Preparation of Porous NiO-Ce 0.8 Sm 0.2 O 1.9 Ceramics for Anode-supported Low-temperature Solid Oxide Fuel Cells

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1 J. Mater. Sci. Technol., 21, 26(6), Preparation of Porous NiO-Ce.8 Sm.2 O 1.9 Ceramics for Anode-supported Low-temperature Solid Oxide Fuel Cells Han Chen, Kui Cheng, Zhicheng Wang, Wenjian Weng, Ge Shen, Piyi Du and Gaorong Han Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 3127, China [Manuscript received May 14, 29, in revised form December 23, 29] Porous NiO-Ce.8 Sm.2 O 1.9 (NiO-SDC) ceramics with homogeneous pore distribution were prepared using polystyrene (PS) spheres as pore templates. NiO-SDC powders were synthesized by a glycine-nitrate method, ultrasonically mixed with PS spheres in ethanol, dried and then pressed into green pellets. The pellets were sintered to yield porous NiO-SDC ceramics. The effects of the sintering temperature on the microstructure and mechanical strength of the ceramics were investigated. NiO-SDC ceramics sintered at 12 and 135 C have interconnected pore structures and high compression strength. When single cells were fabricated using porous NiO-SDC ceramics as anode-supported layers, the peak power density of the cells at 6 C was 333 and 353 mw cm 2 for ceramics sintered at 12 and 135 C, respectively. The results indicated that these porous ceramic materials are promising for anode substrates for solid oxide fuel cells. KEY WORDS: Porous ceramic; NiO-Ce.8Sm.2O 1.9 (NiO-SDC); Polystyrene spheres; Microstructure; Anode-supported solid oxide fuel cells 1. Introduction With the development of the solid oxide fuel cells (SOFCs), much attention has been focused on cells that can operate at temperatures below 7 C to reduce system costs and improve their reliability, portability, and operational life [1,2]. However, the decrease in operating temperature leads to the decreases in ionic conductivity and therefore cell performance. Since the ohmic resistance of oxide electrolytes, which is proportional to the electrolyte thickness, is believed to be one of the main hindrances of the cell performance, the electrode-supported cells with a thin-film electrolyte have attracted increasing attention, especially anode-supported cells [3 5], for which the mechanical strength of the anode is as significant as the electrical properties. In SOFCs, anodes need to have an interconnected Corresponding author. Prof., Ph.D.; Tel.: ; Fax: ; address: wengwj@zju.edu.cn (W.J. Weng). pore structure providing large surface area for electrochemical reactions and a continuous framework to maintain sufficient mechanical strength. Multiscale porous ceramics are ideal for anode applications because macropores promote rapid gas transport through the porous electrode and nanopores provide a high surface area for gas adsorption/desorption and more catalytically active sites for electrode reaction. An effective method is to use pore-formers which are mixed with anode powders to form a porous ceramic after sintering. Flour, rice starch, and graphite were widely used as pore-formers [6,7]. Owing to their irregular shape, the structures of pore are difficult to control. Many researchers had used polymer spheres as templates to prepare well-ordered porous films as electrodes in electrolyte-supported cells [8,9]. The resulting pore structure in these thin electrodes was appropriate for high-performance anodes because of the large surface area and triple-phase boundaries (TPBs) for gas-solid reactions.

2 524 H. Chen et al.: J. Mater. Sci. Technol., 21, 26(6), In the present study, polymethyl methacrylate (PMMA) and polystyrene (PS) spheres were used as the pore template respectively to prepare porous bulk NiO-Ce.8 Sm.2 O 1.9 (NiO-SDC) ceramics as the anodes in anode-supported cells. The effects of the sphere kinds and the preparation conditions on the microstructure were investigated. Single cells with a porous NiO-SDC anode were fabricated and tested in the range C. 2. Experimental 2.1 NiO-SDC powders preparation and characterization Nanosized NiO-SDC (65 wt pct NiO) powders were prepared by a glycine-nitrate method [1]. Stoichiometric amounts of Ni(NO 3 ) 2 6H 2 O, Ce(NO 3 ) 3 6H 2 O and Sm(NO 3 ) 3 6H 2 O (all AR grade) were dissolved in deionized water to form a 1. mol/l mixed aqueous solution. Glycine was added to the solution under stirring (NO 3 /glycine molar ratio 2:1). The resulting solution was heated to evaporate water, yielding a viscous resin. On further heating, spontaneous ignition occurred and a voluminous, dark, sponge-like ash was obtained. The ash was calcined at 7 C for 4 h to remove residual carbon. The calcined powder was then ball-milled for 48 h. Sm.5 Sr.5 CoO 3 (SSC) powders for the cathode were prepared using a similar procedure. The powder was characterized by X-ray diffraction (XRD, D/Max-RA, Rigaku, Japan) analysis using CuKα radiation. The as-combusted particle morphology and powder size were observed by transmission electron microscopy (TEM, JEM-CX, JEOL, Japan). The particle size distribution was measured on a Malvern Zeta-Sizer (Nano-S9, Malvern, UK). 2.2 Preparation of porous ceramics Monodisperse PMMA and PS spheres with size close to that of NiO-SDC agglomerated particles were adopted as pore templates. NiO-SDC powders and polymer spheres (7.:3.4 for PMMA, and 7.:3. for PS, weight ratio) were ultrasonically dispersed in ethanol for at least 6 h. The mixture was then dried in a vacuum oven at 55 C for 6 h. The dried powders were pressed into pellets (15 mm in diameter and 1 mm thick) under 2 MPa. The green pellets were heat-treated in air at 4 C for 4 h, and sintered at 8, 1, 12 or 135 C for 4 h at a heating rate of 1 C/min. The morphology of the porous ceramics was observed by field-emission scanning electron microscope (FESEM, SIRION-1, FEI, Holland). The anode porosity was characterized by Archimedes density measurements. The compression strength of NiO-SDC ceramic columns (11. mm diameter and.5 mm thick) after polishing was measured in a universal testing machine (Z1, Zwick, Germany). 2.3 Cell fabrication and characterization Single cells were fabricated according to an anodesupported cell structure. The sintering temperature for the electrolyte film on the anode was set at more than 12 C to obtain a dense film. A co-pressing and co-sintering method [11] was adopted to prepare bi-layer pellets with an electrolyte film on the anode. The NiO-SDC and PS powders were first pressed at 1 MPa into pellets ( 15 mm diameter and 1 mm thick) as the anode supporting layer. Then.2 g SDC powder was added to the pellets as the electrolyte and pressed at 2 MPa. The bi-layer pellets were co-sintered at different temperatures. A slurry consisting of SSC/SDC (3 wt pct SDC) and ethylcellulose was applied to the electrolyte side of the sintered bi-layer pellets by spin-coating and fired at 95 C in air for 2 h to form a porous cathode with about 2 µm in thickness and.22 cm 2 in area. The cells with bi-layer pellets sintered at 12 and 135 C were denoted as cells I and II, respectively. The single cells were mounted onto a quartz tube. Silver paste was used to seal the anode compartment. A silver current collector was painted in a circular pattern in the center of the cathode using silver paste. Two silver wires were attached to the anode and cathode with silver paste. Measurements were conducted from 45 to 65 C while the anode and cathode were fed with humidified (3% H 2 O) H 2 /air as fuel/oxidant. Current density voltage (I V) curves were determined for the cells using a potentiostat (CHI6C, CH Instrument, USA) at a constant fuel flow rate of 6 ml/min. The cell impedance was measured in the frequency range from.1 Hz to 1 khz in an AC impedance spectrometer interfaced to a computer. 3. Results and Discussion 3.1 NiO-SDC powders The XRD pattern (Fig. 1) indicates that the calcined powders consisted of NiO and SDC phases, as expected. TEM micrographs show that the calcined powders are highly porous with a foam-like microstructure (inset of Fig. 2), and ball-milling powders contain fine particles with a primary particle size of 2 4 nm (Fig. 2), an average agglomerated particle size of 17.3 nm and a narrow size distribution (Fig. 3). These powders are suitable for preparing SOFC anodes, since the TPBs length and electronic/ionic conductivity strongly depend on the particle size [12]. According to the size of NiO-SDC agglomerated particles, PMMA and PS spheres with the diameter of 3 35 nm (inset of Fig. 4(a) and (b)) were adopted as pore templates.

3 H. Chen et al.: J. Mater. Sci. Technol., 21, 26(6), SDC NiO Intensity / a.u / deg. Fig. 1 XRD pattern of anode powders prepared by glycine-nitrate method Fig. 2 TEM micrographs of NiO-SDC particles by ballmilling (the inset shows the morphology of the powders before ball-milling) Volume / % Diameter / nm Fig. 3 Particle distribution of NiO-SDC composite powders prepared by the glycine-nitrate method 3.2 Effect of different polymer spheres on the microstructure The FESEM micrographs (Fig. 4(a) and (b)) show the pore structures derived from different polymer spheres. The NiO-SDC ceramic derived from PMMA pore-formers after 12 C sintering has an inhomogenous pore structure and is not suitable for anode. Fig. 4 FESEM micrographs of NiO-SDC ceramics sintered at 12 C: (a) PMMA spheres as poreformers, (b) PS spheres as pore-formers (the insets show the PMMA and PS spheres) While the NiO-SDC ceramic derived from PS poreformers reveals good pore structure with homogeneous pore distribution. Since each kind of the spheres could be well separated by ultrasonically dispersing in ethanol (inset of Fig. 4(a) and (b)), the difference in pore structure could be attributed to the aggregation of PMMA spheres which takes place during mixing due to weaker affinity with oxide particles than PS spheres. 3.3 Effect of sintering temperature on the microstructure Detailed microstructural changes of ceramics used PS spheres as pore-formers with sintering temperature are shown in Fig. 5. Although the ceramics sintered at 8 and 1 C have a porous structure (Fig. 5(a) and (b)), they have poor mechanical properties (Table 1). This could result from the difference in sintering reactivity between NiO and SDC [5]. NiO powders have a lower sintering temperature than SDC powders which begin to sinter at approximately 1 C [13]. This sintering mismatch leads to the formation of a weakly sintered skeleton with low volume shrinkage (Table 1). As the sintering temperature increased, the resulting ceramics exhibit reasonably good mechanical

4 526 H. Chen et al.: J. Mater. Sci. Technol., 21, 26(6), Table 1 Sintering properties of the NiO-SDC ceramic sintered at different sintering temperature Sintering temperature Volume shrinkage Porosity/% Compression strength / C /% Before reduction After reduction /MPa Fig. 5 FESEM micrographs of NiO-SDC ceramics sintered at 8 C (a), 1 C (b), 12 C (c) and 135 C (d) Fig. 6 Microstructure of SDC electrolytes for different cells: (a) cell I, (b) cell II strength (Table 1), but the porous structure remains (Fig. 5(c) and (d), Table 1). The increase in compression strength is attributed to the formation of a continuous SDC network at more than 12 C that is favorable for ionic conductivity. Figure 6 shows the cross-sectional views of the SDC electrolyte. The SDC electrolytes in the bi-layer pellets sintered at 12 and 135 C have high density, and the thickness are both about 2 µm. 3.4 Cell performance Based on the mechanical strength of the anode (Table 1), temperatures of 12 and 135 C were selected for fabrication of cells I and II, respectively. The voltage and power density as a function of the current density and the impedance spectra for the cells are shown in Figs. 7 and 8, respectively. The peak power density at 6 C (Fig. 7) for cells

5 H. Chen et al.: J. Mater. Sci. Technol., 21, 26(6), Voltage / V Voltage / V (a) 55 o C 6 o C 65 o C Current density / ma cm (b) 55 o C 6 o C 65 o C Current density / ma cm Power / mw cm -2 Power / mw cm -2 Fig. 7 Cell voltages (open symbols) and power densities (solid symbols) as a function of current density of the cell I (a) and cell II (b) tested at different temperature -lmz / cm Cell I Cell II ReZ / cm 2 Fig. 8 Impedance spectra measured at 6 C in open circuit conditions for different cells I and II is 333 and 353 mw cm 2, respectively, indicating that the two cells perform comparatively. Cell performance is closely related to the microstructures of the anode, electrolyte and cathode. In the present study, it is assumed that the cathode microstructure does not change because the formation conditions are the same for both cells. From the impedance spectra (Fig. 8), two significant data can be obtained, the ohmic resistance (R b ) and the area specific resistance (ASR). R b is determined from the high-frequency intercepts with the real axis and primarily reflects the electrolyte resistance. ASR is the difference between the intercepts on the real axis at high and low frequency, and represents the interfacial polarization resistance between the electrode and the electrolyte. Cell II has lower R b values than cell I (Fig. 8). Based on the same mass of SDC used as electrolyte, the result implies that larger grain size [14] is attained at 135 C. The lower ASR value in cell II might be due to the higher electronic and ionic conductivity of its anode induced by better linking of NiO and SDC particles, respectively, and the better interface between the electrode and the electrolyte as the bi-layer pellet sintered at 135 C, although lower porosity (Table 1) slightly increases reaction resistance on electrochemical polarization and the impedance of gas diffusion because of low surface areas and less catalytically active sites [15]. Generally, a higher electrolyte density [16] can lead to a higher open circuit voltage (OCV). However, The OCV of cell II is slightly lower than that of cell I. Considering high electrolyte density in both cells, this might be caused by the activity of electrode. The higher electric conductivity in the anode of cell II causes higher leaking current, thus lower OCV [17]. Based on cell I with the approximately equal performance as cell II, cell I could be considered as better one because a low temperature sintering leads to a low mismatching between anode and electrolyte, resulting in a good fuel cell stability. 4. Conclusion The glycine/nitrate method has yielded fine NiO- SDC powders with reliable crystalline phases and narrow size distribution. The kind of polymer spherical pore-former affects the microstructure of NiO-SDC ceramics dramatically. Porous ceramics with the required microstructure and mechanical strength could be prepared using PS spheres as a pore template. The sintering temperature has a great influence on the microstructure and compression strength of the resulting ceramics. NiO-SDC ceramics sintered at 12 and 135 C show high compression strength and the corresponding cells have desirable cell performance. Acknowledgements This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (No. Y4837) and the research fund of Shenzhen Key Laboratory of Functional Polymers. REFERENCES [1 ] B. Zhu: J. Power Sources, 21, 93, 82. [2 ] T. Matsui, M. Inaba, A. Mineshige and Z. Ogumi: Solid State Ion., 25, 176, 647. [3 ] H. Tu and U. Stimming: J. Power Sources, 24, 127, 284.

6 528 H. Chen et al.: J. Mater. Sci. Technol., 21, 26(6), [4 ] J. Van herle, R. Ihringer, R. Vasquez Cavieres, L. Constantin and O. Bucheli: J. Eur. Ceram. Soc., 21, 21, [5 ] F. Tietz, F.J. Dias, D. Simwonis and D. Stöver: J. Eur. Ceram. Soc., 2, 2, 123. [6 ] J.J. Haslam, A. Pham, B.W. Chung, J.F. DiCarlo and R.S. Glass: J. Am. Ceram. Soc., 25, 88, 513. [7 ] J.Y. Hu, Z. Lü, K.F. Chen, X.Q. Huang, N. Ai, X.B. Du, C.W. Fu, J.M. Wang and W.H. Su: J. Membr. Sci., 28, 318, 445. [8 ] Y.L. Zhang, S.W. Zha and M.L. Liu: Adv. Mater., 25, 17, 487. [9 ] A. Lashtabeg, J. Drennan, R. Knibbe, J.L. Bradley and G.Q. Lu: Microporous Mesoporous Mat., 29, 117, 395. [1] R.R. Peng, C.R. Xia, Q.X. Fu, G.Y. Meng and D.K. Peng: Mater. Lett., 22, 56, 143. [11] C.R. Xia and M.L. Liu: Solid State Ion., 21, 144, 249. [12] F.H. Wang, R.S. Guo, Q.T. Wei, Y. Zhou, H.L. Li and S.L. Li: Mater. Lett., 24, 58, 379. [13] J.G. Li, T. Ikegami and T. Mori: Acta. Mater., 24, 52, [14] Y.J. Kang, H.J. Park and G.M. Choi: Solid State Ion., 28, 179, 162. [15] J.H. Nam and D.H. Jeon: Electrochim. Acta., 26, 51, [16] L.J. Zhao, X.Q. Huang, R.B. Zhu, Z. Lu, W.W. Sun, Y.H. Zhang, X.D. Ge, Z.G. Liu and W.H. Su: J. Phys. Chem. Solids, 28, 69, 219. [17] Q. Huang, B.W. Wang, W. Qu and R. Hui: J. Power Sources, 29, 191, 297.