135 and 1049 cm1. The appearance of the latter band confirmed a clear distinction in the structures of electrolytes I and II and the bonding of silsesquioxane units. The IR bands observed in the spectrum of a model compound 1-methylimidazolium acetate (MIm+OAc) in Fig. 4 could be correlated with the bands in electrolyte II (1710, 1512, 1367, 1272, 1111, 932, 880, 663, 621, and 449 cm1 in Fig. 3b). It was found that the IR spectrum of the electrolyte with AcOH (electrolyte II) did not reveal distinct bands of acetate ions, which can be noted at 1734, 1599 (most intense), and 1436 cm1 in the spectrum of lithium acetate (Fig. 5b). Bands at 1710, 1409, and 1272 cm1 (electrolyte II in Fig. 3), approaching to the frequencies of acetic acid (1712, 1412, and 1292 cm1 in Fig. 6) were found instead. This proved that hydrogen bonding developed in this electrolyte, with the H-atom residing closer to the acetate anion than to the imidazolium cation. The formation of hydrogen bonds was confirmed by the presence of three broad and low-intensity bands—A, B, C—between 3200 and 1800 cm1, characteristic of acid salts, e.g. KH2AsO4, KH2PO4 [21], and KH(CH3COO)2 [22]. Differences in the structures of TMSPIm-based electrolytes I and II also explained the higher conductivity of electrolyte I (TMSPIm+TFA). The presence of TFA anions close to imidazolium cations did not disturb the jumping of protons over the imidazolium rings. As pointed out in Section 3.1,

ARTICLE IN PRESS A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135

131

1162

-67.0

T3 -57.3 -55.2

Intensity

1779 1463 b: TFA

a: TMSPIm+TFA

T2 -70.5

1712 0.25

Absorbance

1221

1758

T3

1292 1412

1238 b: TMSPIm+AcOH

a: AcOH

4000

3000

2000

1000

Wavenumber [cm-1] Fig. 6. IR spectra of acids: (a) trifluoroacetic acid and (b) acetic acid.

more rapid transfer was found in protonated imidazole than in neutral imidazole [20]. In electrolyte II (TMSPIm+AcOH), the protons of imidazolium rings formed hydrogen bonds with the acetate anion, which moderately decreased the mobility of the protons. Addition of lithium acetate did not influence the spectral region of the siloxane bands (electrolyte III in Fig. 3c), indicating that the structure of the silsesquioxanes remained the same as in electrolyte II (TMSPIm+AcOH in Fig. 3b). However, the presence of lithium acetate salt could be demonstrated from the strong IR band at 1585 cm1 and increased absorption in the region 1400–1460 cm1 in which the LiOAc band at 1436 cm1 was superimposed (Figs. 3c and 5). This suggested that LiOAc salt dissolved in the matrix of the electrolyte, leading to increased viscosity of electrolyte III, compared to electrolyte II. In addition to IR spectroscopy, 29Si NMR spectroscopy was used for structural investigations of the electrolytes applied in EC devices. 29Si chemical shifts of trialkoxysilanes (TMSPIm in Fig. 1A) appeared in four groups of lines marked by Tn(i,j), where n represents the number of siloxane bonds (n ¼ 0–3), i the number of –OH, and j the number of –OR groups on silicon atom [23]. With ageing, i.e. in the course of hydrolysis (solvolysis) and condensation reactions, the signals shift from the T1 to the T2 and T3 regions [4,5,23–25]. The T3 signal that represents trialkoxysilanes bound by three siloxane bonds was noted in the 29 Si NMR spectra of both the measured electrolytes TMSPIm+TFA (electrolyte I) and TMSPIm+AcOH (electrolyte II) (Fig. 7). This indicated the presence of a highly condensed siloxane structure, either cube- or ladderlike silsesquioxane species [4,5,17], as was already inferred from the IR spectra (Fig. 3). IR spectroscopy was, however, the technique that could differentiate between the two kinds of species, showing the prevalence of cubelike species in the case of TMSPIm+TFA and the

-30

-40

-50 -60 Chemical shift [ppm]

-70

-80

Fig. 7. 29Si NMR spectra of: (a) TMSPIm+TFA (electrolyte I)— analysed on the day of its preparation and (b) TMSPIm+AcOH (electrolyte II)—analysed 1 month after its preparation.

coexistence of cube- and ladder-like species in the TMSPIm+AcOH electrolyte. The 29Si NMR measurements also revealed that when non-aged electrolyte was analysed, T2 signals appeared in the spectrum in addition to the T3 signal (Fig. 7a). The T2 signal suggested the presence of various cyclic or open cube-like species with residual uncondensed functional groups (–OMe, silyl ester) [4,5,23–25]. When such a freshly prepared electrolyte was applied in the EC device, 24 h drying enabled the continuation of the condensation processes and the formation of the fully condensed silsesquioxane species. The adhesive properties of the electrolytes were excellent, preventing rupture of the dried EC cells. 3.3. In situ UV–vis spectroelectrochemical measurements Initially, the optical and electrical properties of EC devices produced from EC WO3 thin films, TMSPIm-based electrolytes I–III, and CeVO4 counter-electrodes sandwiched between two SnO2:F-glass substrates were tested. The range of CV measurements (2.5 to 2.5 V) was determined by CV cycling of the symmetric SnO2:F-glass/ electrolyte/SnO2:F-glass cells (Fig. 8). The potential window of 2.5 V was larger than that of the ionic liquid 1-butyl-3-methyl-imidazolium-tetrafluoroborate admixed with the sol–gel precursors MTMOS+H2O+H3PO4, i.e. 1.5 V [12]. The in situ UV–vis measurements of WO3/ electrolyte/CeVO4 devices revealed that the main features were similar (Fig. 9). When scanned from the initial potential (IP) of 0 V to the vertex potential of 2.5 V, the CV curves revealed two cathodic current waves (probably related to two different bounding sites in the WO3 film): the low-current density wave between 0 and 0.8 V and the dominating broad current wave around 1.8 V. The cathodic current waves were accompanied by a transmittance decrease that occurred as a result of the blue

ARTICLE IN PRESS A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135

132

colouration of the optically active WO3 electrode. In the reverse scan, the most intense anodic current wave developed approximately between 1.5 and 0.8 V, while the second, considerably less intense one, appeared around 1.4 V. Both current waves were connected with the increase in transmittance, i.e. bleaching of the WO3 films. The scan rate of 50 mV/s was too high and did not allow a clear

distinction between the two bleaching steps when the monochromatic transmittance at 634 nm was monitored (Fig. 9). However, such a distinction could be evidenced from the UV–vis spectra recorded at various potentials during CV measurement (Fig. 10). It was found, at least in regards to the second cycle, that larger optical changes accompanied the first anodic current wave. Namely, cycling of these EC devices revealed that the stability was not sufficiently good. Through the sixth cycle, the magnitude of the current density and the transmittance change slightly decreased (Fig. 9). The scanning of the electrode was performed at a scanning rate of 50 mV/s. Comparison of these EC devices can also be performed with regard to their photopic transmittance Tvis, i.e. their optical modulation in the visible spectral range weighted by the luminous efficiency of the light-adapted human eye [18,26]. In the bleached states, the devices constructed with a CeVO4 counter-electrode were found to be characterised by Tvis(bl) ranging from 54.9% to 60.1% (Fig. 11), the lowest value being determined by the EC device with electrolyte III (TMSPIm+AcOH+Li+). Similarly, Tvis values of the coloured states varied from 15.6% to 22.2%, with the lowest value again characterising the device with electrolyte III. The measurements revealed that the difference between Tvis(bl) and Tvis(col) remained constant for electrolyte II, while it decreased gradually for electrolytes I and III.

0.12 I: TMSPIm+TFA II: TMSPIm+AcOH

0.08

0.00 -0.04 -0.08 -0.12 -3

-2

-1

0 E [V]

1

2

3

Fig. 8. Potential window of TMSPIm-based electrolytes determined in the SnO2:F-glass/electrolyte/SnO2:F-glass systems. Scan rate used was 10 mV/s.

j [mA cm-2]

j [mA cm-2]

0.0 IP 2nd cycle 6th cycle

-0.5

-3

-2

-1

0

1

2

0.5

0.0 IP 2nd cycle 6th cycle

-0.5

3

-3

-2

-1

E [V]

0

1

2

Transmittance [%]

IP

60 40 2nd

20

cycle

6th cycle

0 -2

-1

0 E [V]

2nd cycle 6th cycle -3

3

-2

-1

1

2

3

0

1

2

3

E [V]

100

80

IP

60 40 2nd

20

cycle

6th cycle

80 IP

60 40

2nd cycle 6th cycle

20 0

0 -3

IP

-0.5

100

80

0.0

E [V]

100 Transmittance [%]

j [mA cm-2]

0.5

0.5

Transmittance [%]

j [mA cm-2]

0.04

-3

-2

-1

0 E [V]

1

2

3

-3

-2

-1

0

1

2

3

E [V]

Fig. 9. In situ UV–vis spectroelectrochemical measurements of EC devices composed of WO3/TMSPIm-based electrolyte/CeVO4: (A, B) TMSPIm+TFA (electrolyte I), (C, D) TMSPIm+AcOH (electrolyte II), and (E, F) TMSPIm+AcOH+Li+ (electrolyte III). (A, C, E) Scanning of two-electrode EC devices was performed with 50 mV/s from initial potential (IP) of 0 to 2.5 V, reversed and scanned to +2.5 V, and then reversed again to a final potential (FP) of 0 V. (B, D, E) Monochromatic transmittance was measured at 634 nm.

ARTICLE IN PRESS A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135

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Fig. 10. UV–vis spectra of EC devices composed of WO3/TMSPIm-based electrolyte/CeVO4 recorded at various potentials during in situ cyclovoltammetric (CV) measurements: (A, B) TMSPIm+TFA (electrolyte I), (C, D) TMSPIm+AcOH (electrolyte II), and (E, F) TMSPIm+ AcOH+Li+ (electrolyte III). Scanning of two-electrode EC devices was performed with 50 mV/s from IP of 0 to 2.5 V, reversed and scanned to +2.5 V, and then reversed again to final potential (FP) of 0 V.

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Electrolyte, counter-electrode Fig. 11. Photopic transmittance Tvis of WO3/TMSPIm-based electrolyte/counter-electrodes EC devices. The black columns denote the coloured states, the grey columns bleached states. The numbers on arrows denote the CV cycle number.

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Fig. 12. In situ UV–vis spectroelectrochemical measurements of EC devices with various counter-electrodes: (A, B) WO3/TMSPIm+AcOH/V2O5, (C, D) WO3/TMSPIm+AcOH+Li+/V2O5, and (E, F) WO3/TMSPIm+AcOH+Li+/V-Ti-oxide. Scanning of two-electrode EC devices was performed with 50 mV/s from an initial potential (IP) of 0 to 2.5 V, reversed and scanned to +2.5 V, and then reversed again to a final potential (FP) of 0 V.

Other counter-electrodes, such as V2O5 and Ti/V-oxide, were tested in EC devices with electrolytes II and III, in order to check their performance (Fig. 12). The measurements revealed significantly lower optical modulation than was found for EC devices with CeVO4 ion-storage films. It was shown that the optical modulation increased with the cycle number, at least for electrolyte III containing Li+ ions, but did not reach the Tvis values of devices with CeVO4 (Figs. 9 and 11). In part, the reason might be sought in the crystalline structure of V2O5 [15] and Ti/Voxide [16] films, with particle sizes—the factor critically influencing the functioning of EC devices with hydrophobic ionic liquids [11]—of up to 50 nm. The particle grain size in CeVO4 films did not exceed 20 nm. 4. Conclusions Proton-conducting electrolytes were synthesised by the sol–gel process using TMSPIm, initiated by the addition of either trifluoroacetic or acetic acid. The addition of carboxylic acids initiated solvolysis and condensation reactions, leading to formation and arrangement of imidazolium cations around silsesquioxane species. Predominantly cube-like silsesquioxane species formed when TFA was used. The electronegative nature of the fluorine ions stabilised the dissociated anion. As a result, the final electrolyte was an ionic liquid derivative with free trifluoroacetic anions, as demonstrated by IR spectro-

scopy. Hydrogen bonding was found to be established in the electrolyte prepared with acetic acid. The H-bonded hydrogen atom shared by the imidazolium nitrogen and acetate anion was positioned closer to the anion, which was reflected in IR band frequencies approaching the values of acetic acid (1712, 1412, and 1292 cm1). By means of cyclic voltammetry, an electrochemical window of 2.5 V was determined for EC devices produced from either electrolyte. The electrochemical stability of EC devices assembled from WO3/either electrolyte/CeVO4 turned out below expectation, partially as a result of the structure of the thin films with intercalation properties— mainly their morphology and particle size. This problem can be solved by stranding or by a network forming sol–gel agent like tetramethoxysilane. Dissolution of lithium salts or strong acids would contribute to the conductivity of the layer with an increased number of charge carriers. Acknowledgments This work was produced within the frame work of Programme P1-0030 and Project J1-5010 supported by the Slovenian Research Agency. V. Jovanovski would like to thank this agency for his Ph.D. grant. References [1] T. Welton, Chem. Rev. 99 (1999) 2071. [2] H. Ohno, M. Yoshizawa, W. Ogihara, Electrochim. Acta 50 (2004) 255.

ARTICLE IN PRESS A. Sˇurca Vuk et al. / Solar Energy Materials & Solar Cells 92 (2008) 126–135 [3] P. Bonhoˆte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Gra¨tzel, Inorg. Chem. 35 (1996) 1168. [4] V. Jovanovski, B. Orel, R. Jesˇ e, A. Sˇurca Vuk, G. Mali, S.B. Hocˇevar, J. Grdadolnik, E. Stathatos, P. Lianos, J. Phys. Chem. B 109 (2005) 14387. [5] B. Orel, R. Jesˇ e, A. Sˇurca Vuk, V. Jovanovski, L. Slemenik Persˇ e, M. Zˇumer, J. Nanosci. Nanotechnol. 6 (2006) 382. [6] B. Garcia, S. Lavalle´e, G. Perron, C. Michot, M. Armand, Electrochim. Acta 49 (2004) 4583. [7] W. Lu, A.G. Fadeev, B. Qi, E. Smela, B.R. Mattes, J. Ding, G.M. Spinks, J. Mazurkiewicz, D. Zhou, G.G. Wallace, D.R. MacFarlane, S.Z. Forsyth, M. Forsyth, Science 297 (2002) 983. [8] W. Lu, A.G. Fadeev, B. Qi, B.R. Mattes, Synth. Met. 135–136 (2003) 139. [9] W. Lu, A.G. Fadeev, B. Qi, B.R. Mattes, J. Electrochem. Soc. 151 (2004) H33. [10] R. Marcilla, F. Alcaide, H. Sardon, J.A. Pomposo, C. Pozo-Gonzalo, D. Mecerreyes, Electrochem. Commun. 8 (2006) 482. [11] M.-H. Delville, G. Campet, S. Duluard, I. Litas, H. Jung, Electrochromic devices based on hydrophobic ionic liquids, in: Abstracts, MRS Fall Meeting, Boston, 28 November –2 December 2005, Materials Research Society, Warrendale, PA, 2005, E2.1, p. 110. [12] Z. Li, H. Liu, Y. Liu, P. He, J. Li, J. Phys. Chem. B 108 (2004) 17512. [13] Y. Zhou, M. Antonietti, Chem. Mater. 16 (2004) 544. [14] U. Opara-Krasˇ ovec, R. Jesˇ e, B. Orel, J. Grdadolnik, G. Drazˇicˇ, Monatshefte Chem. 133 (2002) 1115. [15] A. Sˇurca, B. Orel, G. Drazˇicˇ, B. Pihlar, J. Electrochem. Soc. 146 (1999) 232.

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[16] A. Sˇurca, S. Bencˇicˇ, B. Orel, B. Pihlar, Electrochim. Acta 44 (1999) 3075. [17] B. Orel, R. Jesˇ e, A. Vilcˇnik, U. Lavrencˇicˇ Sˇtangar, J. Sol–Gel Sci. Technol. 34 (2005) 251. [18] F.W. Billmeyer, M. Saltzman, Principles of Color Technology, second ed., Wiley, New York, 1981, p. 2 (Chapter 2). [19] A. Potier, The hydrogen bond and chemical parameters favouring proton mobility in solids, in: Ph. Colomban (Ed.), Proton Conductors, Solids, Membranes and Gels—Materials and Devices, Chemistry of Solid State Materials, vol. 2, Cambridge University Press, Cambridge, MA, 1992, pp. 1–17. [20] A. Kawada, A.R. AcFhie, M.M. Labes, J. Chem. Phys. 52 (1970) 3121. [21] Ph. Colomban, A. Novak, Proton conductors: classification and conductivity, in: Ph. Colomban (Ed.), Proton Conductors, Chemistry of Solid State Materials, vol. 2, Cambridge University Press, Cambridge, MA, 1992, pp. 38–60. [22] A. Novak, Hydrogen bonding in solids, correlation of spectroscopic and crystallographic data, in: J.-H. Fuhrhop, G. Blauer, T.J.R. Weakley, A. Novak (Eds.), Structure and Bonding, Large Molecules, vol. 18, Springer, Berlin, 1974, pp. 177–216. [23] F. Brunet, J. Non-Cryst. Solids 231 (1998) 58. [24] A. Jitianu, A. Britchi, C. Deleanu, V. Badescu, M. Zaharescu, J. Non-Cryst. Solids 319 (2003) 263. [25] Y. Sugahara, S. Okada, S. Sato, K. Kuroda, C. Kato, J. Non-Cryst. Solids 167 (1994) 21. [26] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier Science, Amsterdam, 1995.