pubs.acs.org/Langmuir © 2010 American Chemical Society
Catalytically Active Structure of Bi Deposited on a Au(111) Electrode for the Hydrogen Peroxide Reduction Reaction Masashi Nakamura,*,† Narumasa Sato,† Nagahiro Hoshi,† and Osami Sakata‡,§ †
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan, ‡Materials Science Division, Japan Synchrotron Radiation Research Institute/SPring-8, Kouto 1-1-1, Sayo, Sayo-gun, Hyogo 679-5198, Japan, and §CREST, Science and Technology Agency (JST), Sanbancho Building 5, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan Received January 8, 2010. Revised Manuscript Received February 28, 2010 The surface structure of underpotentially deposited Bi has been determined on Au(111) in perchloric acid solution using surface X-ray diffraction (SXD), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. SXD analysis and STM images reveal that the catalytically active structure for the hydrogen peroxide reduction reaction (HPRR) is the (2 2)-Bi honeycomb structure with θBi = 0.50. The stability is supported by DFT calculations. The hydrated perchlorate anion is located in the center of the honeycomb structure without hydrogen peroxide. DFT calculations predict that the Bi honeycomb structure promotes the dissociation of the O-O bond of hydrogen peroxide. Hydrogen peroxide expels the hydrated perchlorate anion, and then HPRR takes place at the honeycomb center.
Introduction The oxygen reduction reaction (ORR) is a fundamental reaction in many fields, such as fuel cell, corrosion, and bioscience. The high overpotential and slow kinetics of ORR decrease the energy efficiency of fuel cell systems significantly.1,2 A very active catalyst for ORR is necessary for the development of the protonexchange membrane fuel cell (PEMFC). ORR proceeds via four electron (eq 1) and two-electron reactions (eq 2) on metal catalysts in acid solutions:1,2 O2 þ 4e - þ 4Hþ f 2H2 O
ð1Þ
O2 þ 2e - þ 2Hþ f H2 O2
ð2Þ
H2 O2 þ 2e - þ 2Hþ f 2H2 O
ð20 Þ
The reaction processes depend on the electrode materials, pH, and electrode potentials.1-4 Pt and Ag electrodes promote the direct four-electron reaction (eq 1). The Au electrode activates the two-electron reduction of oxygen to hydrogen peroxide (eq 2) but inactivates the additional two-electron reduction to water (eq 20 ).3-5 In PEMFC, the four-electron reaction is preferable to the two-electron reaction for the following reasons: (a) the fourelectron reaction has a higher energy efficiency and (b) hydrogen peroxide, which is produced from the two-electron reaction, damages the polymer electrolyte membrane. Previous studies show that the deposition of heteroatoms on an Au electrode enhances the activity for the four-electron reduc*Corresponding author. E-mail:
[email protected]. (1) Gattrell, M.; Macdougall, B. In Handbook of Fuel Cells; Vielstich, W., Gasteiger, H. A., Lamm, A., Eds.; John Wiley & Sons: New York, 2003; Vol. 2, pp 443-446. (2) Adzic, R. In Electroanalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 197-242. (3) Markovic, N. M.; Adzic, R. R.; Cahan, B. D.; Yeager, E. B. J. Electroanal. Chem. 1994, 377, 249. (4) Bllzanac, B. B.; Ross, P. N.; Markovic, N. M. Electrochim. Acta 2007, 52, 2264. (5) Alvarez-Rizatti, M.; Juttner, K. J. Electroanal. Chem. 1983, 144, 351. (6) Adzic, R. R.; Tripkovic, A.; Markovic, N. M. J. Electroanal. Chem. 1980, 114, 37.
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tion.5-8 In particular, underpotentially deposited (upd) Bi atoms activate the additional hydrogen peroxide reduction reaction (HPRR). The activity depends on the coverage of Bi (θBi). The open structure of Bi at intermediate coverage activates the HPRR.8-12 The structure of upd Bi was studied extensively on the low-index planes of Au using atomic force microscopy (AFM),11 scanning tunneling microscopy (STM),9,13 and surface X-ray diffraction (SXD).12-14 The upd Bi atoms form several well-defined structures on Au single-crystal surfaces.9,11 The voltammogram of Au(111) shows two upd peaks at 0.23 and 0.15 V in HClO4 containing 5 mM Bi2O3, as shown in Figure 1. The charges of the first (red area) and second peaks (blue area) are 308 and 110 μC cm-2, respectively. The values of θBi after the first and second upd peaks are estimated to be 0.46 and 0.63, respectively, assuming a three-electron reaction (Bi3þ√þ 3e- f Biad). The 2 2 and uniaxially incommensurate p 3 structures of Bi were found on the Au(111) electrode using STM 11,13 The 2 2 structure is active for HPRR whereas and AFM. √ the p 3 structure is inactive.15 Previous AFM and STM studies found that one Bi atom is adsorbed at the hollow site in the 2 2 unit cell.11,13 The value of θBi is 0.25 on the (2 2)-Bi structure; however, the voltammogram in Figure 1 gives θBi = 0.46. Although θBi estimated from the voltammogram may be uncertain because of the anion displacement √ by Bi, the inconsistency is too large. The value of θBi of p 3 is consistent with that obtained from the voltammogram. Gewirth et al. studied (7) Amadelli, R.; Molla, J.; Bindra, P.; Yeager, E. J. Electroanal. Chem. 1981, 128, 2706. (8) Jutter, K. Electrochim. Acta 1984, 29, 1597. (9) Hara, M.; Nagahara, Y.; Yoshimoto, S.; Inukai, J.; Itaya, K. Jpn. J. Appl. Phys. 2004, 43, 7232. Hara, M.; Nagahara, Y.; Yoshimoto, S.; Inukai, J.; Itaya, K. Electrochim. Acta 2006, 51, 2327. (10) Jeffrey, C. A.; Harrington, D. A.; Morin, S. Surf. Sci. 2002, 512, L367. (11) Chen, C.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 5439. (12) Tamura, K.; Ocko, B. M.; Wang, J. X.; Adzic, R. R. J. Phys. Chem. B 2002, 106, 3896. (13) Chen, C.; Kepler, K. D.; Gewirth, A. A.; Ocko, B. M.; Wang, J. J. Phys. Chem. 1993, 97, 7290. (14) Tamura, K.; Wang, J. X.; Adzic, R. R.; Ocko, B. M. J. Phys. Chem. B 2004, 108, 1992. (15) Oh, I.; Biggin, M. E.; Gewirth, A. A. Langmuir 2000, 16, 1397.
Published on Web 03/08/2010
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Figure 2. STM image of Au(111) in 1 M HClO4 þ 5 mM Bi2O3 at 0.21 V vs Ag/AgCl. The yellow and blue spheres show Au and Bi atoms, respectively. The bias voltage and set-point current were (a) -0.059 V and 1.2 nA and (b) -0.076 V and 1.7 nA, respectively. Figure 1. Cyclic voltammogram of Au(111) in 1 M HClO4 þ -1
5 mM Bi2O3. The scanning rate is 0.002 V s .
HPRR on upd Bi on Au(111) using density functional theory (DFT) calculations.16,17 The intermediate state for HPRR depends on the structure of the Bi adlayer. Although previous SXD studies indicate the 2 2 periodicity of Bi, detailed structural analysis was not done. It is necessary to reveal the structure in detail for the elucidation of the reaction processes of HPRR. In this letter, we have determined the detailed structure of the Bi adlayer and coadsorbed species on the Au(111) electrode using SXD, STM, and DFT calculations. A new (2 2)Bi honeycomb structure activating HPRR has been found. The active site for HPRR is reinvestigated.
Experimental Section An Au(111) disk crystal (Surface Preparation Laboratory, The Netherlands) and the (111) facet of a single-crystal bead of Au were used for measurements. The sample was annealed in an H2 flame and then cooled to room temperature in an Ar atmosphere. The annealed surface was protected with a droplet of ultrapure water (Milli-Q Advantage) and transferred to the electrochemical cell. The electrolyte solution was prepared with HClO4 (Merck Suprapur), Bi2O3 (Aldrich), H2O2 (Santoku Chemical), and ultrapure water. The reference electrode was Ag/AgCl for all of the measurements. In-situ STM measurements were carried out using a Nanoscope E (Digital Instruments). W wires were electrochemically etched in 4 M KOH and coated with clear nail polish so as to minimize the Faradaic current. SXD measurements were performed with a multiaxis diffractometer at BL13XU (SPring-8) for surface and interface structure determination.18 The X-ray wavelength used was 0.061 nm. The diffracted beam was measured using the symmetric ω = 0 mode. Integrated intensities were measured with rocking scans and then were corrected for Lorentz and polarization factors. A hexagonal surface coordinate system was used for the Au(111) crystal in which the reciprocal √ wave vector √ is Q = Ha* þ Kb* þ Lc*, where a* = b* = 4π/ 3a, c* = 2π/ 6a, a = 0.2885 nm, and L is along the direction normal to the surface. Structure refinements were conducted using the least-squares method with the ANA-ROD program.19 Optimization of the structural parameters, scale (16) Li, X.; Gewirth, A. A. J. Am. Chem. Soc. 2003, 125, 7086. (17) Li, X.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 5252. (18) Sakata, O.; Furukawa, Y.; Goto, S.; Mochizuki, T.; Uruga, T.; Takeshita, K.; Ohashi, H.; Ohta, T.; Matsushita, T.; Takahashi, S.; Tajiri, H.; Ishikawa, T.; Nakamura, M.; Ito, M.; Sumitani, K.; Takahashi, T.; Shimura, T.; Saito, A.; Takahashi, M. Surf. Rev. Lett. 2003, 10, 543. (19) Vlieg, E. J. Appl. Crystallogr. 2000, 33, 401.
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factor, surface fraction factor, occupancy factor, and roughness factor was performed. All of the parameters were refined simultaneously using crystal truncation rods and fractional order rods. The integrated intensity of 620 reflections along 338 CTRs and 282 fractional order rods was collected. The structure factors were averaged by assuming the P3m1 space group and yielded 329 independent reflections with a reproducibility of 13%. DFT calculations were carried out with the projector augmented wave Vienna ab initio simulation program (PAW-VASP).20 The structure model was composed of a six-layer Au slab with a 2 2 surface unit cell. The slabs were separated by approximately 1 nm of vacuum. The first Brillouin zone was sampled with a 6 6 1 k-point mesh within the Monkhorst-Pack scheme.21 A plane-wave cutoff of 400 eV has been used. Electron exchange and correlation were described within the PW91 generalized gradient approximation (GGA).22 The calculated lattice constant of bulk Au was 0.418 nm, which was within the error of 2.4% for the experimental value.
Results and Discussion STM and SXD measurements were carried out at 0.21 V versus Ag/AgCl, at which significant reduction current was observed in the solution containing hydrogen peroxide. A previous SXD study revealed that the 2 2 periodicity remains unchanged during ORR.12 Figure 2a shows a high-resolution STM image of Au(111) in 1 M HClO4 þ 5 mM Bi2O3. The STM image exhibits an ordered honeycomb pattern, and the distance between the neighboring spots is 0.35(2) nm. The spots are assigned to Bi atoms adsorbed on Au(111). The periodicity corresponds to the (2 2)-Bi honeycomb structure with θBi = 0.50 as shown in the hard-sphere model in Figure 2a. This coverage is comparable to the value estimated from the charge on the first upd peak (red area in Figure 1). Previous STM and AFM studies reported the (2 2)-Bi structure with θBi = 0.25.11,13 At 0.21 V, we also observed a similar image at different bias voltages of the STM tips as shown in Figure 2b. The difference in these STM images is unclear. The 2 2 image corresponding to θBi = 0.25 may originate from coadsorbed species overlaid on upd Bi. A upd metal often forms an ion-pair complex with a coadsorbed anion.23 A similar honeycomb structure was reported on upd Cu on Au(111) in sulfuric (20) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558. Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251. Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169. Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1785. (21) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (22) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. (23) Nakamura, M.; Matsunaga, K.; Kitahara, K.; Ito, M.; Sakata, O. J. Electroanal. Chem. 2003, 554, 175.
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Nakamura et al. Table 1. Comparison of the Adsorption Energy Ead (eV) and Structural Data for upd Bi on Au(111) by DFT Calculations (2 2)-Bi top
(2 2)-Bi hcp
(2 2)-Bi fcc
(2 2)-Bi honeycomb fcc þ hcp
2.760 3.610 3.695 3.849 -Ead/atoma 261 276 275 296 dAu-Bi (pm)b c 212 212 211 238 zAu-Bi (pm) a The adsorption energy is defined as the energy difference per Bi atom between the adsorbed system and the sum of the Au slab and the free atom. b The bond length between Au and Bi. c The vertical distance between averaged Au and Bi layers.
Figure 3. (a) Structure factors from crystal truncation rods and the fractional order rod of the (2 2)-Bi honeycomb on Au(111). Dashed and solid lines are calculated structure factors from the optimized (2 2)-Bi honeycomb and the chlorine-incorporated (2 2)-Bi honeycomb model, respectively. Dotted lines are calculated structure factors from the (2 2)-Bi model with θBi = 0.25. (b) Schematic top (left) and side (right) views of the chlorine-incorporated (2 2)-Bi honeycomb on Au(111) as determined by SXD.
acid solution.24,25 The honeycomb structure of upd Cu is stabilized by the accommodation of the (bi)sulfate anion in the honeycomb center. The detailed (2 2)-Bi honeycomb structure has been determined using SXD. Figure 3a shows the crystal truncation rod (CTR) and fractional order rod scattering intensities that originate from the (2 2)-Bi honeycomb structure in 1 M HClO4 þ 5 mM Bi2O3 at 0.21 V. The profile of specular reflectivity (00 rod) is sensitive to the layer distance of the surface and the coverage of adsorbate.26,27 The dips between Bragg peaks (at L = 1.5 and 4.2) suggest the interference of the Au substrate and the Bi layer with θBi = 0.5. In the middle of the two Bragg peaks, the amplitude of scattered X-rays from the Au substrate is given by 1/2 fAu. When the topmost Au layer displaces the Bi layer with θBi = 0.5, the amplitude is given as follows: fsurf = 1/2 fAu - fAu þ 1/2 fBi= -1/2 fAu þ 1/2 fBi.28 Because the atomic number of Bi is close to that of Au, the amplitude is canceled out. We confirmed the validity of our (2 2)-Bi honeycomb model including the in-plane structure using the CTRs and fractional order rod scattering intensities. The honeycomb model is composed of two Bi atoms at hcp and fcc hollow sites on Au(111), as shown in Figure 2a. The favored adsorption site of Bi is the hollow site rather than the atop site, as will be shown later. The structural parameters of Bi and the first Au layer were refined by leastsquares refinement. The goodness of fit (χ2) of the optimized (2 2)-Bi honeycomb model is 2.0; however, the optimization of the (2 2)-Bi model with θBi = 0.25 gives χ2 = 4.8. The fractional order rods as well as the reflectivity result also support the validity of the honeycomb (2 2)-Bi structure as shown in Figure 3a. (24) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R. Phys. Rev. Lett. 1995, 75, 4472. (25) Nakamura, M.; Endo, O.; Ohta, T.; Ito, M.; Yoda, Y. Surf. Sci. 2002, 514, 227. (26) Feidenhans’l, R. Surf. Sci. Rep. 1989, 10, 105. (27) Robinson, I. K.; Tweet, D. J. Rep. Prog. Phys. 1992, 55, 599. (28) Takahashi, M.; Hayashi, Y.; Mizuki, J.; Tamura, K.; Kondo, T.; Naohara, H.; Uosaki, K. Surf. Sci. 2000, 461, 213.
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Because the surface fraction of the (2 2)-Bi honeycomb is estimated to be 0.998 from SXD analysis, this Bi layer does not include the (2 2)-Bi phase with θBi = 0.25. The counter anion may be coadsorbed on upd Bi as described in the paragraph reporting the STM results. The perchlorate anion should be included in the (2 2)-Bi honeycomb structure. The position of chlorine has been determined, giving a final χ2 of 1.8. Figure 3b shows the optimized structure of the chlorineincorporated (2 2)-Bi honeycomb structure on Au(111). The structure factors of (1/2, 0) and (0, 1/2) rods oscillate along L, as shown in Figure 3a. The chlorine-incorporated model reproduces this oscillation well. The chlorine of the perchlorate anion is located at the honeycomb center at a distance of 0.61(1) nm from the first layer of Au. The nearest-neighbor distance between upd Bi and chlorine was 0.47(1) nm. These distances support the fact that the perchlorate anion does not bind to upd Bi directly. This point is described further below. The model, including oxygen atoms, did not improve the value of χ2. Oxygen atoms of the perchlorate anion and hydrated water may be randomly orientated in the honeycomb structure. We could not find a coadsorbed hydroxyl group (OHad) via the infrared reflection absorption spectroscopy (IRAS) measurement. A previous study, however, indicates that OH is coadsorbed with Bi with θOH = 0.17 according to the chronocoulometry in 1 mM Bi3þ þ 0.1 M HClO4 at various pH values (1.04 e pH e 2.65).29 We tried to confirm the reproducibility of this report. However, Bi3þ was precipitated as bismuth hydroxide after pH adjustment with NaOH because Bi3þ cannot exist in aqueous solution stably at [Hþ] < 0.5 M.30,31 The adjustment of pH causes a decrease in the concentration of Bi3þ in the solution; we are afraid that the surface concentration of OH- cannot be estimated accurately with this method. DFT calculations were performed to confirm the honeycomb structure of Bi and the coadsorption structure of the hydrated perchlorate anion. First, the adsorption energy of Bi was estimated on Au(111), as shown in Table 1. Bi favors the hollow site on Au(111), and there is not much structural difference between fcc and hcp sites. The Bi adsorption energy of the (2 2)-Bi honeycomb structure is more stable by 0.15 eV per Bi than that of (2 2)-Bi at the fcc hollow site. In the case of upd Cu in H2SO4 solution, the (bi)sulfate anion stabilizes the honeycomb structure of Cu.24,25 However, upd Bi favors the honeycomb structure without the coadsorbed anion. The bond length of Bi-Bi is 0.33 nm, which is comparable to that in the bulk phase (0.31 nm). These results suggest that neighboring Bi atoms interact attractively and form the honeycomb structure. According to the previous DFT study by Gewirth et al., the (2 2)-Bi structure with θBi = 0.5 is more unstable than the (2 2)-Bi with θBi = 0.25.16 (29) Niece, B. K.; Gewirth, A. A. Langmuir 1996, 12, 4909. (30) Grener, F.; Sillen, L. G. Acta Chem. Scand. 1947, 1, 631. (31) Pokric, B.; Pucar, Z. J. Inorg. Nucl. Chem. 1973, 35, 3287.
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Figure 4. Schematic top and side views of (a) the hydrated perchlorate anion and (b) hydrogen peroxide on the (2 2)-Bi honeycomb as determined by DFT calculations.
They used a model of θBi = 0.5 where all of the Bi atoms are adsorbed on fcc sites (i.e., not 2 2 but 2 1). In their model, the nearest-neighbor Bi-Bi distance is 0.29 nm, which is shorter than the bulk value. The short Bi-Bi distance may cause a repulsive interaction, resulting in the instability of the 2 1 structure. The structure of the hydrated perchlorate anion adsorbed on upd Bi has been investigated using DFT calculations. Figure 4a shows the optimized model of the hydrated perchlorate anion on the (2 2)-Bi honeycomb structure. Because the orientation of the perchlorate anion and the water of hydration cannot be determined by SXD, the position of the water of hydration and the orientation of the perchlorate anion are expected to be disordered. Therefore, the optimized model is one of the possible structures of the hydrated perchlorate anion. The water of hydration is located 0.25 nm above the top of the upd Bi. The Au-Cl and the Bi-Cl distances are 0.64 and 0.49 nm, respectively. These values are consistent with the SXD results. We also considered a model where the perchlorate anion is adsorbed directly onto the honeycomb center of Bi. Three oxygen atoms of the perchlorate anion are bonded to Bi atoms, as is the case for the (bi)sulfate anion adsorbed on upd Cu on Au(111). The distance between Au and the chlorine of the perchlorate anion is 0.51 nm, which is shorter than that determined by SXD. This result also supports the fact that the perchlorate anion is not directly adsorbed on Bi. The adsorption structure of hydrogen peroxide on the (2 2)Bi honeycomb has been examined in order √ to elucidate the active site for HPRR. The close-packed p 3 structure without the vacancy is inactive for HPRR; the O-O bond of hydrogen
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peroxide cannot be dissociated. The (2 2)-Bi honeycomb is a key structure in the activation of HPRR. The model, in which hydrogen peroxide is adsorbed at the honeycomb center with the O-O axis parallel to the surface, is optimized by minimizing the total energy. Figure 4b shows the optimized structure of hydrogen peroxide on upd Bi. The total energy of the optimized structure is more stable by 2.75 eV than the sum of free hydrogen peroxide and the (2 2)-Bi honeycomb on Au(111). The O-O distance expands from 0.15 (the value of the isolated molecule) to 0.29 nm. Hydrogen peroxide is dissociated to two adsorbed OHs (OHad) on upd Bi. The OHad’s form OH-O hydrogen bonds. The Au-O distances of the two OHad’s are 0.37 and 0.41 nm. The distances are too long for OHad to interact with the Au atom. Gewirth et al. investigated the HPRR mechanism on the (2 2)-Bi structure using DFT calculations.16 They also suggested the reaction pathway through OHad. In our model, the hydrated percholorate anion is located at the honeycomb center. In the solution containing hydrogen peroxide, however, hydrogen peroxide may replace the hydrated perchlorate anion in the honeycomb structure. The adsorption energy of the hydrated perchlorate anion on the (2 2)-Bi honeycomb surface is lower by 0.1 eV than that of hydrogen peroxide. The percholorate anion will be replaced by hydrogen peroxide spontaneously during HPRR. The O-O bond is dissociated in the honeycomb center because of the attractive force of upd Bi. The honeycomb structure of upd Bi is important in the effective dissolution of the O-O bond of hydrogen peroxide.
Conclusions The detailed structure of upd Bi on Au(111) has been determined using SXD. The Bi atoms form the (2 2)-Bi honeycomb structure at 0.21 V. The (2 2)-Bi honeycomb structure activates the hydrogen peroxide reduction reaction. SXD and DFT calculations show that the hydrated perchlorate anion is located at the center of the honeycomb structure. Hydrogen peroxide may expel the perchlorate anion from the surface during HPRR. DFT calculations reveal that OHad is adsorbed on upd Bi as an intermediate species of HPRR. Hydrogen peroxide goes into the honeycomb center where the O-O bond is dissociated. Acknowledgment. SXD measurements were supported by the Japan Synchrotron Radiation Research Institute (JASRI) under proposal numbers 2007A104, 2008A1227, and 2008B1395. This work was supported by a grant-in-aid (KAKENHI) for young scientists (B, no. 20710082), the Iketani Science and Technology Foundation, and the New Energy and Industrial Technology Development Organization (NEDO).
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