Electrochimica Acta 51 (2006) 4243–4249
Gel polymer electrolyte based on poly(acrylonitrile-co-styrene) and a novel organic iodide salt for quasi-solid state dye-sensitized solar cell Jihuai Wu a,∗ , Zhang Lan a , Dongbo Wang a , Sancun Hao a , Jianming Lin a , Yunfang Huang a , Shu Yin b , Tsugio Sato b b
a Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Received 1 September 2005; received in revised form 22 November 2005; accepted 30 November 2005 Available online 17 January 2006
Abstract A gel polymer electrolyte based on poly(acrylonitrile-co-styrene) as polymer matrix and N-methyl pyridine iodide salt as I− source was prepared. Controlling the concentration of polymer matrix of poly(acrylonitrile-co-styrene) at 17.5 wt.%, mixing the binary organic solvents mixture ethylene carbonate and propylene carbonate with 6:4 (w/w), and the concentration of N-methyl pyridine iodide and iodine with 0.5 and 0.05 M, respectively, the gel polymer electrolyte attains the maximum ionic conductivity (at 30 ◦ C) of 4.63 mS cm−1 . Based on the gel polymer electrolyte, a quasi-solid state dye-sensitized solar cell was fabricated and its overall energy conversion efficiency of light-to-electricity of 3.10% was achieved under irradiation of 100 mW cm−2 . © 2005 Elsevier Ltd. All rights reserved. Keywords: Quasi-solid state dye-sensitized solar cells; Gel polymer electrolyte; N-Methyl pyridine iodide; Poly(acrylonitrile-co-styrene); Ionic conductivity
1. Introduction Dye-sensitized solar cells (DSSCs) are one of the promising candidates for the next generation of solar cells because of their simple structure with relatively high conversion efficiencies, inexpensive fabrication procedures in contrast to amorphous silicon [1,2]. Although DSSCs based on liquid electrolytes have already achieved high conversion efficiencies [3], they also caused some substantial problems to put DSSCs into practical uses. For example, their high volatilities, solvent losses occur during long-term operations, resulting in decreases in DSSCs performances [4]. To overcome these problems, much effort has been made to replace the liquid electrolytes with solid or quasi-solid type charge transport materials [5–8]. Compared with other kinds of charge transport materials, the gel polymer electrolytes have some advantages including high ionic conductivities which are achieved by “trapping” a liquid electrolyte in polymer cages
∗
Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22693999. E-mail address:
[email protected] (J. Wu).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.11.047
formed in a host matrix, good contacting and filling properties of the nanostructured electrode and counter electrode. Therefore, the gel polymer electrolytes have been attracting intensive attention. Up to the present, several types of gel electrolytes based on different kind of polymers have already been used in quasi-solid state dye-sensitized solar cells [8–10]. On the other hand, gel polymer electrolyte is constructed by trapping liquid electrolytes which usually contain organic solvents and inorganic salts such as propylene carbonate (PC), ethylene carbonate (EC) or acetonitrile (ACN), lithium iodide (LiI), sodium iodide (NaI) and potassium iodide (KI). However, there are some problems in these gel systems. For example, these organic solvents are relatively bad solvents for alkaline iodide salts and crystallization of alkaline iodide salts in these gel systems have been a source of deterioration of the cell [11,12]. Therefore many groups make great efforts to substitute metal cations by organic cations or using ionic liquids as the source of I− [13–16]. In this paper, a novel organic iodide salt N-methyl pyridine iodide was synthesized, by using the N-methyl pyridine iodide as I− source, poly(acrylonitrile-co-styrene) as polymer matrix, a gel polymer electrolyte was prepared. Owing to N-
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methyl pyridine iodide is a kind of organic iodide salt, it will be easier to dissolve the N-methyl pyridine iodide salt in organic solvents than the alkaline iodide salts. On the other hand, Nmethyl pyridine iodide has large cation, I− can easily separate from the molecule [17]. Therefore, the gel polymer electrolyte containing the novel iodide salt will attain relatively high conductivity. Quasi-solid state dye-sensitized solar cells prepared by using the gel polymer electrolyte will show relatively good performance. 2. Experimental 2.1. Materials Poly(acrylonitrile-co-styrene) was commercially obtained from chemicals company, China and used without further purification. Tetrabutyltitanate, titanium tetrachloride, pyridine, methyl iodide, iodine, ethylene carbonate (EC) and propylene carbonate (PC) were all A.R. grade and all purchased from Xi Long Chemicals. All reagents were used without further treating before using. Conducting glass plates (FTO glass, fluorine doped tin oxide over-layer, sheet resistance 8 cm−2 , purchased from Hartford Glass Co., USA) were used as a substrate for precipitating TiO2 porous film on and were cut into 2 cm × 1.5 cm sheets. Sensitizing dye is cis-dithiocyanate-N,N -bis-(4-carboxylate4-tetrabutylammoniumcarboxylate-2,2 -bipyridine) ruthenium (II) (N719, Ruthenium TBA 535, Solaronix SA). 2.2. Synthesis of N-methyl pyridine iodide and preparation of gel polymer electrolyte N-methyl pyridine iodide was synthesized by adding 0.5 mol pyridine into 0.5 mol methyl iodide slowly under vigorous stirring until the system became solid state at ambient condition. The salt was washed up with anhydrous ethanol to remove excess unreacted solvents and then the salt was kept in the vacuum oven at 40 ◦ C for 24 h. Fig. 1 shows the chemical equation of the reaction. Appropriate amount of N-methyl pyridine iodide and iodine were added into binary organic solvents mixture containing ethylene carbonate (EC) and propylene carbonate (PC) under stirring to form a homogeneous liquid electrolyte. The suitable amount of poly(acrylonitrile-co-styrene) was added into the above liquid electrolyte. Then the resulting mixture was heated in a closed flask at 120 ◦ C to dissolve the polymer matrix, followed by cooling down to room temperature to form gel state electrolyte.
Fig. 1. The synthetic reaction chemical equation of N-methyl pyridine iodide.
2.3. Assembling of the quasi-solid state dye-sensitized solar cell Nanoporous TiO2 film was manufactured by the following procedure. Tetrabutyltitanate (20 ml) was rapidly added to distilled water (200 ml) and a white precipitate was formed immediately. The precipitate was filtered using a glass frit and washed three times with 100 ml distilled water. The filter cake was added to nitric acid aqueous solution (0.1 M, 200 ml) under vigorous stirring at 80 ◦ C until the slurry became a translucent bluewhite liquid. The resultant colloidal suspension was autoclaved at 200 ◦ C for 12 h to form milky white slurry. The resultant slurry was concentrated to 1/4 of its volume, then PEG-20000 (10 wt.% slurry) and a few drops of emulsification regent of Triton X-100 was added to form a TiO2 colloid. A conducting glass sheet (FTO) was immersed in an isopropanol solution for 48 h to remove any impurities. Then it was cleaned in Triton X-100 aqueous solution, washed with ethanol, and treated with 50 mM TiCl4 aqueous solution at 70 ◦ C for 30 min to make a good mechanical contact between TiO2 layer and conducting glass matrix. A plastic adhesive tape was fixed on the four sides of conducting glass sheet to restrict the thickness and area of TiO2 porous film. The TiO2 colloid was dropped on the FTO glass plate by using a doctor scraping technique. The process was done for three times to form a thick TiO2 film about 6–8 m. The TiO2 film was treated with 50 mM TiCl4 aqueous solution again and washed with distilled water before sintered at 450 ◦ C in air for 30 min. Finally, the TiO2 porous film was sintered by firing the conducting glass sheet at 450 ◦ C in air for 30 min. After cooled to 80 ◦ C, the TiO2 film was immersed in a 2.5 × 10−4 M absolute ethanol solution of cis-[(dcbH2 )2 Ru(SCN)2 ] for 24 h to absorb the dye adequately, then the dyesensitized TiO2 film was washed up with anhydrous ethanol and dried in moisture-free air. Quasi-solid state dye-sensitized solar cell was assembled by injecting the melted gel polymer electrolyte into the aperture between the TiO2 porous film electrode (anode electrode) and a Pt plated conducting glass sheets (cathode electrode, prepared by electrodepositing). The two electrodes were clipped together and a cyanoacrylate adhesive was used as sealant to prevent the electrolyte solution from leaking. Epoxy resin was used for further sealing the cell to measure the stability of the cell. 2.4. Measurements Fourier transform infrared spectrum (FTIR) of sample was measured with a Nicolet Nexus 470 spectrometer, the sample being in the form of KBr pellets. The ionic conductivity of gel polymer electrolytes was measured by using model DDB-6200 digitized conductivity meter (Shanghai Reici Instrument Factory, China). The instrument was calibrated with 0.1 M KCl aqueous solution prior to experiments. The photovoltaic test of quasi-solid state dye-sensitized solar cells was carried out by measuring the J–V character curves under irradiation of white light from a 100 W xenon arc lamp (XQ-100W, Shanghai Photoelectricity Device Company, China)
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Fig. 2. FTIR spectrum of pyridine (a) and N-methyl pyridine iodide (b).
under ambient atmosphere. The incident light intensity and the active cell area were 100 mW cm−2 and 0.8 cm2 , respectively. The photoelectronic performances [i.e. fill factor (FF) and overall energy conversion efficiency (η)] were calculated by the following equations [18]: FF =
Vmax Jmax Voc Jsc
η (%) =
Vmax Jmax Voc Jsc FF × 100 = × 100 Pin Pin
(1)
(2)
3.2. Influence of the concentration of polymer matrix on the conductivity of electrolyte Fig. 3 shows the dependence of the ionic conductivity at 30 ◦ C on the polymer concentration in the gel polymer electrolyte. It is found that the ionic conductivity decreases with the increase of the polymer concentration. It is known that gel polymer electrolyte is formed by trapping liquid electrolyte in polymer cages. When adding larger amount of polymer in gel polymer electrolyte, the cages in gel polymer electrolyte become smaller which result in hindering ionic movement, so the ionic conductivity decreases with the increase of the polymer.
where Jsc is the short-circuit current density (mA cm−2 ), Voc is the open-circuit voltage (V), Pin is the incident light power, and Jmax (mA cm−2 ) and Vmax (V) are the current density and voltage in the J–V curves, respectively, at the point of maximum power output. 3. Results and discussion 3.1. Infrared spectra of N-methyl pyridine iodide The FTIR spectrum of N-methyl pyridine iodide is shown in Fig. 2. In order to compare with the pure pyridine, the FTIR spectrum of pyridine is also shown in there. It can be found that the FTIR spectrum of N-methyl pyridine iodide is relatively similar with the spectrum of pyridine except some different absorption peaks. Because of the influence of N, the absorption of alkyl C–H blending in the group of H3 C–N can present a characteristic absorption between 1460 and 1430 cm−1 [19]. So the absorption of 1442 cm−1 can be used to prove the compound has a group of H3 C–N. The C–H stretching vibration in the group of CH3 is dominant at 2987 and 2966 cm−1 .
Fig. 3. Dependence of the conductivity on the concentration of poly(acrylonitrile-co-styrene) matrix in the gel polymer electrolyte. Gel polymer electrolyte consists of 0.5 M N-methyl pyridine iodide, 0.05 M I2 and binary organic solvents mixture containing EC and PC [5/5 (w/w)].
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Fig. 4. . Influence of EC concentration on the ionic conductivity of gel polymer electrolyte. Gel polymer electrolytes contains 0.5 M N-methyl pyridine iodide and 0.05 M I2 .
Fig. 5. Effect of the concentration of N-methyl pyridine iodide on the conductivity (at 30 ◦ C) for the gel polymer electrolyte, the concentration of I2 is 10 mol% of N-methyl pyridine iodide.
It is noticeable that with the decrease of the polymer concentration in the system, the ionic conductivity increases but the system changes from gel state to viscosity polymer solution. The viscosity polymer solution cannot hold liquid electrolyte stably as gel polymer electrolyte. Therefore, the polymer concentration of 17.5 wt.% was chosen, at this concentration, a gel state polymer electrolyte with high stability and high ionic conductivity of 4.11 mS cm−1 can be formed and kept.
binary organic solvents containing EC and PC [6:4 (w/w)] was chosen in the following experiments.
3.3. Influence of the compositions of solvents on the conductivity of electrolyte Fig. 4 exhibits the dependence of the ambient conductivity on EC content in the binary organic solvents mixture containing EC and PC. As shown in Fig. 3, the amount of EC content has great influence on the ambient ionic conductivity of gel polymer electrolyte. The ambient ionic conductivity increases when EC content is increased from 30 to 60 wt.% and achieves the maximum at 60 wt.%. However, above 60 wt.%, the ionic conductivity begins to decrease. The following reason can used to explain the influence of the compositions of solvents on the ionic conductivity of the gel polymer electrolyte. Although EC has high dielectric constant [εr = 90 (40 ◦ C)], pure EC is tend to crystal and is solid state in ambient temperature. In order to obtain higher ionic conductivity, the relatively low dielectric constant solvent of PC [εr = 65 (25 ◦ C)] is used to solve EC to form binary organic solvents mixture [17,20]. On account of high dielectric constant of EC, the more amount of EC used, the higher dielectric constant of binary organic solvents mixture can be attained. It is known that the high dielectric constant is good for salts to separate to form free solvation ions and afford more charge carrier [20,21]. Therefore, the ionic conductivity increases with increase in amount of EC in the binary organic solvents mixture. However, higher content of EC will lead it to crystal, so the ionic conductivity decreases with further increase of EC. The highest ionic conductivity can be obtained when the system contains 60 wt.% of EC, so the
3.4. Influence of the concentration of N-methyl pyridine iodide on the conductivity Variation of the ionic conductivity at 30 ◦ C as a result of changing N-methyl pyridine iodide concentration is shown in Fig. 5. The ionic conductivity originally increases and attains the maximum. The highest ionic conductivity about 4.63 mS cm−1 is attained at the concentration of N-methyl pyridine iodide of 0.5 M. Then the ionic conductivity decreases with the increase in the concentration. This tendency can be explained by the pair-ions model [20] and the phase disengagement between polymer matrix and liquid electrolyte, which is caused by effect of ions. Fig. 6 shows the state of ions in solvent according to pair-ions model. When adding a small amount of N-methyl pyridine iodide into gel polymer electrolyte, it can form homogenous system. The state of ions in the gel polymer electrolyte is solvation. In this case, the higher N-methyl pyridine iodide concentration can give the more charge carriers and result in increasing of the value of the ambient ionic conductivity. However, when adding more than 0.5 M N-methyl pyridine iodide into gel polymer electrolyte, the state of the most ions in the gel polymer electrolyte changes from solvation ions to contact ions. It can be explained by the reactions
Fig. 6. The state of ions in solvent according to pair-ions model.
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and ionic conductivity of gel polymer electrolytes. As is apparent from Fig. 4, the Arrhenius Eq. (3) can be used to describe the conductivity–temperature behavior of the gel polymer electrolyte: −Ea (3) σ(T ) = A exp RT where Ea is the activation energy, R is the molar gas constant, A is a constant and T is absolute temperature. 3.6. Influence of N-methyl pyridine iodide and iodine on photoelectronic properties of DSSCs
Fig. 7. Temperature dependence of the ionic conductivity (σ) for gel polymer electrolyte. Gel polymer electrolytes with 0.5 M N-methyl pyridine iodide and 0.05 M I2 .
(I) and (II) where S represents the organic liquid solvent. In this case, the dominant reaction changes from (II) to (I). It is known that the contact ions are hard to move which are bad for ionic conductivity. Therefore, the value of ambient ionic conductivity decreases especially on the situation of adding more than 1 M N-methyl pyridine iodide into the gel polymer electrolyte. On the other hand, the higher N-methyl pyridine iodide concentration in the gel polymer electrolyte causes the shrinkage of the polymer chains and a phase disengagement between polymer matrix and liquid electrolyte occurs which results in many ions remaining in liquid electrolyte. It causes further decrease of the ionic conductivity of the gel polymer electrolyte. +
−
+ −
+
−
+
(C6 H8 N )Sn + I → (C6 H8 N I )Sn−m + mS (C6 H8 N )Sn + I → (C6 H8 N )Sn I
−
(I) (II)
3.5. Influence of temperature on the conductivity of electrolyte As shown in Fig. 7, the ionic conductivity increases with the increase of temperature. A reasonable explanation is that the polymer matrix is amorphous and has large amounts of free volume cages. These free volume cages increase with increase in temperature just as described in the free-volume model [22]. This makes for increasing of the ionic mobility
In order to investigate the influence of the concentration of N-methyl pyridine iodide on the photoelectronic properties of the dye-sensitized solar cell, a series of cells were fabricated by sandwiching the gel polymer electrolyte between two electrodes. It can be seen in Table 1, the maximum photoelectronic values were obtained by using the highest ionic conductivity of gel polymer electrolyte containing 0.5 M N-methyl pyridine iodide. It can be concluded that the photoelectronic performance of the cells was deeply dependent on the N-methyl pyridine iodide concentration in gel polymer electrolyte. Higher Jsc was attained by using higher ionic conductivity of gel polymer electrolyte. The open-circuit voltage decreases with increase in N-methyl pyridine iodide concentration. The behavior of the open-circuit voltage with varying Nmethyl pyridine iodide concentration can be explained by following redox reactions and relation [4]: I2 + I− ↔ I3 − I3 − + 2e−
(a)
dark cathode
3I−
(b)
3I− −→ I3 − + 2e−
(c)
−→
photoanode
dark reaction on TiO2
I3 − + 2e− (CB) −→ 3I− Iinj kT Voc = ln e ncb ket [I3 − ]
(d) (4)
where k is the Boltzmann constant, T is the absolute temperature, e is the electron energy, Iinj is the flux of charge resulting from sensitized injection and ncb is the concentration of electrons at the TiO2 surface, ket is reaction rate constant of I3 − dark reaction on TiO2 , [I3 − ] is the concentration of I3 − in gel polymer electrolyte.
Table 1 The photoelectronic properties of the quasi-solid state dye-sensitized solar cellsa N-Methyl pyridine iodide concentration (M)
Ionic conductivity (mS cm−1 )
Jsc (mA cm−2 )
Voc (mV)
FF
η (%)
0.1 0.25 0.5 1 2
1.28 2.83 4.63 4.43 1.44
3.18 6.16 7.82 6.64 2.82
737 720 708 679 645
0.46 0.43 0.56 0.45 0.52
1.08 1.91 3.10 2.03 0.95
a Cells were assembled with gel polymer electrolytes containing various concentration of N-methyl pyridine iodide and iodine. The proportion of N-methyl pyridine iodide and iodine was kept in 10/1 (mol/mol).
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Fig. 8. Isc and Voc curve vs. iodine concentration for gel polymer electrolytes containing 0.5 M N-methyl pyridine iodide.
The higher content N-methyl pyridine iodide and iodine causes the higher concentration of I3 − in the gel polymer electrolyte according to reaction (a). And the reaction (d) becomes important [23]. It is known that Voc decreases with increase in current of dark reaction. According to relation (4), the opencircuit voltage decreases with increase of [I3 − ]. The influence of iodine concentration in gel polymer electrolyte with 0.5 M N-methyl pyridine iodide on the photoelectronic performance of quasi-solid state dye-sensitized solar cells was exhibited in Fig. 8. Jsc increases rapidly by adding small amount of iodine and the maximum is achieved by adding 0.05 M iodine into gel polymer electrolyte. While for larger iodine concentration Jsc decreases with increase in iodine concentration. Voc initially decreases with adding 0.025 M iodine in gel polymer electrolytes. For further increase of iodine concentration from 0.025 to 0.2 M, Voc changed in a relatively small range. The variation of photovoltaic performance with different amount of iodine also can be explained by the above redox reactions and relation. Increase of iodine concentration results in favor of I3 − according to reaction (a). In the first instance, a critical level of I3 − is necessary for cell functioning, reactions (b) and (c) are put in effect, so the Jsc rapidly increases and soon achieves a maximum. However, when adding more amount of iodine in gel polymer electrolyte, the reaction (d) becomes important [23], resulting in decrease of Jsc . For the variation of Voc , [I3 − ] increases by adding amount of iodine. According to relation (4), the open-circuit voltage decreases with increase of [I3 − ]. However, when the amount of iodine changes from 0.025 to 0.05 M, the reactions (b) and (c) are all put in effect and kept balance, so the dark reaction is comparatively feeble. This is the reason for Voc changed in a relatively small range. For further increase of iodine, the dark reaction becomes important, and [I3 − ] becomes larger, so the Voc decreases rapidly. 3.7. Photovoltaic characterization of quasi-solid state dye-sensitized solar cells The photocurrent versus photovoltage characteristics for the cell by sandwiching the gel polymer electrolyte with 0.5 M N-
Fig. 9. Photocurrent–photovoltage curve for the quasi-solid state solar cell fabricated using gel polymer electrolyte with 0.5 M N-methyl pyridine iodide and 0.05 M iodine.
methyl pyridine iodide and 0.05 M I2 was shown in Fig. 9. For an incident solar light intensity of 100 mW cm−2 , the open circuit voltage (Voc ) and the short circuit current density (Jsc ) values were 708 mV and 7.82 mA cm−2 , respectively. The fill factor (FF) and overall energy conversion efficiency (η) of the cell were calculated to be 0.56 and 3.10%. The energy conversion efficiency is almost the same value compared with the PANbased gel polymer electrolyte, which uses tetrapropyl ammonium iodide (Pr4 N+ I− ) as I− source. But it is lower than other authors reported by using ionic liquid [13,24]. It may be due to the low quality of TiO2 film. The wet cell based on liquid electrolyte containing 0.5 M KI, 0.05 M I2 in PC solvent and using the same TiO2 film was also prepared for comparison. The highest energy conversion efficiency was 4.86%. Therefore, by further optimizing the TiO2 film and gel polymer electrolyte, higher energy conversion efficiency will be obtained. The stability of the quasi-solid state cell was also measured by comparing with the cells without extra sealed and with extra sealed. After 2 months, the overall energy conversion efficiency of the cell without extra sealed obtained only 39% of the fresh one. However, the cell with extra sealed kept 81% of the fresh one. 4. Conclusions A gel polymer electrolyte based on poly(acrylonitrile-costyrene) as polymer matrix and N-methyl pyridine iodide salt as I− source was prepared. The influence of the compositions and concentration of polymer matrix, solvent and N-methyl pyridine iodide, as well as temperature on the ionic conductivity of the gel polymer electrolyte was investigated. When the concentration of polymer matrix of poly(acrylonitrile-co-styrene) is controlled at 17.5 wt.%, the binary organic solvent is mixed with EC and PC in the ratio of 6:4 (w/w), and the concentration of N-methyl pyridine iodide and iodine are adjusted as 0.5 and 0.05 M, respectively, the maximum ionic conductivity (at 30 ◦ C) of 4.63 mS cm−1 was achieved. Based on the novel gel polymer electrolyte, a quasi-solid state dye-sensitized solar cell
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was fabricated and its overall energy conversion efficiency of light-to-electricity of 3.10% was achieved under irradiation of 100 mW cm−2 . Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (no. 50572030, no. 50372022 and no. 50082003). References [1] A. Hagffeldt, M. Gratzel, Chem. Rev. 95 (1995) 49. [2] A. Goetzberger, J. Luther, G. Willeke, Sol. Energy Mater. Sol. Cells 74 (2002) 1. [3] M.K. Nazeeruddin, A. Kay, I. Rodicio, M. Gr¨atzel, J. Am. Chem. Soc. 115 (1993) 6382. [4] N. Papageorgiou, W. Maier, M. Gratzel, J. Electrochem. Soc. 144 (1997) 876. [5] B. O’Regan, F. Lenzmann, R. Muis, et al., Chem. Mater. 14 (2002) 5023. [6] U. Bach, D. Lupo, P. Comte, M. Gr¨atzel, Nature 395 (1998) 583. [7] A.F. Nogueira, M.A. De Paoli, I. Montanari, J. Phys. Chem. B 105 (2001) 7517. [8] F. Cao, G. Oskam, C.P. Searson, J. Phys. Chem. B 99 (1995) 17071.
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