Electrochemical and Solid-State Letters, 10 共6兲 F23-F25 共2007兲
F23
1099-0062/2007/10共6兲/F23/3/$20.00 © The Electrochemical Society
Acceleration of Redox Diffusion and Charge-Transfer Rates in an Ionic Liquid with Nanoparticle Addition Toru Katakabe, Ryuji Kawano, and Masayoshi Watanabe*,z Department of Chemistry and Biotechnology, Yokohama National University, Hodogaya-ku, Yokohama 240-8501, Japan The exchange-reaction-based diffusion and the interfacial charge-transfer rates of an iodide/triiodide redox couple in an ionic liquid were enhanced by the addition of SiO2 nanoparticles, although the composites successively became gels and solids with increasing nanoparticle composition. Because of this acceleration of the charge transport and the interfacial charge-transfer rates, dye-sensitized solar cells using the composite electrolytes achieved high photon-to-current conversion efficiencies, comparable to those using the neat ionic liquid electrolyte. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2720636兴 All rights reserved. Manuscript submitted December 21, 2006; revised manuscript received February 9, 2007. Available electronically March 28, 2007.
Dye-sensitized solar cells 共DSSCs兲 have attracted much attention as clean energy source for the next generation.1,2 The fabrication processes of DSSCs do not include vacuum processes, so the costs of DSSCs can be made much lower than those of Si solar cells. Recently, the achievement of a photoelectron conversion efficiency of 11% was reported for DSSCs.3 The emphasis on studies of DSSCs is moving from the electron-transfer kinetics in the photoanode to the durability and safety of electrolytes. In typical DSSCs, iodide/triiodide redox couples have been used as solutes, and organic solvents, such as acetonitrile, have been used as solvents. However, the irradiation of sunlight makes solar cells heat to 80°C so that the organic solvents are not the best choice in terms of durability and safety. The possibility of replacement of the organic solvents by ionic liquids 共ILs兲 has been studied in order to solve these problems.4,5 ILs are liquids comprised entirely of ions and have unique properties such as nonvolatility, nonflammability, high electrochemical stability, high ionic conductivity, and gel-forming property with polymers.6-9 These properties of ILs are desirable as electrolyte of DSSCs. We found that in spite of the relatively high viscosity of ILs, an iodide/triiodide redox couple diffused fast by the unique transport mechanism, i.e., the exchange-reaction-based diffusion such as I− + I−3 → I−3 + I−,10,11 which contributed to the total diffusion only in the ILs and not in molecular solvents.11 The ILs in DSSCs can afford higher transport of the redox couple than molecular solvents if compared at the same viscosities.10,11 Solidification of the electrolytes would further enhance longterm stability and improve the fabrication process. In ILs, the exchange-reaction-based diffusion would become higher in a locally concentrated domain of the redox couple, since it is the secondorder reaction. This hypothesis could be confirmed by comparing the charge transport in two ILs, where one is a smectic liquid crystalline IL and the other is an isotropic IL.12 Although the viscosity of the former is higher, the exchange-reaction-based diffusion for the former becomes higher than that for the latter,12 because the phase separation between the ionic groups and alkyl groups in the former makes local concentration of the redox couple higher. This fact indicates that the exchange-reaction-based diffusion appears to be less affected by solidification of the electrolytes. This study deals with the solidification of an IL with the addition of nanoparticles. The effects of the addition of nanoparticles to liquid electrolytes have been widely studied.13-21 Interestingly, the addition of nanoparticles less affects the transport properties of ILs,14,16,18-21 although the appearance successively becomes gel and solid with the addition. The DSSC performance is also slightly affected16,20,21 or in some cases enhanced to some extent.18,19 However, the detailed reasons for these unique phenomena have not been
* Electrochemical Society Active Member. z
E-mail:
[email protected]
well understood. We aimed at understanding changes in the charge transport and the interfacial charge transfer of an iodide/triiodide redox couple with the addition of nanoparticles to the IL, in relevance to high-performance solid-state solar cells.
Experimental 1-Ethyl-3-methylimidazolium bis共trifluoromethane sulfone兲imide 共EMImTFSI兲 was used as IL, which was synthesized as described in our previous reports.8,9 SiO2 nanoparticles 共Aerosil 200, Nippon Aerosil, average diam 12 nm兲 were added to EMImTFSI. The SiO2 nanoparticles and EMImTFSI were thoroughly mixed to form gels or solid powders. For making the electrolytes electrochemically active, 1-ethyl-3-methylimidazolium iodide 共EMImI兲 and I2 were added to EMImTFSI. The ionic conductivity and the charge-transfer resistance at the electrode/electrolyte interface for EMImTFSI and these composite electrolytes with or without the redox couple were measured by means of electrochemical impedance spectroscopy 共EIS兲 using Pt-coated stainless steel electrodes 共area 50 mm2兲 over the frequency range of 0.01 Hz to 1 MHz at an amplitude of 0.03 V. Diffusivity of the redox couple was estimated from the limiting current obtained from microelectrode cyclic voltammetry 共CV兲 using three-electrode cells,22 where a 12 m diam Pt disk 共working electrode兲, a tip of Ag wire 共quasi-reference electrode兲, and a tip of Pt wire 共counter electrode兲 were exposed from an insulating plane.22 The CV measurements were conducted under Ar atmosphere by contacting the three-electrode cells with liquid or composite electrolytes. When powder-like composite electrolytes were used for the measurements, the composite electrolytes were compressed into disks and the contacts between the electrolyte and the electrodes were ensured by applying pressure. For both EIS and CV measurements, an electrochemical workstation 共BAS, ALS-600兲 was used. DSSCs used in this study were fabricated as follows: Ti-nanoxide T paste 共Solaronix SA兲 was coated on fluorine-doped tin oxide 共FTO兲 glass plates 共Asahi Glass兲 and sintered at 450°C. A piece of the TiO2-coated FTO glass plate 共photoanode, 5 ⫻ 5 mm兲 was dipped into a cis-di共thiocyanate兲-N,N⬘-bis共2,2⬘-bipyridyl-4,4⬘-dicarboxylate兲ruthemium共II兲 共Kojima Kagaku兲 solution 共0.3 wt % in an acetronitrile/tert-butylalcohol mixed solvent 共1:1 by weight兲 at room temperature. Pt-sputtered FTO glass plates were employed as counter electrodes. A neat ionic liquid electrolyte for the DSSCs was prepared by adding EMImI and I2 to a mixture of EMImTFSI and 1-ethyl-3-methylimidazolium thiocyanate 共EMImSCN兲, where EMImSCN was added to increase the open-circuit voltage. The DSSCs using the neat ionic liquid electrolyte were fabricated using a conventional method.5 The composite electrolytes for the DSSCs were prepared by adding the nanoparticles to the neat electrolytes and spread over photoanodes, and counter electrodes were pressed onto the photoanodes and fixed by cell holders. Solar cell tests were performed by using a solar simulator 共Yamashita Denso, YSS-50A,
F24
Electrochemical and Solid-State Letters, 10 共6兲 F23-F25 共2007兲
Figure 1. 共Color online兲 Appearance and ionic conductivity at 25°C for composite electrolytes composed of EMImTFSI and SiO2 nanoparticles.
AM 1.5, 100 mW cm−2兲 connected with the electrochemical workstation. Results and Discussion The solidification of the IL easily occurred with the addition of the SiO2 nanoparticles to EMImTFSI. Figure 1 shows changes in the appearance and ionic conductivity of the composite electrolytes with the addition of the nanoparticles. The appearance changes from a liquid to a gel with the 0.05 weight ratio addition of the nanoparticles to EMImTFSI, and the further addition makes the IL white powders. When the weight ratio is 0.10 it looks like wet powders, while it becomes dry powders when the ratio is 0.2. Interestingly, the ionic conductivity remains constant in spite of the great change in the appearance. In these composite electrolytes, the ions retain fast dynamics like the neat IL, although they are solids in appearance. In order to know the charge transport of the iodide/triiodide redox couple in these composite electrolytes, microelectrode CV was measured. Figure 2 shows typical examples of the voltammograms for a neat IL electrolyte 共top兲 and a composite electrolyte 共SiO2 weight ratio of 0.2兲 共bottom兲. The appearance of the composite electrolytes, when the redox couple was added, was similar to that in the absence of the redox couple 共Fig. 1兲. By comparing the voltammograms, it is remarkable that the limiting current of the composite electrolyte is significantly larger than that of the neat IL electrolyte, although the former is a solid electrolyte and the latter is a liquid electrolyte. The limiting current corresponds to the diffusion-limited transport of the iodide/triiodide redox couple in these electrolytes. The increasing ratios of the limiting current with the addition of the nanoparticles are shown in the inset of Fig. 2. The highest limiting current is given at nanoparticle weight ratios of 0.1–0.2, which is ca. 1.4 times larger than that of the neat IL electrolyte. It is of great interest to know why the limiting current increases with the addition of the nanoparticles, although the electrolyte turns from a liquid to a solid and the volume fraction of the electroactive IL electrolyte becomes lower. Because the ionic conductivity is not seriously affected by the addition of the nanoparticles 共Fig. 1兲, the increase in the limiting current appears to be attributed to the contribution of the exchangereaction-based diffusion of the redox couple. When physical diffusion and exchange-reaction-based diffusion are conjugated, the limiting current obtained from microelectrode CV is expressed by the following equations10,11 Ilim = 4nFDapprc = 4nF共Dphys + Dex兲rc
关1兴
Figure 2. Microelectrode CVs 共 v = 10 mV/s兲 at 25°C for EMImTFSI and a composite electrolyte 共EMImTFSI:SiO2 = 5:1 by weight兲, where 1 M EMImI and 0.1 M I2 are dissolved in EMImTFSI. Increasing the ratio of limiting current for the iodide/triiodide redox couple in the composite electrolytes normalized by that in EMImTFSI 共inset兲.
Dex = 1/6kex␦2c
关2兴
where Dapp, Dphys, and Dex are apparent, physical, and exchangereaction-based diffusion coefficients, respectively, n is the number of transferred electrons, F is the Faraday constant, r is the microdisk electrode radius, c is concentration of the redox couple, kex is the exchange-reaction rate constant, and ␦ is the center-to-center intersite distance at the exchange reaction. The Dapp values 共total diffusion rate兲 of the neat IL and the composite electrolyte are calculated as 1.97 ⫻ 10−7 and 2.64 ⫻ 10−7 cm2 s−1, respectively, from the Ilim values in Fig. 2 and Eq. 1. In Eq. 1, Dphys is assumed to be constant with the addition of the SiO2 nanoparticles, because there is no change in the ionic conductivity 共Fig. 1兲. Thus, the increase in the limiting current 共Fig. 2兲, can be attributed to the contribution of Dex to the total diffusion processes 共Dapp兲. As can be seen from Eq. 2, Dex is a function of kex, ␦, and c; however, it appears plausible that the main contributor to the increase in Dex is an increase in c with the addition of the nanoparticles. The addition of the SiO2 nanoparticles enhances the local concentration of the redox couple, which accelerates the second-order exchange reaction. The SiO2 nanoparticles used in this study are hydrophilic and negatively charged due to the presence of surface SiO− groups. EMIm cations in the IL electrolytes are adsorbed onto the surface of the SiO2 particles by the electrostatic interaction, which in turn may accumulate the iodide/triiodide couple. The iodide/triiodide-rich region may form continuous paths in the composite electrolytes, which could be a reason for the enhanced Dex. Such possibility has also been implied by others.18,20 The voltammetric shapes in Fig. 2 indicate that the addition of the nanoparticles also accelerates the electron-transfer rate at the Pt-electrode interfaces as well as the bulk transport of the redox couple 共vide supra兲. Figure 3 shows the Nyquist plots for the composite electrolytes containing the redox couple sandwiched between Pt-coated stainless steel electrodes. The high-frequency ends of the Nyquist plots correspond to the electrolyte resistance 共31–32 ⍀兲, which is independent of the nanoparticle compositions and is con-
Electrochemical and Solid-State Letters, 10 共6兲 F23-F25 共2007兲
F25
Figure 4. Comparison of photocurrent density vs voltage curves between DSSCs using composite electrolyte 共SiO2 nanoparticle:ionic liquid mixture ⫽1:10 by weight兲 and using the ionic liquid mixture electrolyte, where 1 M EMImI and 0.1 M I2 are dissolved in the ionic liquid mixture 共EMImTFSI:EMImSCN⫽ 5:1 by volume兲. Figure 3. Nyquist plots at 25°C for composite electrolytes composed of EMImTFSI and SiO2 nanoparticles, where 1 M EMImI and 0.1 M I2 are dissolved in EMImTFSI, sandwiched between two Pt-coated stainless steel electrodes.
MEXT, Japan, and by a NEDO research grant under the METI, Japan. Yokohama National University assisted in meeting the publication costs of this article.
sistent with the results in Fig. 1. The low-frequency spurs in the Nyquist plots correspond to the Warburg impedance, which appears to reflect the formation of concentration gradient of the redox couple. The middle frequency arcs correspond to the charge-transfer resistance for the reversible reactions, I−3 + 2e− = 3I−. Interestingly, the charge-transfer resistance of the composite electrolytes decreases with the addition of the SiO2 nanoparticles, which indicates an increase in the charge-transfer reaction rate for I−3 + 2e− = 3I−. It is remarkable that the addition of the nanoparticles induces the acceleration of not only the bulk charge transport of the redox couple but also of the interfacial charge-transfer rate. All of the above results encouraged us to apply the composite electrolytes to DSSCs. A comparison was made between the performance of the DSSC using a composite electrolyte and that using a neat ionic liquid electrolyte 共Fig. 4兲. The short-circuit photocurrent density 共Jsc兲, open-circuit voltage 共Voc兲, and fill factor 共FF兲 of a DSSC using the neat ionic liquid electrolyte are 11.2 mA cm−2, 580 mV, and 0.59, respectively, yielding an overall energy conversion efficiency 共兲 of 3.7%. The corresponding device parameters 共Jsc, Voc, FF, and 兲 of a DSSC using the composite electrolyte are 10.4 mA cm−2, 592 mV, 0.62, and 3.7%, respectively. The DSSC using the powder-like composite electrolyte exhibits a similar performance to that using the liquid electrolyte. To improve the solidstate DSSC performance, the interface between the dyednanocrystalline TiO2 and the composite electrolytes would need to be well designed. Acknowledgments This research was supported in part by a Grant-in-Aid for Scientific Research 共no. A-16205024 and no. 452-17073009兲 from the
References 1. B. O’Regan and M. Grätzel, Nature (London), 353, 737 共1991兲. 2. M. Grätzel, Nature (London), 414, 338 共2001兲. 3. M. K. Nazeeruddin, F. D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, and M. Grätzel, J. Am. Chem. Soc., 127, 16835 共2005兲. 4. P. Bonhôte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, and M. Grätzel, Inorg. Chem., 35, 1168 共1996兲. 5. R. Kawano, H. Matsui, C. Matsuyama, A. Sato, M. A. B. H. Susan, N. Tanabe, and M. Watanabe, J. Photochem. Photobiol., A, 164, 87 共2004兲. 6. Ionic Liquids in Synthesis, P. Wasserscheid and T. Welton, Editors, Wiley-VCH, Weinheim, Germany 共2003兲. 7. T. Welton, Chem. Rev. (Washington, D.C.), 99, 2071 共1999兲. 8. H. Tokuda, S. Tsuzuki, M. A. B. H. Susan, K. Hayamizu, and M. Watanabe, J. Phys. Chem. B, 110, 19593 共2006兲. 9. M. A. B. H. Susan, T. Kaneko, A. Noda, and M. Watanabe, J. Am. Chem. Soc., 127, 4976 共2005兲. 10. R. Kawano and M. Watanabe, Chem. Commun. (Cambridge), 2003, 330. 11. R. Kawano and M. Watanabe, Chem. Commun. (Cambridge), 2005, 2107. 12. N. Yamanaka, R. Kawano, W. Kubo, T. Kitamura, Y. Wada, M. Watanabe, and S. Yanagida, Chem. Commun. (Cambridge), 2005, 740. 13. H. J. Walls, P. S. Fedkiw, T. A. Zawodzinski, Jr., and S. A. Khan, J. Electrochem. Soc., 150, E165 共2003兲. 14. K. Hasegawa, M. Tatsumisago, and T. Minami, J. Electrochem. Soc., 146, 3359 共1999兲. 15. A. J. Bhattacharyya and J. Maier, Adv. Mater. (Weinheim, Ger.), 16, 811 共2004兲. 16. P. Wang, S. M. Zakeeruddin, P. Comte, I. Exner, and M. Grätzel, J. Am. Chem. Soc., 125, 1166 共2003兲. 17. E. Statahatos, P. Lianos, S. M. Zakeeruddin, P. Liska, and M. Grätzel, Chem. Mater., 15, 1825 共2003兲. 18. H. Usui, H. Matusi, N. Tanabe, and S. Yanagida, J. Photochem. Photobiol., A, 164, 97 共2004兲. 19. T. Kato, A. Okazaki, and S. Hayase, Chem. Commun. (Cambridge), 2005, 363. 20. T. Kato, T. Kado, S. Tanaka, A. Okazaki, and S. Hayase, J. Electrochem. Soc., 153, A626 共2006兲. 21. H. Yang, C. Yu, Q. Song, Y. Xia, F. Li, Z. Chen, X. Li, T. Yi, and C. Huang, Chem. Mater., 18, 5173 共2006兲. 22. M. Watanabe, H. Nagasaka, and N. Ogata, J. Phys. Chem., 99, 12294 共1995兲.
