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Morphology and Catalytic Performance of Flake-Shaped NiO-YttriaStabilized Zirconia (YSZ) Particles with Nanocrystalline YSZ Grains Yuzhou Wu,†,‡ Wei Wang,† Kun Wang,‡ Yao Zeng,‡ Dehua Dong,§ Zongping Shao,*,† and Huanting Wang*,‡ †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing University of Technology, Nanjing, 210009, China ‡ Department of Chemical Engineering, Monash University, Clayton, VIC, 3800, Australia § Fuels and Energy Technology Institute, Curtin University of Technology, Perth, WA 6845, Australia ABSTRACT: The flake-shaped NiO-yttria-stabilized zirconia (YSZ) particles with nanocrystalline YSZ grains were synthesized using the sucrose-concentrated H2SO4 dehydration reaction, and their microstructure was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). To evaluate the properties of the flakeshaped NiO-YSZ particles as the anode materials for solid oxide fuel cells, the reduction temperature of the flake-shaped NiOYSZ particles and catalytic activity on the methane steam/CO2 reforming reactions of the H2-reduced particles as well as the electrochemical impedance spectra of the YSZ supported symmetrical cells with the electrodes made from these particles were examined in comparison with the mixed commercial NiO-YSZ. HRTEM revealed that the nanocrystalline YSZ was dispersed in the NiO matrix and distributed on the surface of the flake-shaped NiO-YSZ particles. The catalytic performance of the flakeshaped NiO-YSZ particles was better than that of the mixed commercial NiO-YSZ in both steam reforming and CO2 reforming of methane. The symmetrical cell made from the flake-shaped NiO-YSZ exhibited a much lower polarization resistance at the operating temperatures below 800 °C than that made from the mixed commercial NiO-YSZ.

1. INTRODUCTION Solid oxide fuel cells (SOFCs) have attracted much attention because they can efficiently convert chemical energy into electricity and show interesting properties such as excellent electrode kinetics and fuel flexibility as compared with polymer electrolyte membrane fuel cells.1−3 In a typical SOFC with an oxygen ion conducting electrolyte, oxygen is reduced into oxygen ions in the cathode region and particularly the three phase boundary (TPB), and then, the oxygen ions are transported through the electrolyte.4,5 Various fuels such as hydrogen, carbon monoxide, and hydrocarbons can be used in SOFCs, and the reactions in the anode are usually more complex than the cathode reactions.6 Generally, the anodic reactions occur at the active TPB which is made up of a porous mixed electrical and oxygen-ionic conductor such as nickel/ yttria-stabilized zirconia (YSZ) cermet.7−9 The gaseous fuel is supplied in the porous structure of the anode, and gas diffusion kinetics is considered as an important parameter for SOFCs, especially when operated on hydrocarbons or at lower operation temperature.10−12 Therefore, the optimization of the anode microstructure has been proven to be an effective approach to enhancing the cell performance via the improvement of fuel supply and enlargement of TPB.13 Considerable effort has been made to tailor the microstructure of the anode. For instance, Wilson et al. reported a three-dimensional reconstruction of a SOFC anode for calculation of critical microstructural features.14 The anode microstructure could be controlled by either using poreforming additive or porous template and or preparing nanostructured anode powders.13,15−18 However, when a pore-former or template is used in the cell fabrication process, © 2012 American Chemical Society

the porous structure often collapses at high sintering temperatures, and other defects likely result from the removal of the considerable amount of pore-former or template. On the other hand, various approaches have been studied to prepare nanostructured anode powder, such as synthesis of nanocomposites via a sol−gel combustion process and preparation of a zeta potential-balanced NiO and YSZ particles by adjusting the pH value of the aqueous slurry.9,19,20 However, the application of these nanocomposites very much depends on their high-temperature sintering behavior. In addition, other attempts to modify the anode structure have been also reported.21−23 For instance, highly porous nickel foams filled with ceramic material were used as a conductive layer in the SOFC anode.21 Kim et al. reported that the highly porous YSZ was obtained after nickel leaching treatment from NiO/YSZ, and the copper-based anode made from this porous YSZ was successfully operated on both hydrogen and n-butane.22 In order to obtain an interpenetrating phase composite-structured anode, the core−shell NiO-YSZ particles reported by Park et al. were prepared via a precipitation method.24 Electro-deposition was also utilized by Jung et al. to fabricate SOFC anodes with high metal loadings and different metal layers in order to avoid carbon deposition and thus enhance performance.23 We have recently developed a sucrose-concentrated H2SO4 reaction assisted route for synthesis of flake-shaped NiO-YSZ particles. An anode with a high porosity (45.6%) was prepared without Received: Revised: Accepted: Published: 6387

January 14, 2012 April 10, 2012 April 17, 2012 April 17, 2012 dx.doi.org/10.1021/ie300123x | Ind. Eng. Chem. Res. 2012, 51, 6387−6394

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Figure 1. continued

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Figure 1. SEM images and EDS elemental mapping images of (a, c) flake-shaped NiO-YSZ and (b, d) mixed commercial YSZ and NiO powder.

magnetic stirring and heating at 60 °C; 38.8 g of sucrose (99.5%, Sigma-Aldrich) was then added into the above solution as a carbon foam source. Organic monomers (acrylamide [AM, 5 g], N,N-methylenebisacrylamide [MBAM, 0.5 g]) and initiator (ammonium persulfate, 0.04 g) were used to form a polymer network. Concentrated sulfuric acid (65 mL, Univar) was introduced to the polymerized metal-ion contained gel. The carbon-rich precursor was dried at 90 °C for 2 h and then sintered at 900 °C for 2 h to remove carbon. The assynthesized NiO-YSZ powder was then mixed with 7 wt % PVB (polyvinyl butyral, Butvar B-98, Sigma) in ethanol (Absolute, Merck) by ball milling at 80 rpm for 8 h, followed by ethanol evaporation at 100 °C. Commercial NiO powder (particle size −325 mesh, Aldrich) and commercial YSZ powder (particle size around 700 nm, Aldrich) were also mixed with 7 wt % PVB in ethanol medium under the same conditions as those for the flake-shaped powder. 2.2. Characterization. The morphology and microstructure of the powders were investigated by using a JEOL 7001F scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) detector and a JEOL-2011 high resolution transmission electron microscope (HRTEM). Hydrogen temperature programmed reduction (H2-TPR) was performed to study the reduction of NiO in the two types of powder. The powder was first fired at 1100 °C for 2 h and then fired at 1200 °C for another 2 h before the H2-TPR investigation. Treated powder (0.03 g) was reduced by a 10 vol % H2−Ar gas mixture. The temperature was ramped at a heating rate of 10 °C min−1 from room temperature up to 930 °C. The effluent gases were monitored by using a BELCAT-A

using pore-forming additives or template. The single cell prepared with such an anode exhibited enhanced cell performance.25 In the present work, we carried out further characterizations of the flake-shaped NiO-YSZ particles and studied the performance of the anode constructed from this type of particle. The objective of this work was to gain a better understanding of the electrode prepared from the flake-shaped NiO-YSZ. Specifically, the catalytic activity of the anode was investigated in methane CO2 reforming and methane steam reforming processes. Electrochemical impedance spectroscopy (EIS) was applied to determine the electrode polarization by using a symmetrical cell. For comparison, the mixture of commercial NiO powder and commercial YSZ powder was also investigated.

2. EXPERIMENTAL SECTION 2.1. Powder Preparation. Two kinds of NiO-YSZ (60:40 by mass) powder (flake-shaped NiO-YSZ and mixed commercial NiO and YSZ) were prepared in this study. Flake-shaped NiO-YSZ powder was synthesized via a sucroseconcentrated H2SO4 assisted process as previously reported.25 In our previous study, well crystallized NiO-YSZ was obtained when sintered at 900 °C for 2 h. The highest porosity was achieved with a second calcination of the NiO-YSZ powder at 1100 °C for 2 h.25 In this work, the same powder preparation procedure was followed: 11.68 g of Ni(NO3)2·6H2O, 99% (Sigma-Aldrich), 0.3596 g of Y(NO3)3·6H2O (99.9%, SigmaAldrich), and 6.60 g of Zr(NO3)4·5H2O (99.99% International Lab) were dissolved in 40 mL of deionized water under 6389

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nickel element is shown in red. The composition difference between the two samples is clearly observed from the EDS images. The flake-shaped particle is zirconium-enriched whereas the accompanying small particles exhibit uniform dispersion of zirconium in nickel (Figure 1c). The overall interdispersion of NiO-YSZ is very well for the flake-shaped powder. In the case of the mixed commercial powder (Figure 1d), YSZ and NiO particles are not uniformly mixed, and individual YSZ and NiO domains can be clearly identified. The SEM and EDS images also reveal that commercial NiO particles have particle sizes of up to around 2.5 μm and commercial YSZ particles are around 200 nm in size (Figure 1d). The HRTEM was used to investigate the detailed structure of the flake-shaped particles. The representative HRTEM images shown in Figure 2 reveal that many highly crystalline

apparatus with a thermal conductivity detector (TCD) detector. A Varian 3800 gas chromatograph equipped with a TCD and a Poraplot Q column was used to evaluate the catalytic performance of the powders in CH4 steam reforming and CH4−CO2 reforming. The methane conversion, X (%), in the steam reforming and CO2 reforming was calculated by eqs 1 and 2, respectively. X(%) =

X(%) =

[CO] + [CO2 ] × 100% [CO] + [CO2 ] + [CH4]

(1)

0.5[CO] × 100% 0.5[CO] + [CH4]

(2)

The selectivity of CO, S (%), was given by S(%) =

[CO] × 100% [CO] + [CO2 ]

(3)

2.3. Fabrication and Testing of Symmetrical Cells. For the symmetrical cell fabrication, YSZ powder (∼8 wt % yttria, and particle size of around 700 nm, Aldrich) was first drypressed into discs and then sintered at 1400 °C for 5 h to obtain dense electrolyte discs. The resulting YSZ electrolyte discs have a thickness of around 700 μm. Second, 1 g of NiOYSZ powder was dispersed into a mixed solvent with appropriate viscosity that is composed of 0.6 mL of glycerin, 2 mL of glycol (99%, Sigma-Aldrich), and 10 mL of isopropanol (99.5%, Sigma-Aldrich),26 followed by ball milling at 80 rpm for 2 h. The resultant slurry was symmetrically deposited onto both sides of the YSZ electrolyte disk by an airdriven spray technique and subsequently sintered at 1100 °C for 2 h and then 1200 °C for another 2 h. The area of the sprayed electrode was 0.77 cm2. For the symmetrical cell testing, silver paste and silver wire were used as the current collector. All electrochemical impedance spectra were collected by an advanced electrochemical system (Model PARSTAT 2273, Princeton Applied Research). The frequency ranged from 0.01 Hz to 1 MHz, and signal amplitude was 10 mV. All samples were tested under open circuit voltage (OCV) condition in dry hydrogen.

3. RESULTS AND DISCUSSION Traditional NiO-YSZ composite was selected as the anode material in both this study and the previous paper.25 Although the ionic conductivity of the typical electrolyte YSZ is relatively lower than the doped-ceria, the YSZ electrolyte could achieve a higher open circuit voltage (OCV), as well as the matched coefficient of thermal expansion with Ni-YSZ and could inhibit Ni coarsening at elevated temperatures.21,27 In our previous study, we reported a novel sucrose-concentrated H2SO4 reaction assisted route for synthesizing ceramic particles. The as-synthesized NiO-YSZ powder was well crystallized, with an average crystallite size of 28.7 nm for NiO and 15.4 nm for YSZ.25 Figure 1 shows the SEM images and the EDS elemental mapping images for the two types of powder. Our synthesized sample mainly consists of flake-shaped NiO-YSZ particles with particle sizes of up to around 20 μm along with small particles of around 200 nm in size (Figure 1a). The mixed commercial powder is in a particle size range of 0.2−2.5 μm (Figure 1b). The elemental distributions for the flake-shaped NiO-YSZ and mixed commercial NiO and YSZ powders are displayed in Figure 1c,d. The zirconium element is shown in green while the

Figure 2. HRTEM image of flake-shaped NiO-YSZ particles showing the distribution of single crystalline YSZ on NiO: (a) low magnification and (b) high magnification with clear lattice fringes.

YSZ nanograins with sizes of around 10−20 nm are uniformly embedded on the surface of NiO-YSZ particles. Clearly visible lattice fringes on the HRTEM image (Figure 2b) indicate YSZ nanograins are highly single crystalline. The d-spacing is measured to be 0.299 nm, which corresponds to the (111) plane of the tetragonal phased ZrO2 according to JCPDS card No. 14-0534.28,29 As a second firing of the anode powder was required for single cell fabrication, the anode powder was first sintered at 6390

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1100 °C for 2 h, and then, a second sintering at 1200 °C for another 2 h was carried out (the same treatment as the symmetrical cell fabrication) for H2-TPR and catalytic activity evaluation. Figure 3 shows the H2-TPR profiles of the two different kinds of NiO-YSZ anode powder after the second sintering

Figure 3. Hydrogen temperature programmed reduction profiles of (a) flake-shaped NiO-YSZ and (b) mixed commercial NiO-YSZ.

process. The mixture of commercial NiO-YSZ powder exhibits lower reduction temperature than flake-shaped NiO-YSZ powder. Two main reduction temperatures for the mixed commercial NiO-YSZ powder are around 260 and 330 °C. They can be ascribed to the reduction temperatures for free NiO particles and those NiO particles sintered with YSZ nanoparticles, respectively.30,31 Also a small amount of NiO is reduced at around 450 °C; it should arise from the reduction process of the cores of NiO particles. However, the reduction peaks shift to around 290, 380, and 500 °C in the TPR profile of flake-shaped NiO-YSZ powder due to the dispersion of nanocrystalline YSZ in NiO. The first peak that corresponds to free NiO reduction is dramatically suppressed whereas the second peak corresponding to the reduction of NiO particles sintered with YSZ is significantly increased. The H2-TPR result is in good agreement with the HRTEM observations. A large coverage of nanocrystalline YSZ grains on flake-shaped NiOYSZ particles indicates the amount of free NiO in the sample is much smaller than that in the mixture of commercial NiO-YSZ sample (Figure 2). The utilization of hydrocarbons such as methane is one of the advantages of solid oxide fuel cell operation. As one of the main components in the conventional SOFCs, nickel is one of the most active catalysts for methane-steam reforming and methane-CO2 reforming. The structure of nickel-YSZ has substantial influence on the catalytic performance.32 Therefore, catalytic performance of the reduced powder for CH4 steam reforming and CH4−CO2 reforming was evaluated in terms of methane conversion rate and CO selectivity. The powder was sintered at 1100 °C for 2 h and then at 1200 °C for 2 h before NiO was reduced to Ni to ensure the powder sample was prepared under the similar conditions to the symmetrical cell fabrication. Both the steam reforming and CO2 reforming of methane were performed at a CH4/H2O ratio of 1:1 and a CH4/CO2 ratio of 1:1. The two powder samples showed a higher catalytic activity for the methane steam reforming reaction at higher temperatures (Figure 4a). The reduced flakeshaped NiO-YSZ exhibited higher methane conversion rate and higher CO selectivity for methane steam reforming in the

Figure 4. Catalytic activity of flake-shaped NiO-YSZ and mixed commercial NiO-YSZ powder after reduction for (a) CH4-steam reforming (CH4/H2O = 1:1) and (b) CH4−CO2 reforming (CH4/ CO2 = 1:1).

testing temperature range of 600 to 850 °C. This can be explained by the microstructure of flake-shaped particles as shown in Figure 2. The nanocrystalline YSZ on the surface of NiO-YSZ particles reduced the Ni coarsening at high temperatures; meanwhile, the gaps between YSZ nanocrystals should enhance gas diffusion and lead to more catalytic reaction sites. Since the catalytic performance is very dependent on the number of the active sites of nickel particles, it is easily understood that the reduced flake-shaped NiO-YSZ achieves better catalytic activity on the methane steam reforming than the reduced commercial NiO-YSZ.33 Although the overall conversion rate and the CO selectivity in the CH4−CO2 reforming are lower than those in the steam reforming, the reduced flake-shaped NiO-YSZ performs better than the reduced commercial NiO-YSZ. Figure 5 shows the electrochemical impedance spectra (EIS) of the reduced symmetrical cells at various temperatures in hydrogen atmosphere under OCV conditions. In a typical EIS, the first intersecting point of the curve and the x-axis represents the ohmic resistance of the cell. In the case of hydrogen oxidation in the Ni-based anode, the reaction process can be described as the following 5 steps: (i) fuel gas transferred outside the anode region, (ii) gas diffusion in the porous anode, (iii) surface adsorption over the nickel surface, (iv) surface diffusion of the H species to the TPB, and (v) oxidation reaction with charge transfer.10,34,35 The total polarization resistance of these steps represented in the EIS is the value between the two intersecting points of the curve and the x-axis. The frequency measured from step (i) to (v) increases from low to high frequency. It is believed that, due to good catalytic activity of Ni and reactivity of hydrogen, the steps of the charge 6391

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As the anode made from the flake-shaped powder possesses a much higher porosity than that prepared from the mixed commercial powder,25 the diffusion polarization resistance of the former should be much smaller than that of the latter at lower temperatures. Furthermore, at lower temperatures, the diffusion polarization resistance dominates in the overall polarization resistance whereas the enhanced conductivity becomes the main contributor at higher temperatures because the diffusion polarization can be reduced to some extent by increasing the temperature. This explains why the polarization resistance of the cell made from flake-shaped NiO-YSZ is slightly larger than that of commercial mixed powder at 900 °C. Baek et al. reported the anodic behavior of YSZ/NiO cermet in the anode-supported symmetrical cell with an YSZ thickness of around 20 μm.34 They observed that the anodic polarization was greatly affected by the water partial pressure, p(H2O), in the hydrogen fuel. The polarization resistance (Rp) was dramatically increased from ∼2.5 to ∼25 Ω cm2 and from ∼6.5 to ∼120 Ω cm2 when p(H2O) decreased from 3% to 0%, at 800 and 600 °C, respectively. Furthermore, Cho et al. investigated the Rp of Ni/YSZ cermet in an YSZ-supported symmetrical cell.39 Their symmetrical cell fabrication process, such as for the coating of the thin-layer NiO/YSZ electrode, was very close to ours. The thickness of the YSZ electrolyte layer was around 500 μm, which was comparable to ours (∼700 μm). They fabricated two kinds of Ni-YSZ electrodes with different particle sizes and nickel distributions onto the YSZ substrates. These two kinds of symmetrical cells were exposed to the moist gas (97% H2−3% H2O), and they obtained a polarization resistance of 6−18 Ω cm2 at 800 °C. In our study, the polarization resistance was measured on an electrolytesupported symmetrical cell. In addition, a water bubbler was used to control the humidity of hydrogen feed for the symmetrical cell test. The experiment was conducted at the lab temperature of ∼7 °C, and the hydrogen was humidified to p(H2O) = 1%. As a result, given the low H2 humidity, the polarization resistance of the cell made from the flake-shaped powder is arguably lower than those reported.34,39 It is noted that the polarization resistances obtained from the electrolyte-supported symmetrical cells in our study are different from those of real cells.40,41 It is because our cells were tested in one chamber under an open circuit condition and without chemical or electrochemical potential gradient. Nevertheless, the low polarization resistance of the anode made from the flake-shaped NiO-YZS powder should be translated to good performance in a real cell. This again suggests that optimizing the electrode microstructure is an effective way to lower the polarization resistance of the electrode and thus improve the fuel cell performance at lower temperatures.

Figure 5. Electrochemical impedance spectra of the electrolyte supported Ni-YSZ electrode symmetrical cell, made from flake-shaped NiO-YSZ and mixed commercial NiO-YSZ, at different temperatures.

transfer and H−H bond breaking of hydrogen over Ni surface are not a major contributor to the anode resistance. This means the polarization resistance arising from surface diffusion is the main component of the anode resistance.10,36−38 Also the surface diffusion can be reduced by optimizing the anode microstructure at lower operating temperatures. In the present study, all the symmetrical cells were fabricated using the same procedures and tested under the same conditions. Therefore, the effect of the ohmic resistance of the electrolyte should be minimal in terms of comparing the anodes at the same temperatures. From the impedance spectra in Figure 5, the anodic polarization of the cell made from flake-shaped NiOYSZ is 2.75, 8.80, 23.1, and 66.1 Ω cm2 at 900, 800, 700, and 600 °C, respectively, whereas the anodic polarization of the cell made from the mixed commercial NiO-YSZ is 1.75, 9.78, 46.5, and 225 Ω cm2 at 900, 800, 700, and 600 °C, respectively. Interestingly, the cell made from flake-shaped NiO-YSZ exhibits lower polarization resistance than the cell made from the mixed commercial powder at all temperatures except for 900 °C. It is noted that the polarization resistance is mainly affected by the surface diffusion rate and material conductivity.

4. CONCLUSIONS The flake-shaped NiO-YSZ particles with nanocrystalline YSZ grains were synthesized, and their properties such as powder morphology, reduction temperature, catalytic activity, and polarization characteristic were studied in comparison with the mixed commercial NiO-YSZ powder. The hydrogen reduction temperature of the flake-shaped NiO-YSZ powder was slightly higher than that of the mixed commercial NiO-YSZ powder, which is due to the large portion of NiO surface being covered by nanocrystalline YSZ grains in the flake-shaped particles. Both the methane conversion rate and the CO selectivity for the flake-shaped Ni-YSZ were around 1.5 times and over 2 times as much as those for the mixed commercial 6392

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Ni-YSZ powder in the methane-steam reforming at 600 °C and in the methane-CO2 reforming at 850 °C, respectively. The electrode polarization resistance of the anode made from the flake-shaped NiO-YSZ was only around a quarter of that made from the mixed commercial NiO-YSZ powder when the temperature dropped to 600 °C. The largely reduced electrode polarization resistance was due to better surface diffusion in the electrode at lower operating temperatures. Therefore, the anode microstructure fabricated from the flake-shaped NiOYSZ powder is beneficial for reducing the operating temperature of SOFCs.

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(14) Wilson, J. R.; Kobsiriphat, W.; Mendoza, R.; Chen, H. Y.; Hiller, J. M.; Miller, D. J.; Thornton, K.; Voorhees, P. W.; Adler, S. B.; Barnett, S. A. Three-dimensional reconstruction of a solid-oxide fuelcell anode. Nat. Mater. 2006, 5, 541−544. (15) Zhang, Y. L.; Zha, S. W.; Liu, M. L. Dual-scale porous electrodes for solid oxide fuel cells from polymer foams. Adv. Mater. 2005, 17, 487−491. (16) Aruna, S. T.; Rajam, K. S. Synthesis, characterisation and properties of Ni/PSZ and Ni/YSZ nanocomposites. Scr. Mater. 2003, 48, 507−512. (17) Keech, P. G.; Trifan, D. E.; Birss, V. I. Synthesis and performance of sol-gel prepared Ni-YSZ cermet SOFC anodes. J. Electrochem. Soc. 2005, 152, A645−A651. (18) Mamak, M.; Coombs, N.; Ozin, G. A. Mesoporous nickel-yttriazirconia fuel cell materials. Chem. Mater. 2001, 13, 3564−3570. (19) Shao, G. Q.; Cai, H.; Xie, J. R.; Duan, X. L.; Wu, B. L.; Yuan, R. Z.; Guo, J. K. Preparation of nanocomposite Ni/YSZ cermet powder by EDTA complexes-gel conversion process. Mater. Lett. 2003, 57, 3287−3290. (20) Vassen, R.; Simwonis, D.; Stover, D. Modelling of the agglomeration of Ni-particles in anodes of solid oxide fuel cells. J. Mater. Sci. 2001, 36, 147−151. (21) Corbin, S. F.; Clemmer, R. M. C.; Yang, Q. Development and characterization of porous composites for solid oxide fuel cell anode conduction layers using ceramic-filled highly porous Ni foam. J. Am. Ceram. Soc. 2009, 92, 331−337. (22) Kim, H.; da Rosa, C.; Boaro, M.; Vohs, J. M.; Gorte, R. J. Fabrication of highly porous yttria-stabilized zirconia by acid leaching nickel from a nickel-yttria-stabilized zirconia cermet. J. Am. Ceram. Soc. 2002, 85, 1473−1476. (23) Jung, S. W.; Vohs, J. M.; Gorte, R. J. Preparation of SOFC anodes by electrodeposition. J. Electrochem. Soc. 2007, 154, B1270− B1275. (24) Park, J. Y.; Lee, J. H.; Kim, J.; Lee, H. W. Preparation and properties of a novel anode of interpenetrating phase composite structure. In Science of Engineering Ceramics III; Ohji, T., Sekino, T., Niihara, K., Eds.; Trans Tech Publications Ltd: Zurich-Uetikon, 2006; Vol. 317−318, pp 905−908. (25) Wu, Y. Z.; Dong, D. H.; Ran, R.; Zeng, Y.; Wang, K.; Shao, Z. P.; Wang, H. T. Synthesis of flake-shaped NiO-YSZ particles for highporosity anode of solid oxide fuel cell. J. Am. Ceram. Soc. 2011, 94, 3666−3670. (26) Zhou, W.; Shao, Z. P.; Ran, R.; Chen, Z. H.; Zeng, P. Y.; Gu, H. X.; Jin, W. Q.; Xu, N. P. High performance electrode for electrochemical oxygen generator cell based on solid electrolyte ion transport membrane. Electrochim. Acta 2007, 52, 6297−6303. (27) Waldbillig, D.; Wood, A.; Ivey, D. G. Thermal analysis of the cyclic reduction and oxidation behaviour of SOFC anodes. Solid State Ionics 2005, 176, 847−859. (28) Sharma, R.; Naedele, D.; Schweda, E. In situ studies of nitridation of zirconia ZrO2. Chem. Mater. 2001, 13, 4014−4018. (29) Khan, S. Z.; Yuan, Y.; Abdolvand, A.; Schmidt, M.; Crouse, P.; Li, L.; Liu, Z.; Sharp, M.; Watkins, K. G. Generation and characterization of NiO nanoparticles by continuous wave fiber laser ablation in liquid. J. Nanopart. Res. 2009, 11, 1421−1427. (30) Robertson, S. D.; McNicol, B. D.; Debaas, J. H.; Kloet, S. C.; Jenkins, J. W. Determination of reducibility and identification of alloying in copper-nickel-on-silica catalysts by temperature-programmed reduction. J. Catal. 1975, 37, 424−431. (31) Takenaka, S.; Umebayashi, H.; Tanabe, E.; Matsune, H.; Kishida, M. Specific performance of silica-coated Ni catalysts for the partial oxidation of methane to synthesis gas. J. Catal. 2007, 245, 392− 400. (32) Qiu, Y. J.; Chen, J. X.; Zhang, J. Y. Influence of nickel distribution on properties of spherical catalysts for partial oxidation of methane to synthesis gas. Catal. Commun. 2007, 8, 508−512. (33) King, D. L.; Strohm, J. J.; Wang, X. Q.; Roh, H. S.; Wang, C. M.; Chin, Y. H.; Wang, Y.; Lin, Y. B.; Rozmiarek, R.; Singh, P. Effect of

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.S.); huanting.wang@monash. edu (H.W.). Phone: +61 3 9905 3449 (H.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Australian Research Council (ARC). Y.W. thanks the Monash University for Monash postgraduate scholarships. H.W. thanks the ARC for a Future Fellowship.

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dx.doi.org/10.1021/ie300123x | Ind. Eng. Chem. Res. 2012, 51, 6387−6394