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2006, 110, 13319-13321 Published on Web 06/16/2006
Electron Tomography of Nafion Ionomer Coated on Pt/Carbon Black in High Utilization Electrode for PEFCs Hiroyuki Uchida,† Jung Min Song,† Shinsuke Suzuki,† Eiko Nakazawa,‡ Norio Baba,§ and Masahiro Watanabe*,| Interdisciplinary Graduate School of Medicine and Engineering and Clean Energy Research Center, UniVersity of Yamanashi, 4 Takeda, Kofu 400-8510, Japan, Hitachi High-Technologies Corporation, 11-1 Ishikawa-cho, Hitachi-naka 312-0057, Japan, and Department of Electrical Engineering, Kogakuin UniVersity, Nakano-cho, Hachioji 192-0015, Japan ReceiVed: May 2, 2006; In Final Form: June 5, 2006
To confirm the superiority of newly developed electrocatalyst layer (ECL) for polymer electrolyte fuel cells, three-dimensional dispersion states of Nafion ionomer in Pt/carbon black agglomerates were analyzed by electron tomography based on multiple TEM images taken at different tilt angles. Uniform distribution of the ionomer has been first observed, proving the high catalyst utilization in the new ECL distinctive from that of the conventional one.
Introduction Polymer electrolyte fuel cells (PEFCs) are attracting much attention as clean and efficient power sources for electric vehicles and residential cogeneration systems. A typical electrocatalyst layer (ECL) consists of Pt/CB (Pt nanoparticles dispersed on carbon black) coated with perfluorosulfonated ionomer (such as Nafion) as the proton conducting network (see Supporting Information). To reduce the Pt loading amount, it is essential to increase the catalyst cluster utilization by optimizing the ECL structure, besides an enhancement in the specific activity of the catalyst itself for the oxygen reduction reaction by alloying Pt with other elements.1,2 In typical Pt/ CB, two different types of pores were observed, i.e., primary pores with a diameter dprim < ca. 0.1 µm and secondary pores with a diameter dsec > ca. 0.1 µm. The primary pore and the secondary pore are considered to correspond to a space within the Pt/CB agglomerates and a space between the agglomerates, respectively.3 Because a large part of Pt electrocatalysts (>90%) is located on the wall of the primary pores inside the Pt/CB agglomerates, an increase in the fill level of the primary pore with the ionomer is important. The ECL has been formed conventionally by coating a catalyst ink (mixture of Pt/CB, ionomer, and solvent) on a gas diffusion layer or on the electrolyte membrane. However, the Pt utilization has been low so far, because the ionomer predominantly deposited on the wall of the secondary pores and could not penetrate into the primary pores. Thus, a large fraction of Pt catalysts uncoated with the PFSI in the primary pores cannot be utilized for the fuel cell reaction. * Corresponding author. Tel: +81 55 220 8620. Fax: +81 55 254 0371. E-mail:
[email protected]. † Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi. ‡ Hitachi High-Technologies Corporation. § Department of Electrical Engineering, Kogakuin University. | Clean Energy Research Center, University of Yamanashi.
10.1021/jp062678s CCC: $33.50
Recently, we developed a novel preparation procedure of the ECL with high Pt utilization.4 It was found that the cathode prepared with the new catalyst ink exhibited very high performance, i.e., high catalyst utilization and improved gas diffusivity. The microstructure analysis by mercury porosimetry indicated that the Nafion ionomer was effectively introduced into the primary pores of Pt/CB agglomerates. It was also demonstrated by scanning transmission electron microscope (STEM) that the ionomer was distributed more uniformly among Pt/CB agglomerates than those prepared in the conventional manner. Observation of the three-dimensional (3D) dispersion state of the ionomer among Pt/CB agglomerates is essential to finding a clue for designing high-performance ECL. We have focused on electron tomography based on multiple TEM images of the same object. This technique has been employed to characterize the 3D structure of samples in biology5,6 and materials science.7 This technique is analogous to X-ray computer tomography (CT) in the medical area. Here, we, for the first time, demonstrate electron tomographic images of the ionomer-coated Pt/CB. Uniform dispersion of the ionomer over the Pt/CB agglomerates was directly observed for the newly developed ECL. Experimental Section Catalyst samples were prepared in the same manner as in our previous paper.4 A commercial Pt/CB (46.3 wt % Pt, TEC10E50E, Tanaka Kikinzoku Kogyo), Nafion ionomer solution (DE-521, E.I. DuPont, 5 wt % Nafion), and solvent (water + 2-propanol) were mixed by a ball mill. This was heated in a N2-filled autoclave at 200 °C, followed by quenching. The resulting paste was vacuum-dried at 90 °C. The weight ratio of Nafion to CB (N/C) was adjusted to the optimum value of 0.7.4 For a comparison, the catalyst paste without the autoclave treatment was also prepared. A small amount of each sample was embedded in an epoxy resin (EPON815, Shell Chemicals), followed by slicing with a diamond knife at a thickness of about © 2006 American Chemical Society
13320 J. Phys. Chem. B, Vol. 110, No. 27, 2006
Figure 1. 2D-TEM images of H+-Nafion-Pt/CB (N/C ) 0.7) with (A) and without (B) autoclave treatment. The bright field images were B/W inverted (negative images) to compare them with those in Figure 2 or 3.
50 nm with an ultramicrotome (Leica Microsystems). These specimens were observed by TEM (H-7650, Hitachi High-Tech. Corp.) at 120 kV acceleration voltage. Electron tomography was applied to obtain the 3D images of the samples. The TEM images of the samples were serially recorded at tilt angles from 60° to -60° degrees with a 1° step (total of 121 images). The position of each image was precisely aligned. Then, the tilt-series images were reconstructed by using EM image prompter software (Hitachi High-Tech. Corp.) with the topography-based reconstruction technique, in which the estimated profiles by stereo-photogrammetry and the density distribution of images were used as the initial approximation of the calculations for reconstruction.8,9 Finally, the effect of the missing wedge of information due to the mechanical limitation of sample tilting angles was minimized in the reconstructed images. Results and Discussion Typical 2D-TEM images of the H+-form Nafion-coated catalyst particles (H+-Nafion-Pt/CB, N/C ) 0.7) prepared with and without the autoclave treatment are shown in Figure 1. White dots of a few nanometers are Pt catalysts supported on CB agglomerates. It is quite difficult to find out the clear difference in the ionomer distribution between the autoclavetreated one (Figure 1A) and the untreated one (Figure 1B), because H+-Nafion and the epoxy resin embedding the specimen exhibited almost the same contrast. Recently, Xie et al. observed H+-Nafion-coated catalyst layers (Pt/CB or
Letters Pt3Cr/CB) by TEM.10 They also commented that imaging of the ionomer network within the catalyst layers was very difficult due to almost the same contrast of the ionomer and the epoxy resin. Then, to distinguish the ionomer from the epoxy resin, the H+-Nafion-Pt/CB was Ag+ ion-exchanged, followed by washing with water to remove excess Ag+. The Ag+-NafionPt/CB particles were embedded in the epoxy resin, followed by slicing. Sulfonate groups ion-exchanged with Ag+ ions having a high electron absorption coefficient allow us to identify the location of the ionomer. Because Ag+ ions show almost the same contrast as Pt catalysts on CB surfaces in the TEM images, the comparison of Ag+-Nafion-Pt/CB in Figure 2 with
Figure 2. 2D-TEM images (tilt angle ) 0) of Ag+-Nafion-Pt/CB (N/C ) 0.7) with (A) and without (B) autoclave treatment.
H+-Nafion-Pt/CB in Figure 1 gives valuable information. It is found that bright aggregates stick on the Pt/CB without the autoclave treatment (Figure 2B), while bright finer spots of 1020 nm are dispersed over the Pt/CB with the autoclave treatment (Figure 2A). We have assigned these bright aggregates or spots as mass of Ag+-Nafion ionomer because no stickers or spots of such sizes were observed on H+-Nafion-Pt/CB in Figure 1. String-like Ag+-Nafion ionomer can also be seen in Figure 2A. It should be noted that N/C in both samples in Figure 2A,B was the same at 0.7, and the only difference is the autoclave treatment for the sample in Figure 2A. Therefore, it is obvious that the Nafion ionomers were able to be distributed more uniformly by the autoclave treatment. It is noteworthy in Figure 2 that we cannot distinguish whether the location of the ionomer was on the surfaces of agglomerates or inside them, because the electron beam passed
Figure 3. 3D reconstructed images of Ag+-Nafion-Pt/CB (N/C ) 0.7) with (A) and without (B) autoclave treatment. Section images (slices parallel to x-z plane) perpendicular to the y-axis at the locations of arrows a to f are shown in panels a to f, respectively.
Letters through the specimen from one direction to give a 2D-projection image like a Roentgen graph. The computer tomography (CT) is a versatile tool to clarify the 3D structure of samples in medical, biological, and materials science applications. Figure 3A,B shows three-dimensionally reconstructed images of Ag+Nafion-Pt/CB, based on multiple (121 tilt series) TEM images on the same specimens observed in Figure 2A,B, respectively. A crucial difference in the distribution of the Ag+-Nafion ionomer is noted for the CT images of a to c and those of d to f in Figure 3. The Ag+-Nafion segregated around the center of the surface layer in images e and f, whereas small masses of Ag+-Nafion are well-dispersed even in the cross section of the agglomerates. The present tomography clearly supports our PEFC experimental result4 that the better proton network gave improved Pt utilization at the catalyst in Figure 3A than the conventional one. Thus, the autoclave treatment evidently promoted the penetration of Nafion ionomer into primary pores of the agglomerate, probably because of the reduction of the effective micelle size of the ionomer or the increase in the molecular flexibility under such a high-temperature treatment. Hence, the electron tomography is a powerful tool to clarify 3D distribution of the ionomer over the Pt/CB agglomerates, which can complement the analyses by the electrochemical method and the porosimetry. Acknowledgment. This work was supported by the fund for “Leading Project; Next Generation Fuel Cells” of Ministry of Education, Science, Culture, Sports and Technology of Japan. We are indebted to Mr. Takashi Yukawa in Hitachi High-Tech. Corp. for arranging the use of the electron tomography. Supporting Information Available: Illustration of ECL and movies of rotating 3D images shown in Figure 3. This material is available free of charge via the Internet at http://pubs.acs.org.
J. Phys. Chem. B, Vol. 110, No. 27, 2006 13321 References and Notes (1) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750. (2) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (3) Watanabe, M.; Tomikawa, M.; Motoo, S. J. Electroanal. Chem. 1985, 195, 81. (4) Song, J.-M.; Suzuki, S.; Uchida, H.; Watanabe, M. Langmuir, in press, DOI: 10.1021/1a060671w. (5) Frank, J. Electron Tomography: Three-dimensional Imaging with the Transmission Electron Microscope; Plenum Press: New York, 1992. (6) Sali, A.; Glaeser, R.; Earnest, T.; Baumeiste´r, W. Nature (London) 2003, 422, 216. (7) (a) Midgley, P. A. ; Weyland. M. Ultramicroscopy 2003, 96, 413. (b) Koster, A. J.; Ziese, U.; Verkleij, A. J.; Janssen, A. H.; de Jong, K. P. J. Phys. Chem. B 2000, 104, 9368. (c) Weyland, M.; Midgley, P. A.; Thomas, J. M. J. Phys. Chem. B 2001, 105, 7882. (d) Janssen, A. H.; Koster, A. J.; de Jong, K. P. J. Phys. Chem. B 2002, 106, 11905. (e) Midgley, P. A.; Thomas, J. M.; Laffont, L.; Weyland, M.; Raja, R.; Johnson, B. F. G.; Khimyak, T. J. Phys. Chem. B 2004, 108, 4590. (f) Arslan, I.; Yates, T. J. V.; Browning, N. D.; Midgley, P. A. Science 2005, 309, 2195. (g) de Jong, K. P.; van den Oetelaar, L. C. A.; Vogt, E. T. C.; Eijsbouts, S.; Koster, A. J.; Friedrich, H.; de Jongh, P. E. J. Phys. Chem. B 2006, 110, 10209. (8) Baba, N.; Kogami, Y.; Nonaka, K.; Oguchi, Y.; Katayama, E. In Proceedings of the 8th Asia-Pacific Conference on Electron Microscopy; Kanazawa, Japan, 2004; p 172. (9) First, the sample-existing region (showing the density distribution) was defined by accurately measured z-positions (vertical to an untilted image plane) for all the fine image segments by tracing them throughout all the tilt-series images with the aid of the template matching technique in image processing. Subsequently, by using every tilt image considered to be a backprojected density distribution, the tomographic reconstruction was performed with a modified simultaneous iterative reconstruction technique (SIRT) under constraint conditions given by the sample-existing region.8 The effect of the missing wedge of information (MWI) was minimized in the reconstructed images, because the topographic measurements similar to multiple stereo-photogrammetry were not affected by the MWI. (10) Xie, J.; Wood, D. L.; More, K. L.; Atanassov, P.; Borupa, R. L. J. Electrochem. Soc. 2005, 152, A1011.
