- Email: [email protected]
Development of micro-tubular SOFCs with an improved performance via nano-Ag impregnation for intermediate temperature operation
Electrochemistry Communications 9 (2007) 1918–1923 www.elsevier.com/locate/elecom
Development of micro-tubular SOFCs with an improved performance via nano-Ag impregnation for intermediate temperature operation Yu Liu a
a,*
, Masashi Mori a, Yoshihiro Funahashi b, Yoshinobu Fujishiro c, Atsushi Hirano
d
Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan b Fine Ceramics Research Association, 2266-99 Anagahora, Shimo-shidami, Moriyama-ku, Nagoya, Aichi 463-8561, Japan c Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology, Shimo-shidami, Moriyama-ku, Nagoya, Aichi 463-9560, Japan d Department of Chemistry, Faculty of Engineering, Mie University, Kamihama-cho, Tsu, Mie 514-8507, Japan Received 8 March 2007; received in revised form 16 April 2007; accepted 4 May 2007 Available online 17 May 2007
Abstract Micro-tubular solid-oxide fuel cell consisting of a 10-lm thick (ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 (ScSZ) electrolyte on a support NiO/ (ScSZ) anode (1.8 mm diameter, 200 lm wall thickness) with a Ce0.8Gd0.2O1.9 (GDC) buffer-layer and a La0.6Sr0.4Co0.2Fe0.8O3d (LSCF)/GDC functional cathode has been developed for intermediate temperature operation. The functional cathode was in situ formed by impregnating the well-dispersed nano-Ag particles into the porous LSCF/GDC layer using a citrate method. The cells yielded maximum power densities of 1.06 W cm2 (1.43 A cm2, 0.74 V), 0.98 W cm2 (1.78 A cm2, 0.55 V) and 0.49 W cm2 (1.44 A cm2, 0.34 V), at 650, 600 and 550 °C, respectively. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Solid-oxide fuel cells; Micro-tubular; Impregnation; Electrochemically catalytic Ag; Power performance
1. Introduction Scandium-stabilized zirconia (ScSZ) exhibits the highest oxide-ion conductivity of all the zirconia systems and is a very attractive electrolyte material in solid-oxide fuel cells (SOFCs) because of its excellent mechanical strength and outstanding thermal/chemical stabilities in both oxidizing and reducing atmospheres [1]. ScSZ thin-film supported by a Ni-cermet under a micro-tubular design has allowed for lower operating temperatures below 700 °C by virtually decreasing the ohmic resistance of the electrolyte [2]. These micro-tubular cells with a superior tolerance for thermal stress and a high feasibility for rapid start-up are suitable for cogeneration and transportation applications [3]. Addi-
*
Corresponding author. Tel.: +81 46 856 2121; fax: +81 46 856 5571. E-mail address: [email protected] (Y. Liu).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.05.003
tionally, a smaller diameter in the millimeter or sub-millimeter range makes it possible to achieve a higher volumetric power density in stacks [4]. In this configuration, polarization losses on the anode side could be controlled to be reasonably low via an appropriate microstructure and nickel content; whilst the cathode, which has high electrical resistance for collecting electric current and low catalytic activity for oxygen reduction (O2 + 4e ! 2O2), strongly determines the cell performance. As a promising candidate, Ag is a well investigated catalyst for oxygen adsorption, desorption, dissociation and diffusion, and is more affordable than the cost-prohibitive Pd and Pt. However, fabrication of the Ag-based electrode appears to be a major obstacle. The low sintering temperature arising from the low melting point of Ag (ca. 962 °C) generally yields to a poor adhesion of the Ag-based electrodes to the electrolyte. On the other hand, increasing the sintering temperature to over 800 °C tends to densify
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
1919
the Ag electrode structure resulting in severe Ag agglomeration and little porosity, which significantly reduces the triple-phase boundaries (TPB) and raises the impedance to gas transport through the electrodes. A wet impregnation is a well-known method in the development of heterogeneous catalysts. A recent review has summarized the application of such a technique to deposit fine particles into the established electrode structure of the SOFC [5]. This allows Ag particles to infiltrate into an established cathode using a relatively low sintering temperature. Although the long-term stability of the impregnated electrode structure remains questionable, the Ag particles with a relatively small size and an improved dispersion show an enlarged reactive area leading to a higher catalytic activity [6,7]. However, during the impregnation, there still remain some problems due to the solution infiltration ability, and the low adhesion between the Ag precursors (mainly Ag+ aqueous solution) and the ceramic matrices. Moreover, the larger Ag particles (over several of micrometers) and the inhomogeneous particle distribution influenced by the electrode microstructure and the synthesis process may further reduce the Ag catalytic activity [6– 8]. In this work, nano-Ag particles were impregnated into an established cathode with a very homogeneous distribution and a good adhesion to the ceramic matrix by a citrate method. Micro-tubular NiO–ScSZ anode supported cells made with the nano-structured cathode showed a significantly improved electrochemical performance.
the interlayer were further dip-coated in the cathode slurry containing the La0.6Sr0.4Co0.2Fe0.8O3d (LSCF, Seimi Chem. Co. Ltd., Japan) and GDC (LSCF:GDC: 60:40 in volume) powders with organic ingredients and graphite powders. These were finally fired at 1050 °C for 2 h. The heating rate was 2.9 °C/min. The introduction of the GDC interlayer, in this case, was to prevent the chemical incompatibility between the LSCF and the ScSZ during fabrication [9].
2. Experimental
A Pt mesh used as the current collector was covered to the cathode section, while Pt wires were well wound at two terminals of the tube as the anode current collectors. These were fixed with Ag paste. 30 vol.% H2 – Ar (saturated with H2O vapor at 20 °C) as fuel and air as oxidant with a flow rate of 100 mL min1 were flowed through the anode and the cathode, respectively. Before measurements, the cells were heated to 800 °C for 30 min in air. The electrochemical measurement was carried out in the temperature range from 550 to 650 °C. No gas leakage in the electrochemical measurements was confirmed using the bubble-type gas flowmeter. Microstructures of the micro-tubular cells were recorded with scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) (Hitachi, S-3500H, Japan).
2.1. Fabrications of the micro-tubular anode supported cells The NiO-(ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 (ScSZ) tubes were extruded from a plastic mass through a die. The mass was created by mixing the commercially available NiO (Sumitomo Metal Industries, Ltd., Japan) and ScSZ (Daiichi Kigenso Kagaku Kogyo, Japan) powders under a desired volumetric ratio (Ni vs. ScSZ: 50:50 in volume after reduction) in water for 1–2 h, with ethyl cellulose as a binder and polymethyl methacrylate (PMMA) bead as a poreformer. A vacuum was applied to the mixing chamber for 10–30 min to remove air from the mass. The mass was left to age overnight, and then extruded using a ram extruder (Miyazaki Tekko, Inc., Japan) with an in-house designed die. After drying, the tubes were cut to a desirable length and dip-coated in the ScSZ slurry, which was prepared by ball-milling the ScSZ powders, organic ingredients, such as binder, dispersant and solvents for 24 h. This was cofired at 1400 °C for 6 h. Porosity of the anode substrate was calculated to be ca. 40% after the sintering based on the pore-former contents. The interlayer slurry consisting of the Ce0.8Gd0.2O1.9 (GDC, Seimi Chem. Co. Ltd., Japan) powders prepared by the way similar to the ScSZ slurry was dip-coated on the electrolyte, and followed a sintering at 1200 °C for 2 h. The anode tubes with the electrolyte and
2.2. Preparation of the functional cathode with nano-Ag impregnation The micro-tubular cells were immersed into the aqueous solution containing AgNO3 (99.8%, Nacalai Tesque Co., Ltd., Japan), ethylene glycol and citric acid (Wako Pure Chemical Industries, Ltd., Japan) for 30 min. The Ag+ concentration was 2.5 mol L1 and the molecular ratio of citric acid to Ag+ was 1.3. Here, an excess of citric acid (>1 mole per mole Ag+) was used in order to obtain a complexation of the cation, and the molecular ratio of ethylene glycol to citric acid was further made in excess (>1.2) to polymerize the solution with a suitable viscosity. During the impregnation, the anode and the electrolyte sections were protected to avoid contact with the solution. The cells were dried at 100 °C for 1 h, and further followed a heat treatment at 800 °C for 5 h. 2.3. Electrochemical measurements
3. Results and discussion During the different co-firing stages, the shrinkages of the cell components were well controlled in order to avoid the potential crack and deformation. Fig. 1a shows the photograph of a typical micro-tubular cell with a wall thickness of 0.2 mm and a diameter of 1.8 mm (Fig. 1b). The active cathode length during the measurement was 2 cm, indicating that the active cell area was 1.13 cm2. After the reduction of NiO to Ni, the anode tube having uniform and spherical pores (ca. 2–3 lm) presents a well-
1920
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
Fig. 1. (a) Photograph of a typical micro-tubular cell; (b, c) SEM micrographs of the cell after power generation: (b) the cross-section, (c) the anode section and (d) the enlarged cathode/buffer-layer/electrolyte/anode view.
1.2
1.2
Cell volatge / V
1.0
1.0 o
650 C 0.8
0.8
o
600 C o
550 C
0.6
0.6
0.4
0.4
0.2
0.2
0.0 0.0
0.5
1.0
1.5
2.0
Power density / W cm-2
0.0 2.5
Current density / A cm-2 1.2
1.2 o
650 C
Cell voltage / V
1.0
1.0
o
600 C 0.8
0.8 0.6
o
0.6
550 C
0.4
0.4
0.2
0.2
0.0 0.0
0.5
1.0
1.5
2.0
Power density / W cm-2
bonded porous network, which is composed of many welldispersed and connected Ni–ScSZ grains, as shown in Fig. 1c. The high porosity of the substrate and the good connection among the Ni–ScSZ grains can not only enlarge the reactive areas, but also remain high electron paths through the anode framework. Microstructure of the cathode and anode layers near the electrolyte where most of the electrochemical reaction occurs is further shown in Fig. 1d. The porous cathode (upper portion, thickness of 15– 20 lm), the porous interlayer (middle upper portion, ca. 1 lm), the dense electrolyte (middle lower portion, ca. 10 lm) and the porous anode (lower portion) regions were clearly contrasted. There is a good adhesion at the interfaces among the anode, the electrolyte, the buffer-layer and the cathode. Although it is expected to obtain a dense structure for the GDC for the improved buffering property, the as-prepared porous interlayer can effectively suppress the chemical incompatibility between the LSCF and the ScSZ (Fig. 4, elemental distribution) [10]. This is highly consistent with several previous reports [2,9]. Furthermore, due to a relatively low sintering temperature (61200 °C) for the GDC buffer-layer, a possible formation of a (Zr, Ce) O2 solid solution, that exhibits much lower ionic conductivities between the ScSZ and the GDC, was not observed in an EDX level (Fig. 4). The open circuit voltages (OCVs) obtained for the tubular cell at 650, 600 and 550 °C were approximately 1.057, 1.060 and 1.065 V, which are agreed well with the theoretical ones. At 0.7 V, cell outputs were 0.51, 0.33 and 0.088 A cm2, at 650, 600 and 550 °C, respectively, as shown in Fig. 2a. The sharp voltage drops were remarkable in the relatively low current density range of 60.5 A cm2.
0.0 2.5
Current density / A cm-2 Fig. 2. Current density–voltage and power density characteristics of the typical NiO–ScSZ supported micro-tubular cells: (a) before and (b) after Ag impregnation, at 550, 600 and 650 °C.
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
The polarization resistances, which primarily came from the cathode, were found to be apparently higher than the ohmic ones [10]. This seriously lowered the voltage at low current density and limited the cell performance. Fig. 2b reveals that the j–V and power density characteristics of the micro-tubular cell were obviously improved by the introduction of the functional cathode. For the single tubular cell without Ag-loading, the peak power densities obtained at 650, 600 and 550 °C were 0.54, 0.31 and 0.11 W cm2, respectively, whereas such values were significantly increased to be 1.06, 0.98 and 0.49 W cm2 after the Ag impregnation, meaning the improved factors as much as 1.5–3.0. The general mechanism of the reaction involved in the Ag impregnation using the citrate process includes several stages. First, a homogeneous chelate between the Ag cations and the citrate anions was formed in the aqueous solution. Water was allowed to evaporate gradually at 100 °C after the Ag+ solution was entirely impregnated into the LSCF–GDC layer. During the subsequent heat treatment (until to 800 °C), the primary reactions involving the reduction of Ag+ by ethylene glycol occurred at around 160 °C. In the so-called polyol process [11], ethylene glycol severed as both the reductant and the solvent, while citric acid acted as a capping/protective reagent. Finally, the strong combustion in the temperature up to 800 °C completely decomposed all the organic compounds and NO 3 . The citrate process highly inhibited the grain growth of the Ag, as shown in Fig. 3b, in which the microstructure of the functional cathode exhibits very fine particles and high porosities. This might lead to a high number of electrochemically active sites, and therefore to an increased cell performance. The morphology of the functional cathode was so fine that it was difficult to exactly distinguish the segregated Ag and the LSCF/GDC ceramic grains. However, comparing with Fig. 3a, the Ag particles shown in Fig. 3b have an average particle size of <200 nm, and are homogeneously distributed and well adhered with the ceramic LSCF/GDC. Elemental distribution from EDX shown in Fig. 4 confirmed that the impregnated nano-Ag had a well distribution without severe agglomeration, and fulfilled the cathode layer until the GDC interlayer. This leads to an increased three-phase boundary for the Ag to favor the electrocatalytic activities of the LSCF/GDC cathode. The contents of the Ag particles in the cathode were approximately 8 wt.% (vs. ceramic) as confirmed by EDX analysis. The j–V characteristics in Fig. 5 further revealed that the sharp voltage drops related to the electrode polarization at the low current density (60.5 A cm2) were significantly alleviated for the Ag-impregnated cell. The role of Ag in the functional cathode is thus considered to improve the catalytic activity, due to the significantly increased TPB in the nano-structured electrode. Additionally, it can enhance the current collecting effects through the cathode. The results are consistent with those reported previously by Wang et al. [5,6]. However, a detrimental effect regarding
1921
Fig. 3. SEM micrographs at the cross-section of the cathode layer: (a) before and (b) after Ag impregnation. Some examples for the locations of the nano-Ag are plotted in (b).
the addition of the Ag using the impregnation was observed by Haanappel et al. [8]. The negative influence in their results probably is attributed to that the decomposition of the Ag based precursor damaged the functional layer due to the severe pore formation during sintering, and the Ag particles were not dispersed finely enough to induce the desired catalytic effect. These observations strongly suggested that the rate of oxygen reduction (catalytic activity) of the Ag-based cathodes could be highly dominated by the three-phase boundaries related to the Ag dispersion and the active surface area [12]. Additionally, as can be seen in Fig. 5, the ohmic overpotentials (IR drops) that were roughly estimated from the slope line at higher current density (>0.5 A cm2) showed a slight improvement after the Ag-loading. The IR drops reflected the resistance of the electrode and the electrolyte to the transfer of electrons and ions. This improvement could be attributed to the fact that the Ag coating enhanced the electron supply through the porous electrode, and reduced the contact resistance between the cathode and the current collectors [13].
1922
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
Fig. 4. SEM micrographs at the cross-section of the cathode/interlayer/electrolyte area and elemental distribution of Ag, Ce, La, Sr and Zr.
1.2
o
1.0 IR loss 0.8 Ag-load
0.6 0.4
Ag-free
Cell voltage / V
0.2 0.0 0.0
4. Conclusion
650 C
0.5
1.0
1.5
2.0
2.5 o
1.2
600 C
Nano-Ag particles were homogeneously impregnated into the established porous LSCF/GDC cathode layer using a citrate method. Maximum power densities of (1.43 A cm2, 0.74 V), 0.98 W cm2 1.06 W cm2 2 2 (1.78 A cm , 0.55 V) and 0.49 W cm (1.44 A cm2, 0.34 V) were obtained at 650, 600 and 550 °C for the micro-tubular Ni–ScSZ anode supported cells made with such a functional cathode. These results demonstrate that the nano-Ag catalyst-infiltrated cathode can be an effective and low-cost application for the anode supported ITSOFCs.
1.0 IR loss
0.8 0.6
This work has been supported by NEDO, Japan, as part of the Advanced Ceramic Reactor Project.
Ag-load
0.4 0.2
Ag-free
References
0.0 0.0
0.5
1.0
Acknowledgement
1.5
2.0
2.5
Current density / A cm-2 Fig. 5. Comparison in the current density–voltage curves of the microtubular cells with the Ag-free and Ag-loading cathodes at 600 and 650 °C.
Although a further study is required in order to determine the long-term stability of the nano-Ag electrode structure, the present operation temperatures of 550–650 °C can effectively alleviate the diffusivity/volatility of Ag. This ensures the Agbased electrode a long-term electrocatalytic effect.
[1] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [2] T.L. Nguyen, T. Honda, T. Kato, Y. Iimura, K. Kato, A. Negishi, K. Nozaki, M. Shiono, A. Kobayashi, K. Hosoda, Z. Cai, M. Dokiya, J. Electrochem. Soc. 151 (2004) A1230. [3] N.M. Sammes, Y. Du, R. Bove, J. Power Sources 145 (2005) 428. [4] T. Suzuki, T. Yamaguchi, Y. Fujishiro, M. Awano, J. Electrochem. Soc. 153 (2006) A925. [5] S.P. Jiang, Mater. Sci. Eng. A 418 (2006) 199. [6] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwashita, M. Dokiya, Solid State Ionics 146 (2002) 203. [7] S. Wang, T. Kato, S. Nagata, T. Kaneko, N. Iwashita, T. Honda, M. Dokiya, Solid State Ionics 152–153 (2002) 477.
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923 [8] V.A.C. Haanappel, D. Rutenbeck, A. Mai, S. Uhlenbruck, D. Sebold, H. Wesemeyer, B. Rowekamp, C. Tropartz, F. Tietz, J. Power Sources 130 (2004) 119. [9] M. Shiono, K. Kobayashi, T.L. Nguyen, K. Hosoda, T. Kato, K. Ota, M. Dokiya, Solid state Ionics 170 (2004) 1. [10] S. Hashimoto, K. Asano, H. Nishino, Y. Liu, M. Mori, Y. Funahashi, Y. Fujishiro, in: The 47th Battery Symposium
1923
in Japan, p1B-07, Tokyo, Japan, November 2006, pp. 20–22. [11] F. Fievet, J.P. Lagier, M. Figlarz, Mater. Res. Soc. Bull. 14 (December) (1989) 29. [12] J.H. Wang, M. Liu, M.C. Lin, Solid State Ionics 177 (2006) 939. [13] C.E. Hatchwell, N.M. Sammes, K. Kendall, J. Power Sources 70 (1998) 85.
Development of micro-tubular SOFCs with an improved performance via nano-Ag impregnation for intermediate temperature operation Yu Liu a
a,*
, Masashi Mori a, Yoshihiro Funahashi b, Yoshinobu Fujishiro c, Atsushi Hirano
d
Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan b Fine Ceramics Research Association, 2266-99 Anagahora, Shimo-shidami, Moriyama-ku, Nagoya, Aichi 463-8561, Japan c Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology, Shimo-shidami, Moriyama-ku, Nagoya, Aichi 463-9560, Japan d Department of Chemistry, Faculty of Engineering, Mie University, Kamihama-cho, Tsu, Mie 514-8507, Japan Received 8 March 2007; received in revised form 16 April 2007; accepted 4 May 2007 Available online 17 May 2007
Abstract Micro-tubular solid-oxide fuel cell consisting of a 10-lm thick (ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 (ScSZ) electrolyte on a support NiO/ (ScSZ) anode (1.8 mm diameter, 200 lm wall thickness) with a Ce0.8Gd0.2O1.9 (GDC) buffer-layer and a La0.6Sr0.4Co0.2Fe0.8O3d (LSCF)/GDC functional cathode has been developed for intermediate temperature operation. The functional cathode was in situ formed by impregnating the well-dispersed nano-Ag particles into the porous LSCF/GDC layer using a citrate method. The cells yielded maximum power densities of 1.06 W cm2 (1.43 A cm2, 0.74 V), 0.98 W cm2 (1.78 A cm2, 0.55 V) and 0.49 W cm2 (1.44 A cm2, 0.34 V), at 650, 600 and 550 °C, respectively. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Solid-oxide fuel cells; Micro-tubular; Impregnation; Electrochemically catalytic Ag; Power performance
1. Introduction Scandium-stabilized zirconia (ScSZ) exhibits the highest oxide-ion conductivity of all the zirconia systems and is a very attractive electrolyte material in solid-oxide fuel cells (SOFCs) because of its excellent mechanical strength and outstanding thermal/chemical stabilities in both oxidizing and reducing atmospheres [1]. ScSZ thin-film supported by a Ni-cermet under a micro-tubular design has allowed for lower operating temperatures below 700 °C by virtually decreasing the ohmic resistance of the electrolyte [2]. These micro-tubular cells with a superior tolerance for thermal stress and a high feasibility for rapid start-up are suitable for cogeneration and transportation applications [3]. Addi-
*
Corresponding author. Tel.: +81 46 856 2121; fax: +81 46 856 5571. E-mail address: [email protected] (Y. Liu).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.05.003
tionally, a smaller diameter in the millimeter or sub-millimeter range makes it possible to achieve a higher volumetric power density in stacks [4]. In this configuration, polarization losses on the anode side could be controlled to be reasonably low via an appropriate microstructure and nickel content; whilst the cathode, which has high electrical resistance for collecting electric current and low catalytic activity for oxygen reduction (O2 + 4e ! 2O2), strongly determines the cell performance. As a promising candidate, Ag is a well investigated catalyst for oxygen adsorption, desorption, dissociation and diffusion, and is more affordable than the cost-prohibitive Pd and Pt. However, fabrication of the Ag-based electrode appears to be a major obstacle. The low sintering temperature arising from the low melting point of Ag (ca. 962 °C) generally yields to a poor adhesion of the Ag-based electrodes to the electrolyte. On the other hand, increasing the sintering temperature to over 800 °C tends to densify
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
1919
the Ag electrode structure resulting in severe Ag agglomeration and little porosity, which significantly reduces the triple-phase boundaries (TPB) and raises the impedance to gas transport through the electrodes. A wet impregnation is a well-known method in the development of heterogeneous catalysts. A recent review has summarized the application of such a technique to deposit fine particles into the established electrode structure of the SOFC [5]. This allows Ag particles to infiltrate into an established cathode using a relatively low sintering temperature. Although the long-term stability of the impregnated electrode structure remains questionable, the Ag particles with a relatively small size and an improved dispersion show an enlarged reactive area leading to a higher catalytic activity [6,7]. However, during the impregnation, there still remain some problems due to the solution infiltration ability, and the low adhesion between the Ag precursors (mainly Ag+ aqueous solution) and the ceramic matrices. Moreover, the larger Ag particles (over several of micrometers) and the inhomogeneous particle distribution influenced by the electrode microstructure and the synthesis process may further reduce the Ag catalytic activity [6– 8]. In this work, nano-Ag particles were impregnated into an established cathode with a very homogeneous distribution and a good adhesion to the ceramic matrix by a citrate method. Micro-tubular NiO–ScSZ anode supported cells made with the nano-structured cathode showed a significantly improved electrochemical performance.
the interlayer were further dip-coated in the cathode slurry containing the La0.6Sr0.4Co0.2Fe0.8O3d (LSCF, Seimi Chem. Co. Ltd., Japan) and GDC (LSCF:GDC: 60:40 in volume) powders with organic ingredients and graphite powders. These were finally fired at 1050 °C for 2 h. The heating rate was 2.9 °C/min. The introduction of the GDC interlayer, in this case, was to prevent the chemical incompatibility between the LSCF and the ScSZ during fabrication [9].
2. Experimental
A Pt mesh used as the current collector was covered to the cathode section, while Pt wires were well wound at two terminals of the tube as the anode current collectors. These were fixed with Ag paste. 30 vol.% H2 – Ar (saturated with H2O vapor at 20 °C) as fuel and air as oxidant with a flow rate of 100 mL min1 were flowed through the anode and the cathode, respectively. Before measurements, the cells were heated to 800 °C for 30 min in air. The electrochemical measurement was carried out in the temperature range from 550 to 650 °C. No gas leakage in the electrochemical measurements was confirmed using the bubble-type gas flowmeter. Microstructures of the micro-tubular cells were recorded with scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) (Hitachi, S-3500H, Japan).
2.1. Fabrications of the micro-tubular anode supported cells The NiO-(ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 (ScSZ) tubes were extruded from a plastic mass through a die. The mass was created by mixing the commercially available NiO (Sumitomo Metal Industries, Ltd., Japan) and ScSZ (Daiichi Kigenso Kagaku Kogyo, Japan) powders under a desired volumetric ratio (Ni vs. ScSZ: 50:50 in volume after reduction) in water for 1–2 h, with ethyl cellulose as a binder and polymethyl methacrylate (PMMA) bead as a poreformer. A vacuum was applied to the mixing chamber for 10–30 min to remove air from the mass. The mass was left to age overnight, and then extruded using a ram extruder (Miyazaki Tekko, Inc., Japan) with an in-house designed die. After drying, the tubes were cut to a desirable length and dip-coated in the ScSZ slurry, which was prepared by ball-milling the ScSZ powders, organic ingredients, such as binder, dispersant and solvents for 24 h. This was cofired at 1400 °C for 6 h. Porosity of the anode substrate was calculated to be ca. 40% after the sintering based on the pore-former contents. The interlayer slurry consisting of the Ce0.8Gd0.2O1.9 (GDC, Seimi Chem. Co. Ltd., Japan) powders prepared by the way similar to the ScSZ slurry was dip-coated on the electrolyte, and followed a sintering at 1200 °C for 2 h. The anode tubes with the electrolyte and
2.2. Preparation of the functional cathode with nano-Ag impregnation The micro-tubular cells were immersed into the aqueous solution containing AgNO3 (99.8%, Nacalai Tesque Co., Ltd., Japan), ethylene glycol and citric acid (Wako Pure Chemical Industries, Ltd., Japan) for 30 min. The Ag+ concentration was 2.5 mol L1 and the molecular ratio of citric acid to Ag+ was 1.3. Here, an excess of citric acid (>1 mole per mole Ag+) was used in order to obtain a complexation of the cation, and the molecular ratio of ethylene glycol to citric acid was further made in excess (>1.2) to polymerize the solution with a suitable viscosity. During the impregnation, the anode and the electrolyte sections were protected to avoid contact with the solution. The cells were dried at 100 °C for 1 h, and further followed a heat treatment at 800 °C for 5 h. 2.3. Electrochemical measurements
3. Results and discussion During the different co-firing stages, the shrinkages of the cell components were well controlled in order to avoid the potential crack and deformation. Fig. 1a shows the photograph of a typical micro-tubular cell with a wall thickness of 0.2 mm and a diameter of 1.8 mm (Fig. 1b). The active cathode length during the measurement was 2 cm, indicating that the active cell area was 1.13 cm2. After the reduction of NiO to Ni, the anode tube having uniform and spherical pores (ca. 2–3 lm) presents a well-
1920
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
Fig. 1. (a) Photograph of a typical micro-tubular cell; (b, c) SEM micrographs of the cell after power generation: (b) the cross-section, (c) the anode section and (d) the enlarged cathode/buffer-layer/electrolyte/anode view.
1.2
1.2
Cell volatge / V
1.0
1.0 o
650 C 0.8
0.8
o
600 C o
550 C
0.6
0.6
0.4
0.4
0.2
0.2
0.0 0.0
0.5
1.0
1.5
2.0
Power density / W cm-2
0.0 2.5
Current density / A cm-2 1.2
1.2 o
650 C
Cell voltage / V
1.0
1.0
o
600 C 0.8
0.8 0.6
o
0.6
550 C
0.4
0.4
0.2
0.2
0.0 0.0
0.5
1.0
1.5
2.0
Power density / W cm-2
bonded porous network, which is composed of many welldispersed and connected Ni–ScSZ grains, as shown in Fig. 1c. The high porosity of the substrate and the good connection among the Ni–ScSZ grains can not only enlarge the reactive areas, but also remain high electron paths through the anode framework. Microstructure of the cathode and anode layers near the electrolyte where most of the electrochemical reaction occurs is further shown in Fig. 1d. The porous cathode (upper portion, thickness of 15– 20 lm), the porous interlayer (middle upper portion, ca. 1 lm), the dense electrolyte (middle lower portion, ca. 10 lm) and the porous anode (lower portion) regions were clearly contrasted. There is a good adhesion at the interfaces among the anode, the electrolyte, the buffer-layer and the cathode. Although it is expected to obtain a dense structure for the GDC for the improved buffering property, the as-prepared porous interlayer can effectively suppress the chemical incompatibility between the LSCF and the ScSZ (Fig. 4, elemental distribution) [10]. This is highly consistent with several previous reports [2,9]. Furthermore, due to a relatively low sintering temperature (61200 °C) for the GDC buffer-layer, a possible formation of a (Zr, Ce) O2 solid solution, that exhibits much lower ionic conductivities between the ScSZ and the GDC, was not observed in an EDX level (Fig. 4). The open circuit voltages (OCVs) obtained for the tubular cell at 650, 600 and 550 °C were approximately 1.057, 1.060 and 1.065 V, which are agreed well with the theoretical ones. At 0.7 V, cell outputs were 0.51, 0.33 and 0.088 A cm2, at 650, 600 and 550 °C, respectively, as shown in Fig. 2a. The sharp voltage drops were remarkable in the relatively low current density range of 60.5 A cm2.
0.0 2.5
Current density / A cm-2 Fig. 2. Current density–voltage and power density characteristics of the typical NiO–ScSZ supported micro-tubular cells: (a) before and (b) after Ag impregnation, at 550, 600 and 650 °C.
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
The polarization resistances, which primarily came from the cathode, were found to be apparently higher than the ohmic ones [10]. This seriously lowered the voltage at low current density and limited the cell performance. Fig. 2b reveals that the j–V and power density characteristics of the micro-tubular cell were obviously improved by the introduction of the functional cathode. For the single tubular cell without Ag-loading, the peak power densities obtained at 650, 600 and 550 °C were 0.54, 0.31 and 0.11 W cm2, respectively, whereas such values were significantly increased to be 1.06, 0.98 and 0.49 W cm2 after the Ag impregnation, meaning the improved factors as much as 1.5–3.0. The general mechanism of the reaction involved in the Ag impregnation using the citrate process includes several stages. First, a homogeneous chelate between the Ag cations and the citrate anions was formed in the aqueous solution. Water was allowed to evaporate gradually at 100 °C after the Ag+ solution was entirely impregnated into the LSCF–GDC layer. During the subsequent heat treatment (until to 800 °C), the primary reactions involving the reduction of Ag+ by ethylene glycol occurred at around 160 °C. In the so-called polyol process [11], ethylene glycol severed as both the reductant and the solvent, while citric acid acted as a capping/protective reagent. Finally, the strong combustion in the temperature up to 800 °C completely decomposed all the organic compounds and NO 3 . The citrate process highly inhibited the grain growth of the Ag, as shown in Fig. 3b, in which the microstructure of the functional cathode exhibits very fine particles and high porosities. This might lead to a high number of electrochemically active sites, and therefore to an increased cell performance. The morphology of the functional cathode was so fine that it was difficult to exactly distinguish the segregated Ag and the LSCF/GDC ceramic grains. However, comparing with Fig. 3a, the Ag particles shown in Fig. 3b have an average particle size of <200 nm, and are homogeneously distributed and well adhered with the ceramic LSCF/GDC. Elemental distribution from EDX shown in Fig. 4 confirmed that the impregnated nano-Ag had a well distribution without severe agglomeration, and fulfilled the cathode layer until the GDC interlayer. This leads to an increased three-phase boundary for the Ag to favor the electrocatalytic activities of the LSCF/GDC cathode. The contents of the Ag particles in the cathode were approximately 8 wt.% (vs. ceramic) as confirmed by EDX analysis. The j–V characteristics in Fig. 5 further revealed that the sharp voltage drops related to the electrode polarization at the low current density (60.5 A cm2) were significantly alleviated for the Ag-impregnated cell. The role of Ag in the functional cathode is thus considered to improve the catalytic activity, due to the significantly increased TPB in the nano-structured electrode. Additionally, it can enhance the current collecting effects through the cathode. The results are consistent with those reported previously by Wang et al. [5,6]. However, a detrimental effect regarding
1921
Fig. 3. SEM micrographs at the cross-section of the cathode layer: (a) before and (b) after Ag impregnation. Some examples for the locations of the nano-Ag are plotted in (b).
the addition of the Ag using the impregnation was observed by Haanappel et al. [8]. The negative influence in their results probably is attributed to that the decomposition of the Ag based precursor damaged the functional layer due to the severe pore formation during sintering, and the Ag particles were not dispersed finely enough to induce the desired catalytic effect. These observations strongly suggested that the rate of oxygen reduction (catalytic activity) of the Ag-based cathodes could be highly dominated by the three-phase boundaries related to the Ag dispersion and the active surface area [12]. Additionally, as can be seen in Fig. 5, the ohmic overpotentials (IR drops) that were roughly estimated from the slope line at higher current density (>0.5 A cm2) showed a slight improvement after the Ag-loading. The IR drops reflected the resistance of the electrode and the electrolyte to the transfer of electrons and ions. This improvement could be attributed to the fact that the Ag coating enhanced the electron supply through the porous electrode, and reduced the contact resistance between the cathode and the current collectors [13].
1922
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923
Fig. 4. SEM micrographs at the cross-section of the cathode/interlayer/electrolyte area and elemental distribution of Ag, Ce, La, Sr and Zr.
1.2
o
1.0 IR loss 0.8 Ag-load
0.6 0.4
Ag-free
Cell voltage / V
0.2 0.0 0.0
4. Conclusion
650 C
0.5
1.0
1.5
2.0
2.5 o
1.2
600 C
Nano-Ag particles were homogeneously impregnated into the established porous LSCF/GDC cathode layer using a citrate method. Maximum power densities of (1.43 A cm2, 0.74 V), 0.98 W cm2 1.06 W cm2 2 2 (1.78 A cm , 0.55 V) and 0.49 W cm (1.44 A cm2, 0.34 V) were obtained at 650, 600 and 550 °C for the micro-tubular Ni–ScSZ anode supported cells made with such a functional cathode. These results demonstrate that the nano-Ag catalyst-infiltrated cathode can be an effective and low-cost application for the anode supported ITSOFCs.
1.0 IR loss
0.8 0.6
This work has been supported by NEDO, Japan, as part of the Advanced Ceramic Reactor Project.
Ag-load
0.4 0.2
Ag-free
References
0.0 0.0
0.5
1.0
Acknowledgement
1.5
2.0
2.5
Current density / A cm-2 Fig. 5. Comparison in the current density–voltage curves of the microtubular cells with the Ag-free and Ag-loading cathodes at 600 and 650 °C.
Although a further study is required in order to determine the long-term stability of the nano-Ag electrode structure, the present operation temperatures of 550–650 °C can effectively alleviate the diffusivity/volatility of Ag. This ensures the Agbased electrode a long-term electrocatalytic effect.
[1] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [2] T.L. Nguyen, T. Honda, T. Kato, Y. Iimura, K. Kato, A. Negishi, K. Nozaki, M. Shiono, A. Kobayashi, K. Hosoda, Z. Cai, M. Dokiya, J. Electrochem. Soc. 151 (2004) A1230. [3] N.M. Sammes, Y. Du, R. Bove, J. Power Sources 145 (2005) 428. [4] T. Suzuki, T. Yamaguchi, Y. Fujishiro, M. Awano, J. Electrochem. Soc. 153 (2006) A925. [5] S.P. Jiang, Mater. Sci. Eng. A 418 (2006) 199. [6] S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwashita, M. Dokiya, Solid State Ionics 146 (2002) 203. [7] S. Wang, T. Kato, S. Nagata, T. Kaneko, N. Iwashita, T. Honda, M. Dokiya, Solid State Ionics 152–153 (2002) 477.
Y. Liu et al. / Electrochemistry Communications 9 (2007) 1918–1923 [8] V.A.C. Haanappel, D. Rutenbeck, A. Mai, S. Uhlenbruck, D. Sebold, H. Wesemeyer, B. Rowekamp, C. Tropartz, F. Tietz, J. Power Sources 130 (2004) 119. [9] M. Shiono, K. Kobayashi, T.L. Nguyen, K. Hosoda, T. Kato, K. Ota, M. Dokiya, Solid state Ionics 170 (2004) 1. [10] S. Hashimoto, K. Asano, H. Nishino, Y. Liu, M. Mori, Y. Funahashi, Y. Fujishiro, in: The 47th Battery Symposium
1923
in Japan, p1B-07, Tokyo, Japan, November 2006, pp. 20–22. [11] F. Fievet, J.P. Lagier, M. Figlarz, Mater. Res. Soc. Bull. 14 (December) (1989) 29. [12] J.H. Wang, M. Liu, M.C. Lin, Solid State Ionics 177 (2006) 939. [13] C.E. Hatchwell, N.M. Sammes, K. Kendall, J. Power Sources 70 (1998) 85.