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Solar Energy 83 (2009) 108–112

An improved preparation of 3-ethyl-1-methylimidazoliumtrifluoroacetate and its application in dye sensitized solar cells

Chengwu Shi a,*, Qian Ge a, Fang Zhou a, Molang Cai a, Xiaoran Wang a,Xiaqin Fang b, Xu Pan b

a School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR Chinab Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, PR China

Received 29 February 2008; accepted 8 July 2008Available online 20 August 2008

Communicated by: Associate Editor Sam-Shajing Sun

Abstract

In this paper, we reported an improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate (EMITA), which proceeded via

efficient reaction of 1-methylimidazole and ethyl trifluoroacetate under solvent-free conditions using Teflon-lined, stainless steel auto-claves. It was shown that the procedure was simple and eco-friendly. The apparent diffusion coefficients of triiodide and iodide in binaryionic liquids, EMITA and 1-methyl-3-propylimidazolium iodide (MPII) with various weight ratios, were demonstrated by cyclic voltam-metry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide slightly increased and those ofiodide decreased with the weight ratio increase of EMITA and MPII. The dye sensitized solar cells with the electrolyte, which was com-posed of 0.13 M I2, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the binary ionic liquid electrolyte of EMITA and MPII (weight ratio 1:2),gave short circuit photocurrent density of 7.88 mA cm�2, open circuit voltage of 0.61 V, and fill factor of 0.67, corresponding to the pho-toelectric conversion efficiency of 3.22% at the illumination (Air Mass 1.5, 100 mW cm�2).� 2008 Published by Elsevier Ltd.

Keywords: 3-Ethyl-1-methylimidazolium trifluoroacetate; Preparation; Autoclave; Dye-sensitized solar cell

1. Introduction

During the past decades, dye sensitized solar cells(DSCs) have attracted much attention as low-cost alterna-tives to conventional p–n junction photovoltaic devices(O’Regan and Gratzel, 1991; Hagfeldt and Gratzel, 1995;Kambe et al., 2002; Dai et al., 2005, 2004; Shi et al.,2005,). Electrolytes are the critical components in DSCs.Some binary ionic liquid electrolytes have been investigateddue to their negligible volatility, low viscosity and highthermostability (Kuang et al., 2006; Fei et al., 2006; Wanget al., 2004; Ryuji et al., 2004). Usually, the preparation ofalkylimidazolium trifluoroacetate is the metathesis reaction

0038-092X/$ - see front matter � 2008 Published by Elsevier Ltd.

doi:10.1016/j.solener.2008.07.002

* Corresponding author. Tel./fax: +86 551 2901450.E-mail address: shicw506@mail.hf.ah.cn (C. Shi).

between alkylimidazolium halide and silver trifluoroacetatein aqueous solution or the quaternization reaction of theappropriate alkyl trifluoroacetate and alkylimidazole deriv-atives via a conventional heating method in refluxingorganic solvents. The metathesis approach is obviouslylimited by the relatively high cost of silver salts and thelarge quantities of solid by-product. Although there is thevery low solubility of these silver halides (AgCl, AgBr,AgI) in aqueous solution, the resulting product has beenshown to contain residual silver (Rika and Yasuhiko,2000; Bonhote et al., 1996; Wasserscheid and Welton,2002). The quaternization procedure requires an excess ofalkyl trifluoroacetate and several days in refluxing organicsolvents to afford reasonable yields (Bonhote et al., 1996).

In this paper, we reported an improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate (EMITA, 35 cP

C. Shi et al. / Solar Energy 83 (2009) 108–112 109

at 20 �C), which was the quaternization reaction of ethyltrifluoroacetate and 1-methylimidazole under solvent-freeconditions by Teflon-lined, stainless autoclaves. The redoxbehavior of I-/I3

- in the binary ionic liquids of EMITA and1-methyl-3-propylimidazolium iodide (MPII) was investi-gated by cyclic voltammetry using a Pt ultramicroelectrode.The photovoltaic performances of DSCs employing thebinary ionic liquid electrolytes with various weight ratiosof EMITA and MPII were measured.

2. Experimental

2.1. Materials

Anhydrous lithium iodide and 4-tert-butylpyrdine(TBP) were purchased from Fluka and used as received.1-Methylimidazole, ethyl trifluoroacetate and 1,1,1-trichlo-roethane were commercially available and were distilledunder vacuum before used. 1-Methyl-3-propylimidazoliumiodide (MPII) was synthesized according to our previousreport (Shi et al., 2007).

2.2. Preparation and characterization

The mixture of 1-methylimidazole (50.0 mmol) andethyl trifluoroacetate (52.5 mmol) was placed in a 57 mLTeflon-lined, stainless steel autoclave (its structure is simi-lar to the literature (Walton, 2002)), and then was heatedin an oven at 100 �C for 3, 6, 12 and 24 h, respectively.The resulting EMITA was decanted and dried under vac-uum at 70 �C for 6 h. Some crude products were washedwith 1,1,1-trichloroethane before drying, and the weightratio of crude products and 1,1,1-trichloroethane was 1:2in every washing.

1H NMR spectra of EMITA were measured on a BrukerDMX 300 spectrometer using acetone-d6 as solvents. Elec-trospray ionization mass spectra (ESI-MS) were recordedon a Thermofinnigan LCQ Advantage Max, electrospraysource, ion trap instrument on a sample diluted inmethanol.

2.3. Measurement

Conductivity of the mixture of EMITA and MPII weremeasured by lab conductivity meter (DDS-307) with twoplatinum black electrode conductance cell (DJS-1, REX,the area of each platinum black electrode was about0.25 cm2, the distance between two platinum black elec-trodes was about 0.50 cm, the cell constant was 0.974,determined by aqueous KCl standard solution). Cyclic vol-tammetry was achieved by using an electrochemical work-station (CHI660B). The working electrode was a 5.0 lmradius Pt disk ultramicroelectrode (CHI107), the auxiliaryelectrode and the reference electrode were a 1.0 mm radiusPt disk electrode (CHI102). A slow scan rate at 5 mV s�1

was used to obtain steady-state current voltage curves.The apparent diffusion coefficient (Dapp) of triiodide and

iodide in the mixture of EMITA and MPII can be calcu-lated from the steady-state current (Iss) according to thefollowing equation, Iss = 4ncaFDapp (Wang et al., 2004,2003). The photovoltaic performances of DSCs were mea-sured with a Keithley 2420 source meter controlled by Test-point software under solar simulator (Xenon lamp, AirMass 1.5, 100 mW cm�2, Changchun institute of OpticsFine Mechanics and Physics, Chinese Academy of Science,calibrated with standard crystalline silicon solar cell).

2.4. DSCs assembly

Briefly, the TiO2 (anatase) colloidal solution was pre-pared by hydrolysis of titanium tetra-isopropoxide (Huet al., 2003). The TiO2 paste was printed on transparentconducting glass sheets (TEC-8, LOF) and sintered in airat 450 �C for 30 min. The film was 10 lm thick, whichwas determined by profilometer (XP-2, AMBIOS Technol-ogy Inc., USA). A 4-lm-thick light scattering layer wasused. After cooling to 80 �C, the TiO2 films were immersedin a anhydrous ethanol solution with 5.0 � 10�4 M cis-dithiocyanate-N,N0-bis-(4-carboxylate-4-tetrabutylammo-nium carboxylate-2,20-bipyridine) ruthenium (II) (N719)overnight. The excess of N719 dye in TiO2 films was rinsedoff with anhydrous ethanol before assembly. The counterelectrode was platinized by spraying H2PtCl6 solution totransparent conducting glass and fired in air at 410 �Cfor 20 min. Then, it was placed directly on the top of thedye-sensitized TiO2 films. The gap between the two elec-trodes was sealed by thermal adhesive (Surlyn, Dupont).The electrolyte was filled from a hole made on the counterelectrode and the hole was later sealed by a cover glass andthermal adhesive films. The total active electrode areas ofDSCs was 0.25 cm2.

3. Results and discussion

3.1. Preparation of EMITA

Fig. 1 shows the 1H NMR of EMITA prepared at100 �C for different reaction times employing a 5% molarexcess of ethyl trifluoroacetate. Table 1 summarized theyield evolution of EMITA with reaction times. With theincrease of reaction times, the content of the residual 1-methylimidazole decreased, the yield increased, and thecolour became darker in the resulting EMITA. Theamount of impurity resulting in darker EMITA was gener-ally small, and was difficult to be detected by 1H NMR.Therefore, 12 h was the optimal reaction time for prepara-tion of EMITA under solvent-free condition at 100 �C.

Fig. 2 shows the 1H NMR of EMITA before and afterwashing with 1,1,1-trichloroethane. It was found that thecontent of the residual 1-methylimidazole in the resultingEMITA decreased with the washing times increased. More-over, some impurity in the resulting EMITA can bedetected by MS, which may be due to the corrosion of Tef-lon-lined, stainless steel autoclaves, and can be removed by

110 C. Shi et al. / Solar Energy 83 (2009) 108–112

washing with 1,1,1-trichloroethane for four times. It wasadvisable that washing was useful for obtaining pureEMITA.

10 8 6 4 2 0

d

c

b

a

Chemical Shift/ppm

Fig. 1. 1H NMR chemical shifts of 3-ethyl-1-methylimidazolium trifluo-roacetate in acetone-d6. Reaction temperature: 100 �C. Reaction time: (a)3 h; (b) 6 h; (c) 12 h and (d) 24 h.

Table 1The yield evolution of 3-ethy-l-methylimidazolium trifluoroacetate withreaction times

1-Methylimidazole(mol)

Ethyl trifluoroacetate(mol)

Time(h)

Yield(%)

1 1.05 3 641 1.05 6 851 1.05 12 931 1.05 24 �100

10 8 6 4 2 0

c

b

a

Chemical shift/ppm

Fig. 2. 1H NMR chemical shifts of 3-ethyl-1-methylimidazolium trifluo-roacetate in acetone-d6: (a) before washing; (b) after washing twice and (c)after washing four times.

3.2. Influence of the weight ratio of EMITA and MPII on the

redox behavior of triiodide and iodide

Fig. 3 illustrated the relationship between the conductiv-ity of binary ionic liquid mixture and the weight ratios ofEMITA and MPII at 20 �C. With the weight ratiosincreased, the mixture conductivity increased, which maybe associated with the decrease of the mixture viscosity.

Table 2 listed the apparent diffusion coefficient of triio-dide and iodide in binary ionic liquid mixtures at 20 �C.With the increase of weight ratios of EMITA and MPII,the apparent diffusion coefficients of triiodide slightlyincreased and those of iodide decreased.

3.3. Photovoltaic performances of DSCs

Table 3 listed the photovoltaic performances of DSCswith electrolytes A–F and Fig. 4 shows the photocurrent–voltage curve of DSCs with electrolyte A, 0.13 M I2,0.10 M LiI, 0.50 M TBP in the mixture of EMITA andMPII (weight ratio 1:2). The open circuit voltage (Voc)increased with the weight ratio increase of EMITA andMPII in the electrolytes A–D. This was consistent withthe literature (Ryuji and Masayoshi, 2003). That is, theequilibrium potentials of I-/I3

- and open-circuit voltagesof DSCs increased with the decrease of total concentrations([I-] + [I3

-]) in the ionic liquid electrolytes. Moreover, theshort circuit photocurrent density, fill factor and overallconversion efficiency decreased from electrolytes A–C,

0 1 2 3 4

2

4

6

8

10

Con

duct

ivity

(m

S/c

m)

m(EMITA) : m( MPII)

Fig. 3. The relationship between the conductivity of binary ionic liquidmixtures and the weight ratios of EMITA and MPII at 20 �C.

Table 2Apparent diffusion coefficient of triiodide and iodide in binary ionic liquidmixtures at 20 �C

m(EMITA): m(MPII) 0.33 0.50 1.00 2.00 4.00

Dapp(I3-, 10�7 cm2 s�1) 2.90 3.01 3.12 3.17 3.22

Dapp(I�, 10�7 cm2 s�1) 1.21 1.09 1.00 0.84 0.55

Table 3Photovoltaic performances of DSCs with electrolytes A–F

Electrolytea Jsc (mA cm�2) Voc (V) FF g (%)

A. m(EMITA): m(MPII) = 0.50, 0.50 M TBP 7.88 0.61 0.67 3.22B. m(EMITA): m(MPII) = 1.00, 0.50 M TBP 6.76 0.63 0.67 2.87C. m(EMITA): m(MPII) = 2.00, 0.50 M TBP 5.31 0.64 0.64 2.16D. m(EMITA): m(MPII) = 4.00, 0.50 M TBP 5.30 0.66 0.66 2.32E. m(EMITA): m(MPII) = 0.50, without TBP 6.19 0.47 0.64 1.88F. m(EMITA): m(MPII) = 2.00, without TBP 4.68 0.58 0.62 1.71

a The other components of electrolytes are the same, 0.13 M I2, 0.10 M LiI. Illumination: Air Mass 1.5, 100 mW cm�2. Cell active area of DSCs:0.25 cm2.

0.0 0.2 0.4 0.60

2

4

6

-2

Voltage / V

Phot

ocur

rent

den

sity

/ m

A cm

Fig. 4. The photocurrent–voltage curve of DSCs with electrolyte A.

C. Shi et al. / Solar Energy 83 (2009) 108–112 111

which may be associated with the decrease of the apparentdiffusion coefficients and content of iodide. Comparingelectrolyte with and without TBP, it was demonstrated thatthe TBP can improve the performance of the DSCs withthe binary ionic liquid electrolytes.

4. Conclusion

In this paper, an improved preparation of EMITAunder solvent-free conditions by Teflon-lined, stainlessautoclaves was described. It was shown that the procedurewas simple and eco-friendly. The DSCs with the electrolyte,which was composed of 0.13 M I2, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the binary ionic liquid electrolyte ofEMITA and MPII (weight ratio 1:2), gave short circuitphotocurrent density of 7.88 mA cm�2, open circuit volt-age of 0.61 V, and fill factor of 0.67, corresponding to thephotoelectric conversion efficiency of 3.22% at the illumi-nation (Air Mass 1.5, 100 mW cm�2).

Acknowledgments

This work is financially supported by Hefei Universityof Technology and the National Key Project of Chinafor Basic Research (2006CB202600).

Appendix A. Supplementary data

Data of 1H NMR and MS, the picture of autoclaves, thephotos of products, stead-state voltammograms can befound in the online version. Supplementary data associatedwith this article can be found, in the online version, atdoi:10.1016/j.solener.2008.07.002.

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