- Email: [email protected]
Electrochemical performance of cone-shaped anode-supported segmented-in-series SOFCs fabricated by gel-casting technique
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical performance of cone-shaped anode-supported segmented-in-series SOFCs fabricated by gel-casting technique Yan Liu a, Yubao Tang a, Jiao Ding a, Jiang Liu a,b,c,* a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China The Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, 381 Wushan Road, Guangzhou 510641, PR China c The Key Laboratory of New Energy Technology for Guangdong Universities, Department of Education of Guangdong Province, Guangzhou 510641, PR China b
article info
abstract
Article history:
Cone-shaped tubular anode substrates were prepared successfully by using gel-casting
Received 12 November 2010
technique. An 8 mol% yttria-stabilized zirconia (YSZ) electrolyte film was coated onto
Received in revised form
the green body of the anode by dip-coating technique and then the bilayer was co-sintered
18 March 2011
at 1350 C for 4 h. La0.7Sr0.3MnO3 (LSM)eYSZ was brush painted on the surface of the YSZ
Accepted 24 March 2011
film as cathode. Such a single NiO-YSZ/YSZ/LSM-YSZ fuel cell was tested from 600 to 800 C
Available online 29 April 2011
with humidified hydrogen (3%H2O, 100 mL/min) as fuel and ambient air as oxidant. Initial results showed that at 800 C the open circuit voltage (OCV) approached 1.0 V and the
Keywords:
maximum power density was 900 mW cm2. The impedance measurements measured
Gel-casting
under open circuit condition revealed that at 800 C the ohmic resistance of the cell was
Cone-shaped
0.1 U cm2 and the electrode polarization resistance was 0.4 U cm2.
Segmented-in-series
A four-cell-stack based on the above-mentioned cone-shaped tubular anode-supported
SOFCs
SOFC was assembled and tested. The OCV and maximum output power at 800 C were 3.2 V
Electrochemical Performance
and 3.4 W, respectively. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) are well known as powergeneration systems with very high efficiency, low pollution and flexible fuel selection [1e5]. Three basic designs have been explored in the development of SOFCs, the electrolytesupported, the cathode-supported, and the anode-supported cells. With the requirement of developing intermediate temperature SOFCs, anode-supported cells are attracting
more attention than the other two designs, because they can be operated at reduced temperature due to lower ohmic resistance than the electrolyte-supported cells and smaller polarization loss than the cathode-supported cells [6e11]. One of the advantages of SOFCs over the other kind of fuel cells is that their single cells can be fabricated into different shapes, so that stacks, such as cone-shaped segmented-in-series stacks [12e18], can be assembled as designed. The coneshaped anode-supported segmented-in-series SOFC design
* Corresponding author. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China. Tel./fax: þ862022236168. E-mail address: [email protected] (J. Liu). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.159
922
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
proposed by Liu [19] can achieve a relatively higher voltage and power in a limited space so that it is specially suitable for portable applications, which need light weight and compact volume. The single cone-shaped anode-supported cells have been fabricated using slip-casting and dip-coating techniques [15e18]. Slip-casting technique has a strict requirement on the suspension used and the thickness of the products is hard to control, while dip-coating requires much time to repeat the dipping-drying process. Gel casting is a novel forming technique in fabricating ceramic parts. It has many advantages such as low cost, good product homogeneity, high green strength, easy machining, and so on. It has been used in fabricating electrode substrates for SOFCs [20e23]. Compared to slip-coating and dip-coating techniques, gel-casting technique is faster. In this paper, we report our work on using gel-casting technique to fabricate cone-shaped tubular NiOeYSZ anode substrates. The fabricating process is described in detail. A single cell and a four-cell-stack have been assembled and electrochemically tested, and their performance has been characterized.
2.
Experimental procedure
2.1. Fabrication of cone-shaped NiOeYSZ anode substrates by gel-casting NiO (A.R., Australia) and 8 mol% yttria-stabilized zirconia (YSZ, Tosoh, Japan) powders were mixed in a weight ratio of NiO:YSZ ¼ 1:1. The NiOeYSZ mixture and 20 wt% graphite as pore-former were mixed in a proper amount of ethanol and ball-milled for 5 h. Acrylamide (AM) (A.R., Chemical Reagents Plant, Guangzhou, China) was used as the monomer. N, N0 -methylenebisacrylamide (MBAM) (A.R., Chemical Reagents Plant, Guangzhou, China) was used as the cross-linker. A polyacrylicammonia (PAA) solution (pH z 9) was used as dispersant which was prepared by mixing the polyacrylic acid (A.R., Chemical Reagents Plant, Guangzhou, China) with ammonia (A.R. Chemical Reagents Plant, Guangzhou, China) in the volume ratio of 1:1. Then a premixed solution was prepared using AM, MBAM, PAA and deionized water, whose weight ratios to the NiOeYSZ powder were 0.2:1, 0.01:1, 0.03:1, and 0.3:1, respectively [20,24,25]. The premixed solution was added to the above-mentioned NiO/YSZ/graphite mixture, followed by further ball-milling for 2 h. The pH value of the slurry was adjusted to 9e10 by ammonia during the ball-milling process. Thus a uniform NiO-YSZ slurry was obtained. Ammonium persulfate ((NH4)2S2O8, APS) (A.R., Chemical Reagents Plant, Guangzhou, China) which acted as an initiator was mixed with the NiOeYSZ slurry thoroughly. Then the mixture slurry was immediately poured into a mould which was dipped by wax on the inside surface in order to make the demoulding easier and make the surface of the anode glazed. After 10 min’s polymerizing, the green body was demoulded and dried at room temperature for 12 h. Subsequently, the as prepared anode substrate was heated at 150 C for 30 min to make the monomers polymerizing further. The picture of some green anode substrates is shown in Fig. 1.
Fig. 1 e The picture of green anode substrates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.2.
Single cell preparation
YSZ electrolyte film was deposited onto the green coneshaped tubular anode substrates using dip-coating technique and then the bi-layers were co-sintered at 1350 C for 4 h with a heating rate of 1 C min1. Cathode powder, La0.7Sr0.3MnO3 (LSM, Ningbo Institute of Physical Chemistry) was mixed with YSZ (Tosoh) in a weight ratio of 6:4. The LSM/YSZ was painted onto the sintered YSZ film. In addition, a pure LSM layer was applied on the top of the LSM/YSZ composite cathode to serve as a currentcollector. The painted cathode was then sintered at 1200 C for 2 h. Silver grids were painted on the surface of both the anode and cathode as current-collector using silver paste (Shanghai Research Institute of Synthetic Resins, Shanghai, China). Four single cells, one with one end closed and the others with both ends open, were connected in series to make a fourcell-stack as shown in Fig. 2. Silver paste was used as sealing
Fig. 2 e The photo of the four-cell-stack.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
923
and electrical connection between the anode of one cell and the cathode of the next cell. The single cell or the four-cell-stack used for test was attached to an alumina tube using silver paste as sealing and jointing material [16]. The cathode area of the single cell was 0.5 cm2. The total cathode area of the stack was 16.2 cm2 (S ¼ S1 þ S2 þ S3 þ S4 ¼ 6.3 þ 3.6 þ 2.7 þ 3.6 ¼ 16.2 cm2).
2.3.
Performance measurements
Moist hydrogen (3vol% H2O) was used as fuel at the anode side in a flow rate of 100 mL min1 and oxygen in the ambient air was used as oxidant at the cathode side. The single cell and the stack were tested at 600, 650, 700, 750 and 800 C, respectively. The cell performance and impedance spectra were measured using IviumStat electrochemical analyzer (Ivium Technologies B.V., Netherlands). A four-probe setup was adopted to eliminate the resistance losses from the current leading wires. The cell currentevoltage (IeV) curves were tested by linear sweep voltammetry at a scanning rate of 5 mV/s. The impedances were measured in the frequency range of 100 kHze0.1 Hz with a signal amplitude of 5 mV under open circuit condition. After electrochemical text, the cell was fractured and examined using scanning electron microscope (SEM).
3.
Results and discussion
3.1.
Electrochemical performance of the single cell
Fig. 3 shows the performance of the single cell at different temperatures. As can be seen, the maximum power density is about 900 mW cm2 at 800 C. The experimental open circuit voltage (OCV) was about 1 V. The OCVs are not as high as the theoretical values at each testing temperature. This may be caused by the gas leakage from the Ag paste sealing or the test setup leakage. The impedance spectra of the cell measured from 600 C to 800 C under open circuit conditions are presented in Fig. 4. For the total cell impedance at a temperature, the intercept on the real axis in the high-frequency range corresponds to the
Fig. 3 e The performance of the single cell at different temperatures.
Fig. 4 e Impedance spectra measured from 600 C to 800 C.
cell’s ohmic resistance while the arcs in the low-frequency range are the electrode polarization resistance including resistance from both the anode and cathode. The impedance spectra show that the ohmic resistance is small and the polarization resistance dominates the overall resistance. This suggests that the performance of the anode-supported SOFCs can be further improved by enhancing the electrode activity and optimizing the electrode/electrolyte interface.
3.2.
Electrochemical performance of the four-cell-stack
Fig. 5 shows the performance of the four-cell-stack at different temperatures. The open circuit voltages (OCVs) are 3.6e3.2 V from 600 C to 800 C. The maximum output power is about 3.4 W at 800 C. It can be seen that the average OCV of each single cell in the four-cell-stack is less than that of the single cell in Fig. 3. This can be explained by two reasons. First, the technique for preparing each cell is not so stable that the OCV and electrochemical performance of the cells composing the stack are not identical. Second, gas leakage across the Ag paste sealing at the connection may lower the OCV of a stack. It also can be seen that there exists high concentration polarization at high current when the cell is operated at higher temperatures, thus affecting the output performance of the stack.
Fig. 5 e The performance of the four-cell-stack.
924
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
Technology of Guangdong Province (grant NO. 2005B50101007) and Department of Education of Guangdong Province (grant NO. B15-N9060210) is gratefully acknowledged.
references
Fig. 6 e SEM micrograph of cross-section of a single cell after tesing.
3.3.
Cell microstructure
The micrograph of the cross-section of the single cell after testing is shown in Fig. 6. As can be seen, the anode substrate made by the gel-casting technique is porous. The YSZ film cosintered with the anode was quite dense, uniform and crackfree, with a thickness of about 20 mm. It adheres well to both the anode and cathode. This is a typical microstructure of an anode-supported SOFC, demonstrating the feasibility of the gel casting and co-sintering techniques in fabricating the SOFCs. However, the microstructure of the anode is not homogenous with large pores distributed. Note that the shape of these large pores is with a much larger size along the direction parallel to the electrolyte layer than that vertical to the electrolyte, which situation is not good for gas diffusion and electrical conduction across the anode. This can explain the concentration polarization mentioned above. Further work is necessary to optimizing the technique.
4.
Conclusion
Gel-casting process is a practical technique for fabricating cone-shaped tubular NiO-YSZ anode substrates for SOFCs. Electrolyte film can be deposited on the green anode substrate and then co-sintered in one step. A single SOFC with a power density of 900 mW cm2 and a four-cell-stack with an output of 3.4 W at 800 C demonstrate the feasibility of gel-casting technique in cone-shaped tubular SOFC single cell and stack fabrication and that the cell performance can be further improved by optimizing the cell assembling technique and improving the microstructure of the anodes.
Acknowledgements Financial support from the National ‘‘863’’ program of China (grant NO. 2007AA05Z136), the Department of Science and
[1] Singhal SC. Advances in solid oxide fuel cell technology. Solid State Ionics 2000;135(1e4):305e13. [2] Ormerod RM. Solid oxide fuel cells. Chem Soc Rev 2003;32(1): 17e28. [3] Tu HY, Stimming U. Advances, aging mechanisms and lifetime in solid-oxide fuel cell. J Power Sources 2004;127(1e2):284e93. [4] Minh NQ. Solid oxide fuel cell technology-features and applications. Solid State Ionics 2004;174(1e4):271e7. [5] Basu RN, Sharma AD, Dutta A, Mukhopadhyay J. Processing of high-performance anode-supported planar solid oxide fuel cell. Int J Hydrogen Energy 2008;33:5748e54. [6] Hatae T, Kakuda N, Taniyama T, Yamazaki Y. Low temperature preparation and performance of Ni/YSZ anode with a multi-layered structure for SOFC. J Power Sources 2004;135(1e2):25e8. [7] Kim SD, Hyun SH. Fabrication techniques of anode-supported YSZ thin electrolytes for intermediate temperature solid oxide fuel cells. Adv Fuel Cells; 2005:199e211. [8] Park JK, Yang SY, Lee T, Oh EM, Yoo YS, Park JW. Microstructure and electrical properties of single cells based on a Ni-YSZ cermet anode for IT-SOFCs. J Korean Ceram Soc 2006;43(12):823e8. [9] Kim SD, Moon H, Hyun SH, Moon J, Kim J, Lee HW. Ni-YSZ cermet anode fabricated from NiO-YSZ composite powder for high-performance and durability of solid oxide fuel cells. Solid State Ionics 2007;178(21e22):1304e9. [10] Leng YJ, Chan SH, Khor KA, Jiang SP. Performance evaluation of anode-supported solid oxide fuel cells with thin film YSZ electrolyte. Int J Hydrogen Energy 2004;29:1025e33. [11] Moon H, Kim SD, Hyun SH, Kim HS. Development of IT-SOFC unit cells with anode-supported thin electrolytes via tape casting and co-firing. Int J Hydrogen Energy 2008;33:1758e68. [12] Sui J, Liu J. Fabrication and performance of a cone-shaped electrolyte-supported solid oxide fuel cell. J Chin Ceram Soc 2006;34(12):2461e5. [13] Sui J, Liu J. An electrolyte-supported SOFC stack fabricated by slip casting technique. Electrochem Soc Trans 2007;7(1):633e7. [14] Sui J, Liu J. Slip-cast Ce0.8Sm0.2O1.9 cone-shaped SOFC. J Am Ceram Soc 2008;91(4):1335e7. [15] Zhang YH, Liu J, Yin J, Yuan WS, Sui J. Fabrication and performance of cone-shaped segmented-in-series solid oxide fuel cells. Int J Appl Ceram Technol 2008;5(6):568e73. [16] Ding J, Liu J. A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack. J Power Sources 2009;193(2):769e73. [17] Bai YH, Liu J, Gao HB, Jin C. Dip coating technique in fabrication of cone-shaped anode-supported solid oxide fuel cells. J Alloys Compd 2009;480:554e7. [18] Bai YH, Liu J, Wang CL. Performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack fabricated by dip coating technique. Int J Hydrogen Energy 2009;34(17):7311e5. [19] Liu J. Cone-shaped tubular anode-supported solid oxide fuel single cells and stacks. Chin. Pat.: CN100347897. [20] Huang WL, Zhu QS, Xie ZH. Gel-cast anode substrates for solid oxide fuel cells. J Power Sources 2006;162:464e8. [21] Prabhakaran K, Melkeri A, Beigh MO, Gokhale NM, Sharma SC. Preparation of a porous cermet SOFC anode substrate by gel casting of NiOeYSZ powders. J Am Ceram Soc 2007;90(2):622e5.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
[22] Dong DH, Gao JF, Liu XQ, Meng GY. Fabrication of tubular NiO/YSZ anode-support of solid oxide fuel cell by gel casting. J Power Sources 2007;165(1):217e23. [23] Pillai MR, Gostovic D, Kim I, Barnett SA. Short-period segmented-in-series solid oxide fuel cells on flattened tube supports. J Power Sources 2007;163(2):960e5.
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[24] Zhang L, Jiang SP, Wang W, Zhang YJ. NiO/YSZ, anodesupported, thin-electrolyte, solid oxide fuel cells fabricated by gel casting. J Power Sources 2007;170(1):55e60. [25] Li W, Han MF. Rheological properties of NiO/YSZ slurry prepared by gel-casting process. Vacuum Electronics 2009;4: 46e9.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical performance of cone-shaped anode-supported segmented-in-series SOFCs fabricated by gel-casting technique Yan Liu a, Yubao Tang a, Jiao Ding a, Jiang Liu a,b,c,* a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China The Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, 381 Wushan Road, Guangzhou 510641, PR China c The Key Laboratory of New Energy Technology for Guangdong Universities, Department of Education of Guangdong Province, Guangzhou 510641, PR China b
article info
abstract
Article history:
Cone-shaped tubular anode substrates were prepared successfully by using gel-casting
Received 12 November 2010
technique. An 8 mol% yttria-stabilized zirconia (YSZ) electrolyte film was coated onto
Received in revised form
the green body of the anode by dip-coating technique and then the bilayer was co-sintered
18 March 2011
at 1350 C for 4 h. La0.7Sr0.3MnO3 (LSM)eYSZ was brush painted on the surface of the YSZ
Accepted 24 March 2011
film as cathode. Such a single NiO-YSZ/YSZ/LSM-YSZ fuel cell was tested from 600 to 800 C
Available online 29 April 2011
with humidified hydrogen (3%H2O, 100 mL/min) as fuel and ambient air as oxidant. Initial results showed that at 800 C the open circuit voltage (OCV) approached 1.0 V and the
Keywords:
maximum power density was 900 mW cm2. The impedance measurements measured
Gel-casting
under open circuit condition revealed that at 800 C the ohmic resistance of the cell was
Cone-shaped
0.1 U cm2 and the electrode polarization resistance was 0.4 U cm2.
Segmented-in-series
A four-cell-stack based on the above-mentioned cone-shaped tubular anode-supported
SOFCs
SOFC was assembled and tested. The OCV and maximum output power at 800 C were 3.2 V
Electrochemical Performance
and 3.4 W, respectively. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) are well known as powergeneration systems with very high efficiency, low pollution and flexible fuel selection [1e5]. Three basic designs have been explored in the development of SOFCs, the electrolytesupported, the cathode-supported, and the anode-supported cells. With the requirement of developing intermediate temperature SOFCs, anode-supported cells are attracting
more attention than the other two designs, because they can be operated at reduced temperature due to lower ohmic resistance than the electrolyte-supported cells and smaller polarization loss than the cathode-supported cells [6e11]. One of the advantages of SOFCs over the other kind of fuel cells is that their single cells can be fabricated into different shapes, so that stacks, such as cone-shaped segmented-in-series stacks [12e18], can be assembled as designed. The coneshaped anode-supported segmented-in-series SOFC design
* Corresponding author. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China. Tel./fax: þ862022236168. E-mail address: [email protected] (J. Liu). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.159
922
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
proposed by Liu [19] can achieve a relatively higher voltage and power in a limited space so that it is specially suitable for portable applications, which need light weight and compact volume. The single cone-shaped anode-supported cells have been fabricated using slip-casting and dip-coating techniques [15e18]. Slip-casting technique has a strict requirement on the suspension used and the thickness of the products is hard to control, while dip-coating requires much time to repeat the dipping-drying process. Gel casting is a novel forming technique in fabricating ceramic parts. It has many advantages such as low cost, good product homogeneity, high green strength, easy machining, and so on. It has been used in fabricating electrode substrates for SOFCs [20e23]. Compared to slip-coating and dip-coating techniques, gel-casting technique is faster. In this paper, we report our work on using gel-casting technique to fabricate cone-shaped tubular NiOeYSZ anode substrates. The fabricating process is described in detail. A single cell and a four-cell-stack have been assembled and electrochemically tested, and their performance has been characterized.
2.
Experimental procedure
2.1. Fabrication of cone-shaped NiOeYSZ anode substrates by gel-casting NiO (A.R., Australia) and 8 mol% yttria-stabilized zirconia (YSZ, Tosoh, Japan) powders were mixed in a weight ratio of NiO:YSZ ¼ 1:1. The NiOeYSZ mixture and 20 wt% graphite as pore-former were mixed in a proper amount of ethanol and ball-milled for 5 h. Acrylamide (AM) (A.R., Chemical Reagents Plant, Guangzhou, China) was used as the monomer. N, N0 -methylenebisacrylamide (MBAM) (A.R., Chemical Reagents Plant, Guangzhou, China) was used as the cross-linker. A polyacrylicammonia (PAA) solution (pH z 9) was used as dispersant which was prepared by mixing the polyacrylic acid (A.R., Chemical Reagents Plant, Guangzhou, China) with ammonia (A.R. Chemical Reagents Plant, Guangzhou, China) in the volume ratio of 1:1. Then a premixed solution was prepared using AM, MBAM, PAA and deionized water, whose weight ratios to the NiOeYSZ powder were 0.2:1, 0.01:1, 0.03:1, and 0.3:1, respectively [20,24,25]. The premixed solution was added to the above-mentioned NiO/YSZ/graphite mixture, followed by further ball-milling for 2 h. The pH value of the slurry was adjusted to 9e10 by ammonia during the ball-milling process. Thus a uniform NiO-YSZ slurry was obtained. Ammonium persulfate ((NH4)2S2O8, APS) (A.R., Chemical Reagents Plant, Guangzhou, China) which acted as an initiator was mixed with the NiOeYSZ slurry thoroughly. Then the mixture slurry was immediately poured into a mould which was dipped by wax on the inside surface in order to make the demoulding easier and make the surface of the anode glazed. After 10 min’s polymerizing, the green body was demoulded and dried at room temperature for 12 h. Subsequently, the as prepared anode substrate was heated at 150 C for 30 min to make the monomers polymerizing further. The picture of some green anode substrates is shown in Fig. 1.
Fig. 1 e The picture of green anode substrates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.2.
Single cell preparation
YSZ electrolyte film was deposited onto the green coneshaped tubular anode substrates using dip-coating technique and then the bi-layers were co-sintered at 1350 C for 4 h with a heating rate of 1 C min1. Cathode powder, La0.7Sr0.3MnO3 (LSM, Ningbo Institute of Physical Chemistry) was mixed with YSZ (Tosoh) in a weight ratio of 6:4. The LSM/YSZ was painted onto the sintered YSZ film. In addition, a pure LSM layer was applied on the top of the LSM/YSZ composite cathode to serve as a currentcollector. The painted cathode was then sintered at 1200 C for 2 h. Silver grids were painted on the surface of both the anode and cathode as current-collector using silver paste (Shanghai Research Institute of Synthetic Resins, Shanghai, China). Four single cells, one with one end closed and the others with both ends open, were connected in series to make a fourcell-stack as shown in Fig. 2. Silver paste was used as sealing
Fig. 2 e The photo of the four-cell-stack.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
923
and electrical connection between the anode of one cell and the cathode of the next cell. The single cell or the four-cell-stack used for test was attached to an alumina tube using silver paste as sealing and jointing material [16]. The cathode area of the single cell was 0.5 cm2. The total cathode area of the stack was 16.2 cm2 (S ¼ S1 þ S2 þ S3 þ S4 ¼ 6.3 þ 3.6 þ 2.7 þ 3.6 ¼ 16.2 cm2).
2.3.
Performance measurements
Moist hydrogen (3vol% H2O) was used as fuel at the anode side in a flow rate of 100 mL min1 and oxygen in the ambient air was used as oxidant at the cathode side. The single cell and the stack were tested at 600, 650, 700, 750 and 800 C, respectively. The cell performance and impedance spectra were measured using IviumStat electrochemical analyzer (Ivium Technologies B.V., Netherlands). A four-probe setup was adopted to eliminate the resistance losses from the current leading wires. The cell currentevoltage (IeV) curves were tested by linear sweep voltammetry at a scanning rate of 5 mV/s. The impedances were measured in the frequency range of 100 kHze0.1 Hz with a signal amplitude of 5 mV under open circuit condition. After electrochemical text, the cell was fractured and examined using scanning electron microscope (SEM).
3.
Results and discussion
3.1.
Electrochemical performance of the single cell
Fig. 3 shows the performance of the single cell at different temperatures. As can be seen, the maximum power density is about 900 mW cm2 at 800 C. The experimental open circuit voltage (OCV) was about 1 V. The OCVs are not as high as the theoretical values at each testing temperature. This may be caused by the gas leakage from the Ag paste sealing or the test setup leakage. The impedance spectra of the cell measured from 600 C to 800 C under open circuit conditions are presented in Fig. 4. For the total cell impedance at a temperature, the intercept on the real axis in the high-frequency range corresponds to the
Fig. 3 e The performance of the single cell at different temperatures.
Fig. 4 e Impedance spectra measured from 600 C to 800 C.
cell’s ohmic resistance while the arcs in the low-frequency range are the electrode polarization resistance including resistance from both the anode and cathode. The impedance spectra show that the ohmic resistance is small and the polarization resistance dominates the overall resistance. This suggests that the performance of the anode-supported SOFCs can be further improved by enhancing the electrode activity and optimizing the electrode/electrolyte interface.
3.2.
Electrochemical performance of the four-cell-stack
Fig. 5 shows the performance of the four-cell-stack at different temperatures. The open circuit voltages (OCVs) are 3.6e3.2 V from 600 C to 800 C. The maximum output power is about 3.4 W at 800 C. It can be seen that the average OCV of each single cell in the four-cell-stack is less than that of the single cell in Fig. 3. This can be explained by two reasons. First, the technique for preparing each cell is not so stable that the OCV and electrochemical performance of the cells composing the stack are not identical. Second, gas leakage across the Ag paste sealing at the connection may lower the OCV of a stack. It also can be seen that there exists high concentration polarization at high current when the cell is operated at higher temperatures, thus affecting the output performance of the stack.
Fig. 5 e The performance of the four-cell-stack.
924
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 2 1 e9 2 5
Technology of Guangdong Province (grant NO. 2005B50101007) and Department of Education of Guangdong Province (grant NO. B15-N9060210) is gratefully acknowledged.
references
Fig. 6 e SEM micrograph of cross-section of a single cell after tesing.
3.3.
Cell microstructure
The micrograph of the cross-section of the single cell after testing is shown in Fig. 6. As can be seen, the anode substrate made by the gel-casting technique is porous. The YSZ film cosintered with the anode was quite dense, uniform and crackfree, with a thickness of about 20 mm. It adheres well to both the anode and cathode. This is a typical microstructure of an anode-supported SOFC, demonstrating the feasibility of the gel casting and co-sintering techniques in fabricating the SOFCs. However, the microstructure of the anode is not homogenous with large pores distributed. Note that the shape of these large pores is with a much larger size along the direction parallel to the electrolyte layer than that vertical to the electrolyte, which situation is not good for gas diffusion and electrical conduction across the anode. This can explain the concentration polarization mentioned above. Further work is necessary to optimizing the technique.
4.
Conclusion
Gel-casting process is a practical technique for fabricating cone-shaped tubular NiO-YSZ anode substrates for SOFCs. Electrolyte film can be deposited on the green anode substrate and then co-sintered in one step. A single SOFC with a power density of 900 mW cm2 and a four-cell-stack with an output of 3.4 W at 800 C demonstrate the feasibility of gel-casting technique in cone-shaped tubular SOFC single cell and stack fabrication and that the cell performance can be further improved by optimizing the cell assembling technique and improving the microstructure of the anodes.
Acknowledgements Financial support from the National ‘‘863’’ program of China (grant NO. 2007AA05Z136), the Department of Science and
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