NANO LETTERS
High-Efficiency Dye-Sensitized Solar Cell Based on a Nitrogen-Doped Nanostructured Titania Electrode
2005 Vol. 5, No. 12 2543-2547
Tingli Ma,*,† Morito Akiyama,† Eiichi Abe,† and Isao Imai‡ National Institute of AdVanced Industrial Science and Technology (AIST), AIST Kyushu, Shuku 807-1, Tosu, Saga 841-0052, Japan, and Research & DeVelopment Center, Toshiba Ceramics Co., Ltd. 30, Soya, Hadano-shi, Kanagawa 257-8566, Japan Received September 22, 2005; Revised Manuscript Received October 30, 2005
ABSTRACT A highly efficient dye-sensitized solar cell (DSC) was fabricated using a nanocrystalline nitrogen-doped titania electrode. The properties of the nitrogen-doped titania powder, film, and solar cell were investigated. The substitution of oxygen sites with nitrogen atoms in the titania structure was confirmed by X-ray photoemission spectroscopy (XPS). The UV−vis spectrum of the nitrogen-doped powder and film showed a visible light absorption in the wavelength range from 400 to 535 nm. An enhancement of the incident photon-to-current conversion efficiency (IPCE) in the range of 380−520 nm and 550−750 nm was observed. An 8% overall conversion efficiency has been achieved. The results of the stability test indicated that the solar cell fabricated by the nitrogen-doped titania exhibited great stability.
Since Gra¨tzel and co-workers developed a new type of solar cells based on the nanocrystalline TiO2 electrode,1-3 dyesensitized solar cells (DSCs) have been attracting much attention because of their high energy conversion efficiency and as a low-cost alternative to commercial solar cells based on silicon.4-8 It is necessary to further improve the energy conversion efficiency in order to commercialize DSCs successfully. Many methods for improving the conversion efficiency of the DSC have been attempted, including the use of some transition metal ions, such as Zn2+-, Fe3+-, and Pb2+-doped nanocrystalline titania photoelectrodes.9 However, the incident photon-to-conversion efficiency (IPCE) and the overall energy conversion efficiency of these solar cells are very low.9 Alternatively, visible-light-active nanocrystalline titania has been investigated extensively regarding the photocatalyst yield.10-16 Introduction of this type of nanocrystalline titania to the system of the DSC has generally been considered to be unnecessary so far, because some organic dyes as sensitizers, which are usually Ru(II) complexes, have been used to harvest the visible light. Although this method broadens the range of the visible light response effectively, the IPCE for the dye-adsorbed semiconductor is relatively low below 500 nm and above 650 nm. In addition, the * Corresponding author. Present address: Department of Chemistry, Faculty of Science, Kyushu University, Ropponmatsu 4-2-1, Chuo-ku, Fukuoka 810-8560, Japan. Tel/fax: +81 92 726 4747. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Research & Development Center, Toshiba Ceramics Co., Ltd. 10.1021/nl051885l CCC: $30.25 Published on Web 11/10/2005
© 2005 American Chemical Society
semiconductors usually use pure TiO2 in which there is some oxygen deficiency in the crystal structure.10-13 It is known that oxygen deficiency can create electron-hole pairs and that the oxidizing holes can either react with the dye and destroy it and/or is scavenged by iodide ions;17 therefore, the lifetime of the dye-sensitized solar cell may be shortened. To solve these problems, we introduced nitrogen-doped titania into the DSC system to enhance the IPCE and to stabilize the solar cell due to the replacement of oxygendeficient titania by visible-light-active nitrogen-doped titania. An 8% conversion efficiency, which was higher than that of SL-D (Solaronix, Ti-Nanoxide D) electrodes, was obtained.18 This paper reports the properties of the nitrogendoped titania powder and film as well as the photoelectrochemical behavior of the dye-sensitized solar cells based on the nitrogen-doped nanocrystalline titania electrode. Nitrogen-doped nanocrystalline titania powders were synthesized by a procedure that we developed. Commercial anatase TiO2 (ST-01, Ishihara Co., Ltd.) powder was heated at 500 °C under a dry N2 gas flow in the presence of a small quantity of carbon for 3 h. The nanoparticles changed to a deep yellow color. The detailed procedure has been described in a patent and a previous paper.19 Regarding the use of the two elements in the doping process and their advantage, we will describe and discuss them in another paper. The titania paste was prepared using 100% poly(ethylene glycol) (PEG) 600 as a dispersant, which is the method developed by our group.20 The detailed fabrication procedure for the paste, photoelectrode, and device has been given in the same paper.20 The sandwich-type solar cell consisted of
Figure 1. X-ray diffractogram for nitrogen-doped titania powder.
Figure 2. XPS spectrum of nitrogen-doped titania powder.
the titania electrode, on which an Ru dye (Solaronix, N719) is adsorbed, and a platinized counter electrode. The thickness and area of the titania electrode were ca. 18 µm and 0.20 cm2, respectively. The electrolyte solution was composed of 0.1 M LiI, 0.3 M 1,2-dimethyl-3-propylimidazolium iodine, 0.05 M I2, and 0.5 M tert-butylpyridine in 3-methoxypropionitrile. The photocurrent-voltage curves were obtained by scanning a bias voltage while measuring the photocurrents or dark currents under white-light irradiation (100 mW/cm2). A 300 W Xe lamp served as a light source. The UV-vis spectra were taken on a JASCO V-550 using an integrating sphere setup. For comparison, we also fabricated and measured DSCs based on pure TiO2 ST-01, P25 (Nippon Aerosil Co., Ltd.), and SL-D (Solaronix, Ti-Nanoxide D) electrodes under the same conditions. However, the film state of the undoped ST-01 titania as a starting material was not good, and the performance of the DSC was quite poor; thus, we do not show the data for the undoped ST-01 titania in this paper. Structural characterization of the titania powder was performed by X-ray diffraction patterns (XRD). Figure 1 displays the data for the nitrogen-doped titania. The results indicated that the crystal phase of the prepared powder was anatase and that no crystal phase of the rutile was observed after annealing at 500 °C. The substitution of the oxygen sites with nitrogen atoms in the titania structure was confirmed by X-ray photoemission spectroscopy (XPS), as shown in Figure 2. Three binding energy peaks were observed at 396.2, 398.3, and 400.4 eV in the N 1s region. Concerning the assignment of the peak feature in XPS for the nitrogen-doped titania, there is some controversy in the literature so far. Recently, Burda et al.21 studied the photoelectron spectroscopy of a series of the nitrogen-doped titania nanoparticles prepared using excess triethylamine treatment of the hydrolysis solution of Ti[OCH(CH3)2]4. They observed a broad binding energy peak for the nitrogen-doped TiO2 nanoparticles in the N 1s region, extending from 397.4 to 403.7 eV, centered at 401.3 eV. It was attributed to O-Ti-N, based on the redox chemistry involved and the XPS peak position of oxygen, titanium, and nitrogen itself.21 They also indicated that there are distinct differences in the XPS spectra between the synthesized nanoparticles and the commercially available P25 samples.21
In an another recent study involving the XPS depth profiling characterization of the surface layer, Gyorgy et al.22 studied the Ti 2p, O 1s, and N 1s regions. They indicated that the presence of Ti-O-N bonds complicates the interpretation of the Ti 2p region because the binding energy range of TiOxNy compounds is superimposed on that of an energy loss feature situated at approximately a 1.7 ( 0.2 eV higher binding energy from the elastic TiN peaks. Diwald23 studied nitrogen-doped titania for rutile single crystals when the doped titiania sample was prepared by sputtering TiO2 with a N2+/Ar+ gas mixture. The nitrogen peak at 396 eV was observed and attributed to a chemically bound N- species within the crystalline TiO2 lattice. However, for the binding energy of 399.6 eV of the nitrogen-doped sample by flowing NH3 at high temperature, Diwald24 suggested that the photocatalytically active N site was attributed to chemically bound hydrogen and interstitial doping into the TiO2 lattice. Their results disagree with the conclusions in an earlier paper.10 Asahi et al.10 observed three peak structures for binding energies at 402, 400, and 396 eV. The two peaks at higher binding energies were attributed to molecularly adsorbed nitrogen species, whereas the peak at 396 eV was assigned to the substitutionally bound N- species in the TiO2 lattice.10 Thereafter, Irie13 and Diwald23,24 reported that the peak at 396 eV in the XPS spectra was attributed to a chemically bound N- species within the crystalline TiO2 lattice. The signal at 396 eV, however, was not always observed.21 Sakthivel25 reported that the N 1s peak was around 404 eV, whereas the signal at 396 eV was completely absent. On the basis of the results mentioned above, it can been seen that the changes in the nitrogen environment of the nitrogen-doped titania due to the difference in preparation methods can induce significant changes in the XPS spectra. On the basis of the arguments described above, for the currently presented nanoparticles, we consider that the signal at 396.2 eV is attributed to a chemically bound N- species within the crystalline TiO2 lattice. In addition, the weak peak around 398.3 eV is derived from the presence of the O-Ti-N linkages in the crystalline TiO2 lattice. This result is slightly different from the conclusions given by Asahi: they attributed the two peaks at 402 and 400 eV to molecularly adsorbed nitrogen species. These assignments are also supported by the results of the previous studies.21
2544
Nano Lett., Vol. 5, No. 12, 2005
Figure 3. UV-vis spectra of the nitrogen-doped and pure titania powder and film.
al.21
Burda et indicated that when nitrogen is substituted for the oxygen in the initial O-Ti-O structure the electron density around N is reduced and the N 1s binding energy in an O-Ti-N environment is higher than that in an N-Ti-N environment.21 Therefore, we think that the feature around 398.3 eV is highly likely to be the O-Ti-N structure. The formation of the Ti-N and O-Ti-N structures is suggested to proceed during the substitution doping process. Alternatively, the signal around 400.4 eV is considered to be a molecularly adsorbed nitrogen species, which absorbs onto the surface and into the interstitial sites of the titania lattice. This result is consistent with the attribution of the molecularly adsorbed nitrogen species in the previous papers.10,26-28 The nitrogen-doped titania powder has a deep yellow color. The change in color of the nanocrystals upon nitrogen incorporation demonstrates a profound effect on their optical response in the visible wavelength range. The UV-vis absorption spectra, which was measured by using an integrating sphere setup for the nitrogen-doped titania powder and film with adsorbed dye, are shown in Figure 3. For comparison, the spectrum of the P25 titania powder is also illustrated. No absorption peak for P25 was observed above the wavelength of 400 nm. However, the nitrogen-doped titania powders exhibited a new absorption peak in the visible light region between 400 and 550 nm. The visible light activity is considered to be due to the nitrogen doping in the titania crystalline structure because the nitrogen doping induced a new state lying close to the valence band edge.29 Furthermore, this adsorption peak was still observed after sintering at 500 °C for 30 min in various gas atmospheres, such as Ar, N2, and air. This result indicated that the nitrogen-doped titania has great thermal stability. FE-SEM images of the nitrogen-doped titania powder are illustrated in Figure 4. The nanocrystalline material shows a needle appearance with a particle size of 15 × 30 nm2 (Figure 4A). The BET analysis shows that the surface area of the powder was 67 m2/g, which is greater than that of the P25 powder (55 m2/g). The FE-SEM micrograph of the nitrogen-doped titania film prepared by PEG 600 shows a crack-free surface structure, Nano Lett., Vol. 5, No. 12, 2005
Figure 4. FE-SEM micrographs of the nitrogen-doped titania powder and film. (A) nitrogen-doped titania powder; (B) P25 film; (C and D) nitrogen-doped titania film; left: under 20000 resolution; right: under 4000 resolution.
Figure 5. Current-voltage curves of the dye-sensitized cell based on the nitrogen-doped and pure titania electrode.
even when the thickness of a screen-printed single layer is above 10 µm at one time. The obtained films have much better adhesion behaviors than those of P25 and SL-D. The FE-SEM micrograph with high resolution revealed a porous network surface structure (Figure 4C), although the surface shape of the nitrogen-doped titania film does not seem very porous under low resolution (Figure 4D). This phenomenon was different from those observed for P25 (Figure 4B) and SL-D, and it was considered to be due to the difference in the titania crystallinity. The BET results show that the nitrogen-doped titania film has a surface area of 85 m2/g and a porosity of 62%, which is greater than the porosity of the SL-D (56%). These results indicate that the prepared nitrogen-doped titania film has a larger surface area and porosity. Figure 5 shows the current-voltage curves of the open cells based on the nitrogen-doped and pure titania photoelectrodes. The performance properties of the DSCs are summarized in Table 1. A pronounced increase in the photocurrent for the DSC based on the nitrogen-doped titania 2545
Table 1. Performance Characteristics of the Dye-Sensitized Cells Based on the Nitrogen-Doped and Pure Titania Electrode DSC
Voc (mV)
jsc (mA/cm2)
FF (%)
η (%)
amount of dye (mol/cm2) (× 10-8)
N-doped P25 SL-D
690 685 667
17.9 13.3 17.2
62 66 61
8 6 7
12.3 9.8 10.3
was observed. A high-energy conversion efficiency of 8% was achieved, which was 33% and 14% higher than that of the P25 and SL-D titania, respectively. However, the influence on the open circuit potential (Voc) by the presence of nitrogen in the titania electrode is not observed in Figure 5. It is evident that the conduction band edge remains unchanged by nitrogen doping, which is in agreement with the results of Taga and Lindquist.10,29 To clarify the reasons why a higher photocurrent for the solar cell based on nitrogen-doped titania can be obtained, we measured the action spectra of the sealed DSCs based on the nitrogen-doped and the pure titania electrodes with the adsorbed Ru(II) complex (N719). The results are shown in Figure 6. Because our technique for sealing the cell was not good, the photocurrent and the conversion efficiencies for the sealed cells became lower than those of the corresponding open cells. The IPCE values were uncorrected for the absorption and scattering of the incident light (ca. 15%) by the glass substrate. From the action spectra in Figure 6, it can be seen that the spectra shape is similar to that of the UV-vis absorption spectrum of the nitrogen-doped titaniafilm/Ru dye. In addition, we clearly observed a significant enhancement in the IPCE of the DSC based on the nitrogendoped titania electrode compared to those of the undoped titaina electrodes between the ranges of 370-530 and 570720 nm, as shown in Figure 6. We considered that the former was due to a contribution of the photoresponse of nitrogendoped titania in the visible light region, and the latter was caused by the light-scattering effect, resulting from the needle shape of the nanoparticles and the large size of the particles due to the nanocrystalline aggregation during sintering. It has been known that the mixing of the nanoparticles with larger particles30 or by applying a scattering layer31,32 on the nanocrystalline film can increase the light-harvesting performance by enhancing the scattering of light as demonstrated by simulations and experimental studies.30-32 The contribution of the scattering light effect to the increase in the IPCE was not prominent and was limited to only approximately 5%.30 Thus, we concluded that the occurrence of visible light absorption due to the nitrogen-doped titania support intrinsically increases the IPCE value. The enhancement in the IPCE due to the photoresponse of nitrogen-doped titania in the visible light region is also supported by the results reported by Lindquist et al.29 They have demonstrated that the photoinduced current due to the visible light activity of the best nitrogen-doped titania electrode prepared by reactive DC magnetron sputtering can increase significantly by approximately 200 times over those of the undoped titania electrodes.29 On the basis of these results, it can be expected that the optimization of the amount of nitrogen doping in 2546
Figure 6. Action spectra of the dye-sensitized cell based on the nitrogen-doped and pure titania electrode.
titania nanoparticles and electrode can further improve the performance of the DSCs. Such work is currently in progress. Here, it should be pointed out that a higher Ru dye uptake was observed on the nitrogen-doped titania film, compared to that for the undoped one. It has been known that a larger surface area of the titania film can increase the amount of dye uptake and further lead to an increase in the conversion efficiency of the dye-sensitized solar cell. As mentioned above, the present nitrogen-doped titania film has a larger surface area. To clarify the contribution to the increase in IPCE based on the dye adsorption, the amounts of dye adsorbed on the three types of photoelectrodes were measured and the data are summarized in Table 1. The result revealed that the amount of dye adsorbed on the nitrogendoped electrode was 1.6 and 1.2 times those for the P25 and SL-D electrodes, respectively. Thus, we considered that the increase in the amount of dye adsorbed in the nitrogen-doped titania electrode caused an enhancement in the short circuit current (jsc). Because our previous study indicated that the dye uptake increased 1.2 times and the IPCE only increased approximately 3%33 for the present study, we can conclude that the intrinsic increase in the IPCE is due to the visible light absorption of the nitrogen-doped titania. On the contrary, the increase in the Ru dye uptake only partly contributes to the enhancement of the IPCE of the dyesensitized solar cell. This result suggests that the effect of nitrogen doping on light absorption is a more important factor in the good performance of the cell. Although improvement in the IPCE due to the enhanced absorption of the organic dye was observed, it is considered not to be a systematic effect, but occurs by chance in this case; the nanoparticles used in this study were incidentally synthesized to be needle crystalline material of small size, and the properties for the powders and films were quite different from those of the P25 and SL-D. Thus, detailed investigations would be necessary for comparison between the nitrogen-doped and undoped titaina prepared in an identical way. Further, a detailed discussion is needed on the electron transfer and recombination in the DSC system based on the nitrogen-doped titania. There was a concern that the visible-light-active titania could possibly accelerate the deterioration of the dye and the electrolyte in the DSC system; therefore, it is important to determine whether introducing the nitrogen-doped titania Nano Lett., Vol. 5, No. 12, 2005
and photodegradation of the organic dye and the electrolyte in nitrogen-doped DSC system are in progress. Acknowledgment. This work was supported partly by the Japan Science and Technology Corporation. We gratefully acknowledge Professor S. Yanagida and Dr. T. Kitamura, Department of Material and Life Science, Graduate School of Engineering, Osaka University, for their helpful discussion. We thank Professor S. Yamada, Department of Materials Physics and Chemistry, Kyushu University, for his support for the FE-SEM measurements. References
Figure 7. Stability for the DSC based on nitrogen-doped titania over 2000 h of continuous illumination with white light of 100 mW/cm2 intensity.
will cause the photodegradation of the organic dye and electrolyte. We performed the studies of the DSCs stability during irradiation for 2000 hours under white light (100 mW/ cm2) at 25 °C. As shown in Figure 7, no photodegradation was observed for the cell involving the nitrogen-incorporated titania structure. This result revealed that the dye-sensitized solar cell fabricated with the nitrogen-doped titania electrode has an excellent stability over a period of 2000 h. A stability test of the DSC for a longer period under thermal stress and detailed photodegradation studies are now in progress. In conclusion, the nitrogen-doped titania nanocrystalline materials with large surface areas were synthesized successfully by a novel method. Three binding energy peaks were observed at 396.2, 398.3, and 400.4 eV in the N 1s region of the XPS. The signals at 396.2 and 398.3 eV were attributed to a chemically bound N- species and the O-Ti-N linkages within the crystalline TiO2 lattice, respectively. In addition, the signal around 400.4 eV was assigned to the molecularly adsorbed nitrogen species. A new absorption was observed for the UV-vis spectrum of the nitrogen-doped titania in the visible light region. The action spectrum of the DSC based on the nitrogen-doped titania was in agreement with the corresponding optical spectrum. We clearly observed a significant enhancement in the IPCE and conversion efficiency of the DSC based on the nitrogendoped titania due to the intrinsic contribution of the photoresponse of the nitrogen-doped titania in the visible light region from 370 to 530 nm. Concomitant with that, we also observed a small contribution to the increase in the IPCE from the light scattering effect in the red light region and the increase in the Ru dye uptake. The high energy conversion efficiency was achieved successfully for the DSC based on the nitrogen-doped nanocrystalline titania electrode. Further detailed studies on improvement in the efficiency due to the optimization of the amount of nitrogen doping Nano Lett., Vol. 5, No. 12, 2005
(1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrey-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825. (4) Gra¨tzel, M. Prog. PhotoVolt. Res. Appl. 2000, 8, 171. (5) Recent AdVances in Research and DeVelopment for Dye-Sensitized Solar Cells; CMC: Japan, 2001. (6) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825. (7) Lagemaat, J.; Park, N.-G.; Frank, A. J.; Boschloo, G. K.; Goossens, A. J. Phys. Chem. B. 2000, 104, 2044. (8) Schlichtho¨rl, G.; Park, N.-G.; Frank, A. J. J. Phys. Chem. B. 1999, 103, 782. (9) Miki, T.; Soga, T.; Umeno, M. Sol. Energy Mater. Sol. Cells 1997, 48, 123. (10) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (11) Inakamura, I.; Negishi, N.; Kutsuna, S.; Ihara, T.; Sugihara, S.; Takeuchi, K. J. Mol. Catal. A: Chem. 2000, 161, 205. (12) Ihara, T.; Miyoshi, M.; Iriyama, Y.; Matsumoto, O.; Sugihara, S. Appl. Catal., B: EnVironmental 2003, 42, 403. (13) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (14) Irie, H.; Washizuka, S.; Yoshio, N.; Hashimoto, K. Chem. Commun. 2003, 1298. (15) Chen, X.; Lou, Y.; Samia, A. C. S.; Burda, C.; Gole, J. L. AdV. Funct. Mater. 2005, 15, 41. (16) Gole, L.; Stout, J. D.; Burda, C.; Lou, Y.; Chen, X. J. Phys. Chem. B 2004, 108, 1230. (17) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2004, 108, 17269. (18) (a) Ma, T.; Fang, X.; Abe, E.; Shou, Q.; Ito, Y. The Autumn Publication Conference on Combination Research, Kitakyushu, Japan, 2003. (b) Ma, T.; Abe, E. XXII International Conference on Photochemistry, Cairns, Australia, 2005. (19) (a) Ma, T.; Abe, E.; Zhou, Z.; Tokuoka, F.; Ito, Y.; Ishii, T. Japan Patent No. 2005-93944. (b) Zhou, Z.; Tokuoka, F.; Ito, Y.; Ishii, T. Kagaku Sochi 2003, 10, 64. (20) Ma, T.; Kida, T.; Akiyama, M.; Inoue, K.; Tsunematse, S.; Yao, K.; Noma, H.; Abe, E. Electrochem. Commun. 2003, 5, 369. (21) Chen, X.; Burda, C. J. Phys. Chem. B 2004, 108, 15446. (22) Gyo¨rgy, E.; Pe´rez del Pino, A.; Serra, P.; Morenza, L. Surf. Coat. Technol. 2003, 173, 265. (23) Diwald, O.; Thompson, T. L.; Goralski, E. G.; Walck, S. D.; Yates, J. K., Jr. J. Phys. Chem. B 2004, 108, 52. (24) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. K., Jr. J. Phys. Chem. B 2004, 108, 6004. (25) Sakthivel, S.; Kisch, H. ChemPhysChem. 2003, 4, 487. (26) Saha, N. C.; Tompkins, H. G. J. Appl. Phys. 1992, 72, 3072. (27) Fuggle, J. C.; Umbach, E.; Menzel, D. R.; Wandelt, K.; Burndle, C. R. Solid State Commun. 1978, 27, 65. (28) Wu, H. Z.; Chou, T. C.; Mishra, A.; Anderson, D. R.; Lampert, J. K.; Gujrathi, S. C. Thin Solid Films 1990, 191, 55. (29) Lindgraen, T.; Mwabora, J. M.; Avendano, E.; Jonsson, J.; Hoel, A.; Granqvist, C.; Lindquist, S. J. Phys. Chem. B 2003, 107, 5709. (30) Ferber, J.; Luther, J. Sol. Energy Mater. Sol. Cells 1998, 54, 265. (31) Usami, A. Chem. Phys. Lett. 1997, 277, 105. (32) Usami, A. Sol. Energy Mater. Sol. Cells 1999, 59, 163. (33) Ma, T.; Inoue, K.; Yao, K.; Noma, H.; Tsunematse, S.; Abe, E.; Yu, J.; Wang, X.; Zhang, B. J. Electrochem. Chem. 2002, 537, 31.
NL051885L 2547
