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Agglomeration behavior of nickel particles on YSZ electrolyte
Solid State Ionics 225 (2012) 65–68
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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Agglomeration behavior of nickel particles on YSZ electrolyte Haruo Kishimoto a,⁎, Akihito Suzuki b, Taro Shimonosono a, Manuel E. Brito a, Katsuhiko Yamaji a, Teruhisa Horita a, Fumio Munakata b, Harumi Yokokawa a a b
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central No. 5, Higashi 1-1-1, Tsukuba, Ibaraki, 305‐8565 Japan Advanced Research Laboratories, Tokyo City University, Todoroki 8-15-1, Setagaya-ku, Tokyo, 158‐0082 Japan
a r t i c l e
i n f o
Article history: Received 7 September 2011 Received in revised form 14 March 2012 Accepted 9 April 2012 Available online 5 May 2012 Keywords: Solid oxide fuel cell (SOFC) Nickel agglomeration Yttria stabilized zirconia (YSZ) Scanning electron microscope (SEM) Scanning probe microscope (SPM)
a b s t r a c t Agglomeration behavior of nickel particles on yttria stabilized zirconia (YSZ), NiO doped YSZ and TiO2 doped YSZ substrates was examined by scanning electron microscopy (SEM) as two dimensional (2D) analysis and scanning probe microscopy (SPM) as three‐dimensional (3D) analysis. For 2D analysis, binarized SEM image was analyzed to examine the agglomeration factor. In 3D analysis, wettability analyzing method was adopted for solid particle on solid substrate, and “apparent” contact angle was evaluated by the following equation; θ ¼ 2 tan
−1
ð2 h=dÞ
where d and h are the observed particle diameter and the observed particle height of each agglomerated particle obtained by SPM analysis. Results from both analysis techniques revealed that NiO doped YSZ tend to enhance the nickel agglomeration, whereas TiO2 doped YSZ shows better wettability especially for larger nickel particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ceramic and metal composite material (so called cermet), which consists of yttria stabilized zirconia (YSZ) and nickel metal, have been generally used as anode for solid oxide fuel cells (SOFCs) to provide oxide ion conducting path inside the electrodes and to adjust the effective thermal expansion coefficient to that of the electrolyte materials. In this anode, nickel sintering in the anode is one of the serious problems in the long time operation of SOFC systems. For example, nickel sintering is enhanced by long time operation [1–6], impurities (such as sulfur components) in the fuel [7–13], P(H2O) and redox cycle [14–17] and interaction with the oxide component in the anode [18,19]. The microstructure of nickel cermet anode is complex, and therefore, it is difficult to analyze the microstructure change in the cermet anode. To analyze the nickel sintering behavior in the cermet anode, it is necessary to carry out the statistical analysis of two dimensional (2D) cross-section scanning electron microscope (SEM) images [17] or three‐dimensional (3D) re-construction analysis with FIB-SEM technique [19–21]. The former case involves the uncertainty of initial microstructure. The latter has to use rather expensive equipments and the well-trained analyzing technique; since
⁎ Corresponding author at: AIST Tsukuba Central 5, Higashi 1-1-1, Tsukuba, 305‐8565 Japan. Tel.: + 81 29 861 4542; fax: + 81 29 861 4540. E-mail address: [email protected] (H. Kishimoto). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.04.010
this is a destructive method, the initial state of the analyzed microstructure is still somewhat uncertain. It would be very effective if the nickel sintering behavior could be examined using a 2D model structure. Our group has developed a method for analyzing gas–nickel‐oxide substrate interaction by examining the agglomeration behavior of nickel thin film on oxide substrates under a controlled condition [22,23]. In the present study, the nickel agglomeration behavior was examined with a similar technique. Nickel agglomeration was produced by annealing the nickel thin film under reducing condition at a SOFC operating temperature as 1173 K in a similar manner. In the present investigation, two analyzing methods were used for agglomerated nickel particles on oxide substrates. One is 2D analysis with binarization of SEM image. In this case, agglomerated nickel particles were statistically analyzed to examine the tendency of nickel agglomeration. The other method is 3D analysis by scanning probe microscope (SPM) image [24]. An evaluation method of “wettability” for liquid particle on solid substrate is adopted for solid particles on solid oxide substrate. In fabrication of the cermet anode, NiO and YSZ are co-sintered at a high temperature in air and NiO dissolves into YSZ lattice [25]. Therefore, the effect of NiO dissolution in YSZ electrolyte on nickel agglomeration behavior was compared with those on YSZ and TiO2 doped YSZ system. It has been reported that TiO2 doping in YSZ is effective for nickel agglomeration [18,24] by measuring the electrochemical performance as well as the contact angles for liquid nickels on TiO2 doped YSZ.
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H. Kishimoto et al. / Solid State Ionics 225 (2012) 65–68
Fig. 1. Binarized backscattered electron image (BEI) for agglomerated nickel particles on zirconia substrates of a) YSZ, b) TiO2 doped YSZ (Ti-YSZ) and c) NiO doped YSZ (Ni-YSZ). White parts and black parts correspond to the nickel particles and zirconia substrates surface, respectively. White lines show the bisected borderline for two neighboring particles.
2. Experimental In the present study, nickel agglomeration behavior was investigated using two dimensional model electrodes. Nickel thin films were prepared on stabilized zirconia substrates. The effect of TiO2 doping on nickel agglomeration was reported for 5 mol% TiO2 doped YSZ [18], and solubility limit of NiO in YSZ is about 1 mol% [25]. Therefore, 8 mol% Y2O3 doped ZrO2 (8YSZ), 5 mol% TiO2–95 mol% 8YSZ (Ti-8YSZ) and 1 mol% NiO–99 mol% 8YSZ (Ni-8YSZ) were used as stabilized zirconia substrates in the present study. The commercial powders of 8YSZ (TZ-8Y, Tosoh Co., Japan), TiO2 (99.9% purity, Wako Pure Chemical Industries, Ltd., Japan) and NiO (Wako Pure Chemical Industries, Ltd., Japan) were mixed in a predetermined ratio by planetary ball mill with ethanol. Mixed powders were shaped into disks and pressed by CIP at 390 MPa. Pressed disks were sintered at 1673 K for 5 h in air. The surface of the sintered samples was mirrored with diamond slurry by polishing machine. Particle size of diamond slurry was 3–5 μm for all substrates and the density of polishing flaw was 1–3 lines in 5 μm × 5 μm area. Nickel thin film was prepared on the polished surface of substrates by RF sputtering method. The thickness of nickel film was controlled around 60 nm by sputtering time. The samples were annealed under a reducing condition of 1% humidified H2 gas for 2 h at 1173 K. The microstructure of agglomerated nickel particles was analyzed by scanning electron microscopy (SEM) and scanning probe microscopy (SPM). 3. Results and discussion 3.1. 2D analysis (SEM data analysis) Fig. 1 shows the binarized backscattered electron image (BEI) for agglomerated nickel particles on zirconia substrates. In this figure,
white parts and black parts correspond to the nickel particles and the zirconia substrate surface, respectively. Well isolated nickel particles were observed for all substrates. In this figure, it is hard to distinguish the difference of agglomeration behavior of the nickel particles on substrates. In Fig. 2, distributions of projected nickel particle area obtained from Fig. 1 were summarized. In crude distinction, distribution of projected nickel particle area shifts towards smaller and larger for Ti-YSZ and Ni-YSZ, respectively, compared with YSZ. The peak is observed around 0.4–0.5 μm for YSZ and those for Ti-YSZ and NiYSZ are around 0.3–0.4 μm and around 0.5–0.7 μm, respectively. Those agglomerated particle sizes were much smaller than the particle size of the YSZ substrates. The average area of projected nickel particles, S1, listed in Table 1, is in the order of Ti-YSZ b YSZ b Ni-YSZ, but the difference is very small. The averaged influenced area, S2, and the agglomeration factors defined as, S2/S1, for each substrate are also summarized in Table 1. The influenced area, S2, corresponds to the area enclosed by bisected borderlines for two neighboring particles (white lines in Fig. 1); it is estimated that nickel particles are formed by the diffusion of the nickel from this area. In this sense, an agglomeration factor, S2/S1, becomes large when nickel particle is resulted from the nickel diffusion within a large area. A large value of S2/S1 indicates that nickel sintering on the substrate is enhanced. The agglomeration factor analysis has revealed that nickel agglomeration was enhanced in the following order; Ti-8YSZ b 8YSZ b Ni-8YSZ. 3.2. 3D analysis (SPM data analysis) Fig. 3 shows the SPM images for agglomerated nickel particles on oxide substrates. By SPM analysis, as well known, we can obtain the three-dimensional (3D; 2D × height) information for agglomerated particles. For all substrates, hemispherical nickel particles were observed. In the principle of SPM analysis, the information of morphology around contact area cannot be necessarily obtained. Therefore, we have to estimate the contact angle between nickel and zirconia substrate by calculation. In this study, the wettability analyzing method was adopted as previously reported [24]; −1
Population / %
θ ¼ 2 tan
ð2 h=dÞ
ð1Þ
where θ is the contact angle, and d and h are particle diameter and particle height of each agglomerated particle obtained by SPM analysis. When θ is small, wettability of nickel on zirconia substrate can be Table 1 Average area of projected nickel particles, S1, averaged influenced area, S2, and agglomeration factor, S2/S1 for the substrates of YSZ, Ti-YSZ and Ni-YSZ.
Projected area of nickel particle / µm 2 Fig. 2. Distributions of projected nickel particle area obtained from backscattered electron images (BEI). Substrates are YSZ, TiO2 doped YSZ (Ti-YSZ) and NiO doped YSZ (NiYSZ). The particles observed in Fig. 1 were analyzed.
Ave. particle size, S1/μm2 Ave. influenced area, S2/μm2 Agglomeration factor, S2/S1
YSZ
Ti-YSZ
Ni-YSZ
0.129 0.497 3.842
0.126 0.440 3.480
0.147 0.611 4.162
H. Kishimoto et al. / Solid State Ionics 225 (2012) 65–68
a) YSZ
b) Ti-YSZ
67
c) Ni-YSZ
Fig. 3. Scanning probe microscope (SPM) images for agglomerated nickel particles on zirconia substrates of a) YSZ, b) TiO2 doped YSZ (Ti-YSZ) and c) NiO doped YSZ (Ni-YSZ).
Ti−YSZ b YSZ b Ni−YSZ This means that wettability for Ti-YSZ is better than that for YSZ and Ni-YSZ shows the worst. This tendency agrees with the results of the 2D analysis on SEM images as indicated above. All data for obtained d and h are plotted in Fig. 4. Most of the agglomerated nickel particles were located within YSZ grains, while there were a few particles on the grain boundaries or on the polishing flaws. The deformed particles were eliminated from the subsequent analysis because these particles must be affected from surface defects such as grain boundaries. In this figure, a constant contact angle, θ, is represented by a straight line from the origin as expressed by Eq. (1); those values for 60, 70, 80, and 90° are shown as dashed lines. For the YSZ and the Ni-YSZ substrates, observed d and h are distributed within an approximately linear relation over a whole observed particle size range. The contact angles are calculated as 77± 5° and 80± 7 for the YSZ and the Ni-YSZ substrates, respectively, in the range of 400 nmb d b 800 nm. For the Ti-YSZ substrate, however, the d–h relation seems to bend around d = 550 nm; the θ value gradually decreases with increasing particle size more than 550 nm, whereas for the smaller particles, the relation is nearly the same as YSZ [24]. 3.3. Effect of doping for the YSZ electrolyte The observed effect of doping to the YSZ substrate on the nickel agglomeration is the same between the 2D analysis and the 3D analysis; that is, nickel agglomeration tends to be enhanced by NiO doping in the YSZ and to be suppressed for the TiO2 doped YSZ except for the smaller nickel particles. Under the present experimental conditions, hemispherical nickel metal particles were formed on the flat surface of the oxide substrates. Therefore, we adopted Young's law to discuss the interaction between nickel particles and oxide substrate surface even though this law should be adapted to the liquid particle on the flat oxide substrate: γ YSZ ¼ γ Ni cos θ þ γNi=YSZ
ð2Þ
where γYSZ, γNi and γNi/YSZ are surface/interface tensions between YSZ and gas, between nickel and gas and between nickel and YSZ, Table 2 Averaged contact angle, θ, for the substrates of YSZ, Ti-YSZ and Ni-YSZ. Contact angle, θ, is calculated from the observed d and h by SPM analysis. θ = 2 tan− 1 (2h/d). Substrate
8YSZ
Ti-8YSZ
Ni-8YSZ
Contact angle (°)
77 ± 5
69 ± 7
80 ± 7
respectively. Within the present observation, the nickel particles can be regarded to have surfaces with a certain curvature. For such nickel particles, we can consider that surfaces have a certain thickness of “surface layer". It is reasonably assumed that such thickness does not depend on nickel particle size and, therefore, particle size effect can be validated from the volume ratio of the “surface layer” to the total volume of the particle. With decreasing particle size, this ratio becomes large so that the energetics of nickel is mainly dominated by the nickel surface energy. This means that the particle shape is mainly controlled by the nickel surface energy, γNi. On the contrary, with increasing size, the volume ratio becomes small so that not only the surface energy, γNi, but also the interface energy, γNi/YSZ, governs the energetics of nickel particles. This can explain why the doping effect appears only for the nickel particles having larger sizes. For the TiO2 doped YSZ, titanium in YSZ lattice is reduced from + 4 to +3 in a reducing gas and, as a result, the electronic conductivity increases by the electron formation on the titanium sites [26–28]. The doping of TiO2, therefore, gives rise to changes in energetics by introduction of a new interaction between Ni–TiO2 (TiO1.5) in the YSZ lattice. On the other hand, electronic conductivity does not increase but rather decrease for the NiO doped YSZ although nickel in the YSZ lattice is also reduced under a reducing condition [29]. These changes in electronic conductivity (or change of conduction electron density) may affect the γNi/YSZ. Lower electronic conductivity may lead to higher γNi/YSZ. It is also reported that nickel in the YSZ lattice is reduced to nickel metal and precipitates as nanoparticles along the grain boundaries [30,31]. In the present experimental
θ =90 o
80 o
70 o
Height, h / nm
regarded as “good”. Averaged θ for zirconia substrates are summarized in Table 2. Averaged θ is in the following order:
60 o
Averaged diameter, d / nm Fig. 4. Relation between average particle diameter, d, and height, h, for the nickel particles obtained from scanning probe microscope (SPM) images. Substrates are YSZ, TiO2 doped YSZ (Ti-YSZ) and NiO doped YSZ (Ni-YSZ).
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H. Kishimoto et al. / Solid State Ionics 225 (2012) 65–68
condition, nickel metal nanoparticles must be precipitated on the electrolyte surface but in the interface, essentially no change takes place. This suggests no change in γNi/YSZ, but in γYSZ. By using the same technique, we are now examining the nickel agglomeration behavior induced by water vapor and fuel impurities such as sulfur. 4. Conclusion Nickel agglomeration behavior on the YSZ electrolyte was analyzed by SEM technique (2D analysis) and SPM technique (3D analysis) and the doping effect of TiO2 and NiO was examined. By the 2D analysis, the tendency of doping effect on nickel agglomeration can be easily and statistically evaluated. The 3D analysis makes it possible to demonstrate the detailed information such as nickel particle size effect on the nickel agglomeration behavior. Nickel agglomeration behavior is influenced by dopant metal oxides. For the 1 mol% NiO doped YSZ, nickel agglomeration tends to be enhanced. For the 5 mol% TiO2 doped 8YSZ, nickel agglomeration is reduced, especially for larger nickel particle size. It is suggested that the balance in surface/interface tension is different between the small (less than sub-micron size) and the large (may be more than 500 nm) nickel particles in the TiO2 doped YSZ. Acknowledgement A part of this study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] T. Iwata, J. Electrochem. Soc. 143 (1996) 1521. [2] A. Ioselevich, A.A. Kornyshev, W. Lehnert, J. Electrochem. Soc. 144 (1997) 3010–3019. [3] A. Faes, A. Hessler-Wyser, D. Presvytes, C.G. Vayenas, J. Van herle, Fuel Cells 9 (2009) 841–851. [4] D. Simwonis, F. Tietz, D. Stöver, Solid State Ionics 132 (2000) 241–251. [5] R. Vaßen, D. Simwonis, D. Stöver, J. Mater. Sci. 36 (2001) 147–151. [6] Z. Jiao, N. Takagi, N. Shikazono, N. Kasagi, J. Power Sources 196 (2011) 1019–1029.
[7] F.N. Cayan, M. Zhi, S.R. Pakalapati, I. Celik, N. Wu, R. Gemmen, J. Power Sources 185 (2008) 595–602. [8] C.A. Coyle, O.A. Marina, E.C. Thomsen, D.J. Edwards, C.D. Cramer, G.W. Coffey, L.R. Pederson, J. Power Sources 193 (2009) 730–738. [9] C. Xu, J.W. Zondlo, H.O. Finklea, O. Demircan, M. Gong, X.-B. Liu, J. Power Sources 193 (2009) 739–746. [10] H. Kishimoto, Y.P. Xiong, K. Yamaji, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, J. Chem. Eng. Jpn. 40 (2007) 1178–1182. [11] H. Kishimoto, T. Horita, K. Yamaji, M.E. Brito, Y.-P. Xiong, H. Yokokawa, J. Electrochem. Soc. 157 (2010) B802–B813. [12] H. Kishimoto, K. Yamaji, Y.-P. Xiong, T. Horita, M.E. Brito, H. Yokokawa, ECS Trans. 17 (2009) 31–35. [13] D.G. Ivey, E. Brightman, N. Brandon, J. Power Sources 195 (2010) 6301–6311. [14] T. Matsui, R. Kishida, J.Y. Kim, H. Muroyama, K. Eguchi, J. Electrochem. Soc. 157 (2010) B776–B781. [15] H. Muroyama, H. Sumi, R. Kishida, J.-Y. Kim, T. Matsui, K. Eguchi, ECS Trans. 35 (2011) 1379–1387. [16] T. Hatae, Y. Matsuzaki, Y. Yamazaki, Solid State Ionics 179 (2008) 274–281. [17] L. Holzer, B. Iwanschitz, Th. Hocker, B. Münch, M. Prestat, D. Wiedenmann, U. Vogt, P. Holtappels, J. Sfeir, A. Mai, Th. Graule, J. Power Sources 196 (2011) 1279–1294. [18] D. Skarmoutsos, A. Tsoga, A. Naoumidis, N. Nikolopoulos, Solid State Ionics 135 (2009) 439–444. [19] J.R. Wilson, M. Gameiro, K. Mischaikow, W. Kalies, P.W. Voorhees, S.A. Barnett, Microsc. Microanal. 15 (2009) 71–77. [20] P.R. Shearing, J. Golbert, R.J. Chater, N.P. Brandon, Chem. Eng. Sci. 64 (2009) 3928–3933. [21] H. Iwai, N. Shikazono, T. Matsui, H. Teshima, M. Kishimoto, R. Kishida, D. Hayashi, K. Matsuzaki, D. Kanno, M. Saito, H. Muroyama, K. Eguchi, N. Kasagi, H. Yoshida, J. Power Sources 195 (2010) 955–961. [22] M. Yoshinaga, H. Kishimoto, M.E. Brito, K. Yamaji, T. Horita, H. Yokokawa, J. Ceram. Soc. Jpn. 119 (2011) 307–309. [23] M. Yoshinaga, H. Kishimoto, K. Yamaji, Y.-P. Xiong, M.E. Brito, T. Horita, H. Yokokawa, Solid State Ionics 192 (2011) 571–575. [24] H. Kishimoto, A. Suzuki, T. Shimonosono, M.E. Brito, K. Yamaji, T. Horita, F. Munakata, H. Yokokawa, J. Power Sources 199 (2012) 174–178. [25] S. Linderoth, N. Bonanos, K.V. Jensen, J.B. B.-Sorensen, J. Am. Ceram. Soc. 84 (2001) 2652–2656. [26] W.L. Worrell, Electrochemical Society Proceedings, vol. 2001–28, 2001, pp. 75–89. [27] K. Kobayashi, S. Yamaguchi, T. Higuchi, S. Shin, Y. Iguchi, Solid State Ionics 135 (2000) 643–651. [28] M. Mori, Y. Hiei, H. Itoh, G.A. Tompsett, N.M. Sammes, Solid State Ionics 160 (2003) 1–14. [29] T. Shimonosono, H. Kishimoto, K. Yamaji, M.E. Brito, T. Horita, H. Yokokawa, Solid State Ionics 225 (2012) 58–61. [30] W.G. Coors, J.R. O'Brien, J.T. White, Solid State Ionics 180 (2009) 246–251. [31] A. Lefarth, B. Butz, H. Störmer, A. Utz, D. Gerthsen, ECS Trans. 35 (2011) 1581–1586.
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Agglomeration behavior of nickel particles on YSZ electrolyte Haruo Kishimoto a,⁎, Akihito Suzuki b, Taro Shimonosono a, Manuel E. Brito a, Katsuhiko Yamaji a, Teruhisa Horita a, Fumio Munakata b, Harumi Yokokawa a a b
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central No. 5, Higashi 1-1-1, Tsukuba, Ibaraki, 305‐8565 Japan Advanced Research Laboratories, Tokyo City University, Todoroki 8-15-1, Setagaya-ku, Tokyo, 158‐0082 Japan
a r t i c l e
i n f o
Article history: Received 7 September 2011 Received in revised form 14 March 2012 Accepted 9 April 2012 Available online 5 May 2012 Keywords: Solid oxide fuel cell (SOFC) Nickel agglomeration Yttria stabilized zirconia (YSZ) Scanning electron microscope (SEM) Scanning probe microscope (SPM)
a b s t r a c t Agglomeration behavior of nickel particles on yttria stabilized zirconia (YSZ), NiO doped YSZ and TiO2 doped YSZ substrates was examined by scanning electron microscopy (SEM) as two dimensional (2D) analysis and scanning probe microscopy (SPM) as three‐dimensional (3D) analysis. For 2D analysis, binarized SEM image was analyzed to examine the agglomeration factor. In 3D analysis, wettability analyzing method was adopted for solid particle on solid substrate, and “apparent” contact angle was evaluated by the following equation; θ ¼ 2 tan
−1
ð2 h=dÞ
where d and h are the observed particle diameter and the observed particle height of each agglomerated particle obtained by SPM analysis. Results from both analysis techniques revealed that NiO doped YSZ tend to enhance the nickel agglomeration, whereas TiO2 doped YSZ shows better wettability especially for larger nickel particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ceramic and metal composite material (so called cermet), which consists of yttria stabilized zirconia (YSZ) and nickel metal, have been generally used as anode for solid oxide fuel cells (SOFCs) to provide oxide ion conducting path inside the electrodes and to adjust the effective thermal expansion coefficient to that of the electrolyte materials. In this anode, nickel sintering in the anode is one of the serious problems in the long time operation of SOFC systems. For example, nickel sintering is enhanced by long time operation [1–6], impurities (such as sulfur components) in the fuel [7–13], P(H2O) and redox cycle [14–17] and interaction with the oxide component in the anode [18,19]. The microstructure of nickel cermet anode is complex, and therefore, it is difficult to analyze the microstructure change in the cermet anode. To analyze the nickel sintering behavior in the cermet anode, it is necessary to carry out the statistical analysis of two dimensional (2D) cross-section scanning electron microscope (SEM) images [17] or three‐dimensional (3D) re-construction analysis with FIB-SEM technique [19–21]. The former case involves the uncertainty of initial microstructure. The latter has to use rather expensive equipments and the well-trained analyzing technique; since
⁎ Corresponding author at: AIST Tsukuba Central 5, Higashi 1-1-1, Tsukuba, 305‐8565 Japan. Tel.: + 81 29 861 4542; fax: + 81 29 861 4540. E-mail address: [email protected] (H. Kishimoto). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.04.010
this is a destructive method, the initial state of the analyzed microstructure is still somewhat uncertain. It would be very effective if the nickel sintering behavior could be examined using a 2D model structure. Our group has developed a method for analyzing gas–nickel‐oxide substrate interaction by examining the agglomeration behavior of nickel thin film on oxide substrates under a controlled condition [22,23]. In the present study, the nickel agglomeration behavior was examined with a similar technique. Nickel agglomeration was produced by annealing the nickel thin film under reducing condition at a SOFC operating temperature as 1173 K in a similar manner. In the present investigation, two analyzing methods were used for agglomerated nickel particles on oxide substrates. One is 2D analysis with binarization of SEM image. In this case, agglomerated nickel particles were statistically analyzed to examine the tendency of nickel agglomeration. The other method is 3D analysis by scanning probe microscope (SPM) image [24]. An evaluation method of “wettability” for liquid particle on solid substrate is adopted for solid particles on solid oxide substrate. In fabrication of the cermet anode, NiO and YSZ are co-sintered at a high temperature in air and NiO dissolves into YSZ lattice [25]. Therefore, the effect of NiO dissolution in YSZ electrolyte on nickel agglomeration behavior was compared with those on YSZ and TiO2 doped YSZ system. It has been reported that TiO2 doping in YSZ is effective for nickel agglomeration [18,24] by measuring the electrochemical performance as well as the contact angles for liquid nickels on TiO2 doped YSZ.
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H. Kishimoto et al. / Solid State Ionics 225 (2012) 65–68
Fig. 1. Binarized backscattered electron image (BEI) for agglomerated nickel particles on zirconia substrates of a) YSZ, b) TiO2 doped YSZ (Ti-YSZ) and c) NiO doped YSZ (Ni-YSZ). White parts and black parts correspond to the nickel particles and zirconia substrates surface, respectively. White lines show the bisected borderline for two neighboring particles.
2. Experimental In the present study, nickel agglomeration behavior was investigated using two dimensional model electrodes. Nickel thin films were prepared on stabilized zirconia substrates. The effect of TiO2 doping on nickel agglomeration was reported for 5 mol% TiO2 doped YSZ [18], and solubility limit of NiO in YSZ is about 1 mol% [25]. Therefore, 8 mol% Y2O3 doped ZrO2 (8YSZ), 5 mol% TiO2–95 mol% 8YSZ (Ti-8YSZ) and 1 mol% NiO–99 mol% 8YSZ (Ni-8YSZ) were used as stabilized zirconia substrates in the present study. The commercial powders of 8YSZ (TZ-8Y, Tosoh Co., Japan), TiO2 (99.9% purity, Wako Pure Chemical Industries, Ltd., Japan) and NiO (Wako Pure Chemical Industries, Ltd., Japan) were mixed in a predetermined ratio by planetary ball mill with ethanol. Mixed powders were shaped into disks and pressed by CIP at 390 MPa. Pressed disks were sintered at 1673 K for 5 h in air. The surface of the sintered samples was mirrored with diamond slurry by polishing machine. Particle size of diamond slurry was 3–5 μm for all substrates and the density of polishing flaw was 1–3 lines in 5 μm × 5 μm area. Nickel thin film was prepared on the polished surface of substrates by RF sputtering method. The thickness of nickel film was controlled around 60 nm by sputtering time. The samples were annealed under a reducing condition of 1% humidified H2 gas for 2 h at 1173 K. The microstructure of agglomerated nickel particles was analyzed by scanning electron microscopy (SEM) and scanning probe microscopy (SPM). 3. Results and discussion 3.1. 2D analysis (SEM data analysis) Fig. 1 shows the binarized backscattered electron image (BEI) for agglomerated nickel particles on zirconia substrates. In this figure,
white parts and black parts correspond to the nickel particles and the zirconia substrate surface, respectively. Well isolated nickel particles were observed for all substrates. In this figure, it is hard to distinguish the difference of agglomeration behavior of the nickel particles on substrates. In Fig. 2, distributions of projected nickel particle area obtained from Fig. 1 were summarized. In crude distinction, distribution of projected nickel particle area shifts towards smaller and larger for Ti-YSZ and Ni-YSZ, respectively, compared with YSZ. The peak is observed around 0.4–0.5 μm for YSZ and those for Ti-YSZ and NiYSZ are around 0.3–0.4 μm and around 0.5–0.7 μm, respectively. Those agglomerated particle sizes were much smaller than the particle size of the YSZ substrates. The average area of projected nickel particles, S1, listed in Table 1, is in the order of Ti-YSZ b YSZ b Ni-YSZ, but the difference is very small. The averaged influenced area, S2, and the agglomeration factors defined as, S2/S1, for each substrate are also summarized in Table 1. The influenced area, S2, corresponds to the area enclosed by bisected borderlines for two neighboring particles (white lines in Fig. 1); it is estimated that nickel particles are formed by the diffusion of the nickel from this area. In this sense, an agglomeration factor, S2/S1, becomes large when nickel particle is resulted from the nickel diffusion within a large area. A large value of S2/S1 indicates that nickel sintering on the substrate is enhanced. The agglomeration factor analysis has revealed that nickel agglomeration was enhanced in the following order; Ti-8YSZ b 8YSZ b Ni-8YSZ. 3.2. 3D analysis (SPM data analysis) Fig. 3 shows the SPM images for agglomerated nickel particles on oxide substrates. By SPM analysis, as well known, we can obtain the three-dimensional (3D; 2D × height) information for agglomerated particles. For all substrates, hemispherical nickel particles were observed. In the principle of SPM analysis, the information of morphology around contact area cannot be necessarily obtained. Therefore, we have to estimate the contact angle between nickel and zirconia substrate by calculation. In this study, the wettability analyzing method was adopted as previously reported [24]; −1
Population / %
θ ¼ 2 tan
ð2 h=dÞ
ð1Þ
where θ is the contact angle, and d and h are particle diameter and particle height of each agglomerated particle obtained by SPM analysis. When θ is small, wettability of nickel on zirconia substrate can be Table 1 Average area of projected nickel particles, S1, averaged influenced area, S2, and agglomeration factor, S2/S1 for the substrates of YSZ, Ti-YSZ and Ni-YSZ.
Projected area of nickel particle / µm 2 Fig. 2. Distributions of projected nickel particle area obtained from backscattered electron images (BEI). Substrates are YSZ, TiO2 doped YSZ (Ti-YSZ) and NiO doped YSZ (NiYSZ). The particles observed in Fig. 1 were analyzed.
Ave. particle size, S1/μm2 Ave. influenced area, S2/μm2 Agglomeration factor, S2/S1
YSZ
Ti-YSZ
Ni-YSZ
0.129 0.497 3.842
0.126 0.440 3.480
0.147 0.611 4.162
H. Kishimoto et al. / Solid State Ionics 225 (2012) 65–68
a) YSZ
b) Ti-YSZ
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c) Ni-YSZ
Fig. 3. Scanning probe microscope (SPM) images for agglomerated nickel particles on zirconia substrates of a) YSZ, b) TiO2 doped YSZ (Ti-YSZ) and c) NiO doped YSZ (Ni-YSZ).
Ti−YSZ b YSZ b Ni−YSZ This means that wettability for Ti-YSZ is better than that for YSZ and Ni-YSZ shows the worst. This tendency agrees with the results of the 2D analysis on SEM images as indicated above. All data for obtained d and h are plotted in Fig. 4. Most of the agglomerated nickel particles were located within YSZ grains, while there were a few particles on the grain boundaries or on the polishing flaws. The deformed particles were eliminated from the subsequent analysis because these particles must be affected from surface defects such as grain boundaries. In this figure, a constant contact angle, θ, is represented by a straight line from the origin as expressed by Eq. (1); those values for 60, 70, 80, and 90° are shown as dashed lines. For the YSZ and the Ni-YSZ substrates, observed d and h are distributed within an approximately linear relation over a whole observed particle size range. The contact angles are calculated as 77± 5° and 80± 7 for the YSZ and the Ni-YSZ substrates, respectively, in the range of 400 nmb d b 800 nm. For the Ti-YSZ substrate, however, the d–h relation seems to bend around d = 550 nm; the θ value gradually decreases with increasing particle size more than 550 nm, whereas for the smaller particles, the relation is nearly the same as YSZ [24]. 3.3. Effect of doping for the YSZ electrolyte The observed effect of doping to the YSZ substrate on the nickel agglomeration is the same between the 2D analysis and the 3D analysis; that is, nickel agglomeration tends to be enhanced by NiO doping in the YSZ and to be suppressed for the TiO2 doped YSZ except for the smaller nickel particles. Under the present experimental conditions, hemispherical nickel metal particles were formed on the flat surface of the oxide substrates. Therefore, we adopted Young's law to discuss the interaction between nickel particles and oxide substrate surface even though this law should be adapted to the liquid particle on the flat oxide substrate: γ YSZ ¼ γ Ni cos θ þ γNi=YSZ
ð2Þ
where γYSZ, γNi and γNi/YSZ are surface/interface tensions between YSZ and gas, between nickel and gas and between nickel and YSZ, Table 2 Averaged contact angle, θ, for the substrates of YSZ, Ti-YSZ and Ni-YSZ. Contact angle, θ, is calculated from the observed d and h by SPM analysis. θ = 2 tan− 1 (2h/d). Substrate
8YSZ
Ti-8YSZ
Ni-8YSZ
Contact angle (°)
77 ± 5
69 ± 7
80 ± 7
respectively. Within the present observation, the nickel particles can be regarded to have surfaces with a certain curvature. For such nickel particles, we can consider that surfaces have a certain thickness of “surface layer". It is reasonably assumed that such thickness does not depend on nickel particle size and, therefore, particle size effect can be validated from the volume ratio of the “surface layer” to the total volume of the particle. With decreasing particle size, this ratio becomes large so that the energetics of nickel is mainly dominated by the nickel surface energy. This means that the particle shape is mainly controlled by the nickel surface energy, γNi. On the contrary, with increasing size, the volume ratio becomes small so that not only the surface energy, γNi, but also the interface energy, γNi/YSZ, governs the energetics of nickel particles. This can explain why the doping effect appears only for the nickel particles having larger sizes. For the TiO2 doped YSZ, titanium in YSZ lattice is reduced from + 4 to +3 in a reducing gas and, as a result, the electronic conductivity increases by the electron formation on the titanium sites [26–28]. The doping of TiO2, therefore, gives rise to changes in energetics by introduction of a new interaction between Ni–TiO2 (TiO1.5) in the YSZ lattice. On the other hand, electronic conductivity does not increase but rather decrease for the NiO doped YSZ although nickel in the YSZ lattice is also reduced under a reducing condition [29]. These changes in electronic conductivity (or change of conduction electron density) may affect the γNi/YSZ. Lower electronic conductivity may lead to higher γNi/YSZ. It is also reported that nickel in the YSZ lattice is reduced to nickel metal and precipitates as nanoparticles along the grain boundaries [30,31]. In the present experimental
θ =90 o
80 o
70 o
Height, h / nm
regarded as “good”. Averaged θ for zirconia substrates are summarized in Table 2. Averaged θ is in the following order:
60 o
Averaged diameter, d / nm Fig. 4. Relation between average particle diameter, d, and height, h, for the nickel particles obtained from scanning probe microscope (SPM) images. Substrates are YSZ, TiO2 doped YSZ (Ti-YSZ) and NiO doped YSZ (Ni-YSZ).
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condition, nickel metal nanoparticles must be precipitated on the electrolyte surface but in the interface, essentially no change takes place. This suggests no change in γNi/YSZ, but in γYSZ. By using the same technique, we are now examining the nickel agglomeration behavior induced by water vapor and fuel impurities such as sulfur. 4. Conclusion Nickel agglomeration behavior on the YSZ electrolyte was analyzed by SEM technique (2D analysis) and SPM technique (3D analysis) and the doping effect of TiO2 and NiO was examined. By the 2D analysis, the tendency of doping effect on nickel agglomeration can be easily and statistically evaluated. The 3D analysis makes it possible to demonstrate the detailed information such as nickel particle size effect on the nickel agglomeration behavior. Nickel agglomeration behavior is influenced by dopant metal oxides. For the 1 mol% NiO doped YSZ, nickel agglomeration tends to be enhanced. For the 5 mol% TiO2 doped 8YSZ, nickel agglomeration is reduced, especially for larger nickel particle size. It is suggested that the balance in surface/interface tension is different between the small (less than sub-micron size) and the large (may be more than 500 nm) nickel particles in the TiO2 doped YSZ. Acknowledgement A part of this study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] T. Iwata, J. Electrochem. Soc. 143 (1996) 1521. [2] A. Ioselevich, A.A. Kornyshev, W. Lehnert, J. Electrochem. Soc. 144 (1997) 3010–3019. [3] A. Faes, A. Hessler-Wyser, D. Presvytes, C.G. Vayenas, J. Van herle, Fuel Cells 9 (2009) 841–851. [4] D. Simwonis, F. Tietz, D. Stöver, Solid State Ionics 132 (2000) 241–251. [5] R. Vaßen, D. Simwonis, D. Stöver, J. Mater. Sci. 36 (2001) 147–151. [6] Z. Jiao, N. Takagi, N. Shikazono, N. Kasagi, J. Power Sources 196 (2011) 1019–1029.
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