- Email: [email protected]
A cost-effective process for fabrication of metal-supported solid oxide fuel cells
international journal of hydrogen energy 35 (2010) 4592–4596
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
A cost-effective process for fabrication of metal-supported solid oxide fuel cells Yonghong Kong, Bin Hua, Jian Pu*, Bo Chi, Li Jian School of Materials Science and Engineering, State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
article info
abstract
Article history:
Metal-supported SOFC cells with Y2O3 stabilized ZrO2 as the electrolyte were prepared by
Received 16 September 2009
a low cost and simple process involving tape casting, screen printing and co-firing. The
Received in revised form
interfaces were well bonded after the reduction of NiO to Ni in the support and the anode.
19 February 2010
AC impedance was employed to estimate the cell polarizations under open circuit condi-
Accepted 20 February 2010
tions. It was found that the electrode polarization resistance was high at low temperatures
Available online 23 March 2010
and became equivalent to the ohmic resistance at higher temperatures near 800 C. The cell performance was evaluated with H2 as the fuel and air as the oxidant, and maximum
Keywords:
power density between 0.23 and 0.80 W/cm2 was achieved in the temperature range of
Solid oxide fuel cell
650–800 C, which confirms the applicability of the cost-effective process in fabrication of
Metal support
metal-supported SOFC cells.
Tape casting
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Co-firing
1.
Introduction
Solid oxide fuel cells (SOFCs) are a highly efficient energy conversion system, directly converting the chemical energy of fossil fuels into electricity at a low level of pollutant emission without combustion and mechanical processes. In the last decade, significant progresses have been made in reduction of SOFC operating temperatures from near 1000 C to an intermediate temperature range of 600–800 C; as a result, the use of metallic components in SOFCs becomes possible with the advantages of easy fabrication, high thermal and electrical conductivity, low material cost, and robust mechanical property. Metal-supported SOFCs are one of the examples showing the application of metallic components in SOFCs.
It is noted that depositing porous electrodes/dense electrolyte (the cell assembly) on porous metal matrix is challenging; various techniques have been explored to fabricate the metal-supported cells. For placing the cell assembly directly on top of the porous metal matrix prepared by powder metallurgy techniques, there are few low temperature technologies that can be considered, such as pulsed laser deposition [1], plasma spraying [2,3], suspension plasma spraying [4], vacuum plasma spraying [5] and colloidal infiltration or spraying [6] with some costly equipment involved. Firing in reducing atmosphere sometimes is unavoidable due to the oxidation of the metal component [6,7]. In such case, diffusion barrier is required to prevent inter-diffusion between the metal support and the electrode [8]. An alternative approach is
* Corresponding author. Tel./fax: þ86 27 87558142. E-mail address: [email protected] (J. Pu). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.106
international journal of hydrogen energy 35 (2010) 4592–4596
to generate the cell assembly on a metal oxide plate, then reduce the metal oxide to form the porous metal support; and the cell assembly is placed in position by pulse laser deposition [9,10]. In the present study, for the consideration of cost, a different approach was taken to fabricate the metal-supported SOFC. NiO was used as the starting material for the support. NiO tapes were prepared by tape casting, on which a multilayer structure of functional anode, electrolyte and cathode was built consecutively by paste screen printing, and finally the green cell was cofired in air. During the start-up of the test, NiO was reduced in reducing atmosphere to convert NiO to Ni. Such cost effectively manufactured cells were subjected to microstructure characterization and performance evaluation. This cell fabrication method involves only conventional ceramic processes, which enables co-firing of the cell components and is suitable for mass production at low cost.
power (I–V–P) curves were obtained using a Solartron 1480A at a sweep rate of 4 mV s1 from OCV to 0.4 V. In order to understand the microstructure of the prepared cells, out-of-cell reduction test was conducted at 750 C for 9 h in an atmosphere of 4% H2 þ 96% N2 (vol.%) gas mixture with a flow rate of 200 mL/min, using a cell without the cathode. The porosity change of the support with reduction was evaluated by the Archimedes method. The microstructure of the reduced cell was examined by using a scanning electron microscopy (SEM, Quanta 200, FEI corporation, Holand). And the phases were determined by X-ray diffraction (XRD, X’Pert PRO, PANalytical B.V. Corporation, Holand) with Cu Ka radiation under the conditions of 40 kV and 40 mA. The scanning rate was 15 min1, and the 2q angle ranged from 20 to 80 .
3. 2.
Experimental
2.1.
Cell preparation
NiO powder (average particle size w;1 mm, Chengdu Haite Advanced Materials Co., Ltd, China) with a small amount of carbon black as pore former was firstly ball milled at room temperature for 24 h in an organic mixture of xylene and ethanol with Menhaden fish oil (Richard E. Mistler, Inc, USA) as the dispersant. Soon after addition of the organic binder and plasticizer (Solutia Inc, USA), another ball mill of 24 h was followed. The ball-milled slurry of NiO was degassed in moderate vacuum, and tape cast and dried in air to form flexible green tape with a thickness of about 0.9 mm. Discs with a diameter of 26 mm were cut from the cast tape, onto which the functional anode, electrolyte and cathode were subsequently paste screen printed in sequence. The functional anode was consisted of NiO (average particle size w1 mm, Inco, USA) and 8% yttria-stabilized zirconia (YSZ, TZ-8Y of Tosoh Corporation, Japan) in a mass ratio of 64:36 and the electrolyte was made of the YSZ. The material for the cathode was a proprietary composite based on the YSZ with small amounts of precious metal. Such prepared cells were debindered and fired at 1450 C in air for 2 h. The heating rate was 1 C min1. The sintered dimensions of the YSZ electrolyte and the cathode were 14 and 8 mm, respectively. The active area of the cell was 0.5 cm2.
2.2.
4593
Results and discussion
The sintered NiO support was relatively dense, achieved about 92% of the theoretic density of NiO after being co-fired with other cell components. After 9 h reduction in the reducing gas, the porosity reached around 26%, close to that presented in porous SS430 and HastelloyX supports [11] but slightly lower than the conventionally expected value of 30–40%. The reduction started from the exposed surface of the NiO support and penetrated progressively to the functional anode layer. Fig. 1 shows the XRD pattern of the reduced cell, obtained from the side of the functional anode layer covered by dense electrolyte. It is noted that the predominant phases were YSZ and Ni; and the intensity of diffraction peaks generated from the unreduced NiO was relatively weak after 9 h reduction. This suggests that the NiO support will be converted to Ni and the NiO–YSZ functional anode converted to Ni–YSZ during the start-up of cell operation. The SEM micrograph of the reduced cell is shown in Fig. 2. All the interfaces between the cell components were well bonded, no crack was observed. Although the coefficient of thermal expansion (CTE) of the YSZ (10.9 106 K1 [12]) and Ni (15–18 106 K1 [13]) is somewhat mismatched, the adherence between the Ni–YSZ
Cell characterization and evaluation
The cell was evaluated electrochemically using a home-made setup described in Refs. [1,3], with ceramic felt and mica sheet as the seal of anode and cathode side, respectively. Pt meshes were placed on both the cathode and anode as the current collector, and Pt wires were attached to the mesh for applying the electrical current and measuring the voltage. The electrochemical measurement was carried out at 650, 700, 750 and 800 C, respectively, with humidified H2 as the fuel and air as the oxidant, and the cell impedance was acquired using a potentiostat/galvanostat (Solartron 1480A) coupled with a 1260 frequency response analyzer. The current–voltage–
Fig. 1 – X-ray diffraction pattern of the cell reduced at 750 C in 4 vol.%H2–N2 gas for 9 h.
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international journal of hydrogen energy 35 (2010) 4592–4596
Fig. 4 – AC impedance spectra of the Ni-supported cell under open circuit condition over a temperature range between 650 and 800 C.
Fig. 2 – Cross-sectional SEM micrograph of the reduced cell, showing thin electrolyte and electrodes on porous Ni substrate.
functional anode and the Ni support was well maintained. Since the cell was graded in terms of Ni from the substrate to the electrolyte with a functional anode of Ni–YSZ cermet in between, the CTE mismatch between the Ni support and the cell assembly would be minimized even for large size cells. The yielding of the ‘‘soft’’ Ni is expected to be able to compensate the strain caused by the CTE mismatch. It can also be seen from Fig. 2 that the YSZ electrolyte was dense with only few observed isolated holes; and both the electrolyte and the functional anode were thin, in between 5 and 10 mm.
Fig. 3 – I–V and I–P curves of metal-supported cell tested at various temperatures between 650 and 800 C with humidified H2 as the fuel and air as the oxidant.
Fig. 3 presents the current–voltage (I–V) and current–power (I–P) curves of the Ni-supported cell in the temperature range between 700 and 800 C. The open circuit voltage (OCV) is above 1.0 V; minor gas leakage might have occur during the cell test so that the OCV was not as high as expected. The maximum power density of the cell was in the range of 0.23– 0.80 W/cm2 at temperatures between 650 and 800 C, which is significantly higher than that reported in Refs. [14–16] where similar cell component materials were used. Fig. 4 is the AC impedance spectra of the Ni-supported cell under open circuit condition at various temperatures between 650 and 800 C; and accordingly the ohmic resistance of the cell Ro and the electrode polarization resistance Rp can be obtained. The Ro is represented by the low frequency intercept of the spectrum at the real axis, and Rp is determined by the difference between the high and low frequency intercepts. The temperature dependence of the resistances is shown in Fig. 5. The Ro decreased from 0.45 to 0.22 U cm2 and the RP decreased from 3.09 to 0.28 U cm2 as temperature increased from 650 to 800 C. The ohmic resistance is mainly contributed by the YSZ used in the cell, which has a lower conductivity at low temperatures.
Fig. 5 – Temperature dependence of ohmic and electrode polarization resistances of the metal-supported cell between 650 and 800 8C.
international journal of hydrogen energy 35 (2010) 4592–4596
4595
Fig. 6 – Effect of pore former on sintering of the NiO support and the YSZ electrolyte printed on the support: (a) without and (b) with 3 wt% addition of the carbon black in the substrate.
It is expected that if electrolyte materials with higher ionic conductivity, such as Gd-doped ceria (GDC) or Sr- and Mgdoped lanthanum gallate (LSGM) are used, the ohmic loss will be greatly decreased, and in turn the cell performance will be enhanced. Brandon et al. [17] have fabricated metalsupported SOFC with GDC as the electrolyte, and a power density of 310 mW cm2 was obtained at 600 C with an Ro as low as 0.08 U cm2. Ishihara et al. [10] achieved a maximum power density of 400 mW/cm2 at 500 C with a metalsupported cell using LSGM electrolyte. Compared with the Rp, the contribution of Ro to the total cell ohmic resistance is less important at low temperatures, and gradually becomes equivalent to the Rp as temperature increases. The Rp contains the polarization resistances from both the anode and cathode, and the cathode polarization is orders of magnitude higher than that of the anode. Therefore, for further improving the cell performance, high conductive electrolyte and low polarization cathode are demanded. From the above results, it is considered that the process that was used to prepare the Ni-supported cell is promising due to its simple nature and low cost. The key challenge to this process is the densification of the electrolyte layer during cofiring, while the electrodes remain porous. Fig. 6 [18] shows the effect of pore former on the sintering of the NiO substrate (originally 26 mm in diameter) and the YSZ electrolyte printed on the substrate (originally 18 mm in diameter) at various temperatures in air for 2 h. Without adding a pore former, as shown in Fig. 6a, the substrate started to shrink at temperature around 700 C and a total shrinkage of 15% was achieved after sintered at 1450 C; whereas the YSZ started to shrink at temperature around 800 C and the shrinkage of the specimen sintered at 1450 C was only 12%. However, with the addition of 3 wt% of carbon black as the pore former of the NiO substrate, both the substrate and electrolyte shrank at almost the same rate as temperature increased and reached a total shrinkage of 21 and 20% at 1450 C (Fig. 6b), respectively. NiO is easier to be sintered than the YSZ, and the densified NiO support at lower temperatures is likely to prevent the YSZ electrolyte layer from being fully sintered. Therefore, it is helpful to use pore formers in the slurry of NiO, such as corn starch or carbon black, to synchronize the shrinkage of the NiO substrate and the YSZ electrolyte during the co-firing. It is also realized that using Ni as the substrate for metalsupported SOFCs may have several other potential issues,
such as the redox capability, the strength and the microstructure stability of the support. The redox capability is always an issue for Ni in SOFCs, no matter it is used in the support or in the anode. The formation of cracks in YSZ electrolyte and micro-cracks in Ni-YSZ anode at the interface between Ni and YSZ was observed as a result of severe redox cycles [19]. As a matter of fact, compared to the conventional Ni–YSZ anode supported cell, pure Ni as a cell support would not cause more significant crack formation in the cell during redox as there are no extra interfaces inside the support. The lack of strength and continuous sintering of the Ni support at the operation temperature should be taken into consideration in future studies.
4.
Conclusions
From this study, the following conclusions can be made: (1) Simple process of tape casting, screen printing and cofiring can be successfully used to fabricate Ni-supported SOFC cells at low cost, achieving a power density between 0.23 and 0.80 W/cm2 in the temperature range of 650–800 C. (2) The coefficient of thermal expansion mismatch between the porous Ni support and the cell assembly is not likely to cause delamination at the interface (at least not in small cells such as the one used in this study). (3) Electrode polarization resistance is the primary component of the total cell resistance at all temperatures, and at high temperatures close to 800 C, the ohmic and polarization losses are equivalent.
Acknowledgements This work was financially supported by the National 863 project under the contract 2006AA03Z227. The authors acknowledge Wei Qu and Cyrille Dece`s-Petit of National Research Council Canada for their assistance in cell testing, and the Analytical and Testing Center of Huazhong University of Science and Technology for SEM and XRD characterizations.
4596
international journal of hydrogen energy 35 (2010) 4592–4596
references [11] [1] Hui S, Yang D, Wang Z, Yick S, Deces-Petit C, Qu W, et al. Metal-supported solid oxide fuel cell operated at 400–600 C. J Power Sources 2007;2:336–9. [2] Hui R, Wang Z, Kesler O, Rose L, Jankovic J, Yick S, et al. Thermal plasma spraying for SOFCs: applications, potential advantages, and challenges. J Power Sources 2007;2:308–23. [3] Hui R, Berghaus J, Dece`s-Petit C, Qu W, Yicka S, Legoux J, et al. High performance metal-supported solid oxide fuel cells fabricated by thermal spray. J Power Sources 2009;191: 371–6. [4] Huanga Q-A, Wang B, Qu W, Hui R. Impedance diagnosis of metal-supported SOFCs with SDC as electrolyte. J Power Sources 2009;191:297–303. [5] Schiller G, Franco T, Henne R, Lang M, Ruckda¨schel R, Otschik P, et al. Current status of metallic substrate supported thin-film SOFC at DLR Stuttgart. Electrochem Soc Proc 2001;16:885–94. [6] Matus Y, Jonghe L, Jacobson C, Visco S. Metal-supported solid oxide fuel cell membranes for rapid thermal cycling. Solid State Ionics 2005;17:443–9. [7] Visco S, Jacobson C, Villareal I, Leming A, Matus Y, Jonghe L. Development of low-cost alloy supported SOFCs. Electrochem Soc Proc 2003;07:1040–50. [8] Brandner M, Bram M, Froitzheim J, Buchkremer HP, Stover D. Electrically conductive diffusion barrier layers for metalsupported SOFC. Solid State Ionics 2008;27–32:1501–4. [9] Joo J, Choi G. Simple fabrication of micro-solid oxide fuel cell supported on metal substrate. J Power Sources 2008;2:589–93. [10] Ishihara T, Yan J, Enoki M, Okada S, Matsumoto H. Ni–Fe alloy-supported intermediate temperature SOFCs using
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
LaGaO3 electrolyte film for quick startup. J Fuel Cell Sci Tech 2008;5:031205. Huang Q-A, Oberste-Berghaus J, Yang D, Yick S, Wang Z, Wang B, et al. Polarization analysis for metal-supported SOFCs from different fabrication processes. J Power Sources 2008;177:339–47. Adams J, Nakamura H, Ingel R, Rice R. Thermal expansion behavior of single-crystal zirconia. J Am Ceram Soc 1985;68: C228–31. Kollie TG. Measurement of the thermal-expansion coefficient of nickel from 300 to 1000 K and determination of the power-law constants near the Curie temperature. Phys Rev B 1977;16:4872–81. Tucker M, Lau G, Jacobson C, DeJonghe L, Visco S. Performance of metal-supported SOFCs with infiltrated electrodes. J Power Sources 2007;171:477–82. Cho H, Choi G. Fabrication and characterization of Nisupported solid oxide fuel cell. Solid State Ionics 2009;180: 792–5. Lee C, Bae J. Fabrication and characterization of metalsupported solid oxide fuel cells. J Power Sources 2008;176: 62–9. Brandon N, Blake A, Corcoran D, Cumming D, Duckett A, ElKoury K, et al. Development of metal supported solid oxide fuel cells for operation at 500–600 degrees C. J Fuel Cell Sci Tech 2004;1:61–5. Yonghong Kong. The oxidation behavior of Fe–Cr–xMn alloys, and preparation and characterization of Ni-supported SOFCs, Thesis for Master of Engineering, Huazhong University of Science and Technology; 2009. Malzbender J, Steinbrech RW. Advanced measurement techniques to characterize thermo-mechanical aspects of solid oxide fuel cells. J Power Sources 2007;173:60–7.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
A cost-effective process for fabrication of metal-supported solid oxide fuel cells Yonghong Kong, Bin Hua, Jian Pu*, Bo Chi, Li Jian School of Materials Science and Engineering, State Key Laboratory of Materials Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
article info
abstract
Article history:
Metal-supported SOFC cells with Y2O3 stabilized ZrO2 as the electrolyte were prepared by
Received 16 September 2009
a low cost and simple process involving tape casting, screen printing and co-firing. The
Received in revised form
interfaces were well bonded after the reduction of NiO to Ni in the support and the anode.
19 February 2010
AC impedance was employed to estimate the cell polarizations under open circuit condi-
Accepted 20 February 2010
tions. It was found that the electrode polarization resistance was high at low temperatures
Available online 23 March 2010
and became equivalent to the ohmic resistance at higher temperatures near 800 C. The cell performance was evaluated with H2 as the fuel and air as the oxidant, and maximum
Keywords:
power density between 0.23 and 0.80 W/cm2 was achieved in the temperature range of
Solid oxide fuel cell
650–800 C, which confirms the applicability of the cost-effective process in fabrication of
Metal support
metal-supported SOFC cells.
Tape casting
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Co-firing
1.
Introduction
Solid oxide fuel cells (SOFCs) are a highly efficient energy conversion system, directly converting the chemical energy of fossil fuels into electricity at a low level of pollutant emission without combustion and mechanical processes. In the last decade, significant progresses have been made in reduction of SOFC operating temperatures from near 1000 C to an intermediate temperature range of 600–800 C; as a result, the use of metallic components in SOFCs becomes possible with the advantages of easy fabrication, high thermal and electrical conductivity, low material cost, and robust mechanical property. Metal-supported SOFCs are one of the examples showing the application of metallic components in SOFCs.
It is noted that depositing porous electrodes/dense electrolyte (the cell assembly) on porous metal matrix is challenging; various techniques have been explored to fabricate the metal-supported cells. For placing the cell assembly directly on top of the porous metal matrix prepared by powder metallurgy techniques, there are few low temperature technologies that can be considered, such as pulsed laser deposition [1], plasma spraying [2,3], suspension plasma spraying [4], vacuum plasma spraying [5] and colloidal infiltration or spraying [6] with some costly equipment involved. Firing in reducing atmosphere sometimes is unavoidable due to the oxidation of the metal component [6,7]. In such case, diffusion barrier is required to prevent inter-diffusion between the metal support and the electrode [8]. An alternative approach is
* Corresponding author. Tel./fax: þ86 27 87558142. E-mail address: [email protected] (J. Pu). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.106
international journal of hydrogen energy 35 (2010) 4592–4596
to generate the cell assembly on a metal oxide plate, then reduce the metal oxide to form the porous metal support; and the cell assembly is placed in position by pulse laser deposition [9,10]. In the present study, for the consideration of cost, a different approach was taken to fabricate the metal-supported SOFC. NiO was used as the starting material for the support. NiO tapes were prepared by tape casting, on which a multilayer structure of functional anode, electrolyte and cathode was built consecutively by paste screen printing, and finally the green cell was cofired in air. During the start-up of the test, NiO was reduced in reducing atmosphere to convert NiO to Ni. Such cost effectively manufactured cells were subjected to microstructure characterization and performance evaluation. This cell fabrication method involves only conventional ceramic processes, which enables co-firing of the cell components and is suitable for mass production at low cost.
power (I–V–P) curves were obtained using a Solartron 1480A at a sweep rate of 4 mV s1 from OCV to 0.4 V. In order to understand the microstructure of the prepared cells, out-of-cell reduction test was conducted at 750 C for 9 h in an atmosphere of 4% H2 þ 96% N2 (vol.%) gas mixture with a flow rate of 200 mL/min, using a cell without the cathode. The porosity change of the support with reduction was evaluated by the Archimedes method. The microstructure of the reduced cell was examined by using a scanning electron microscopy (SEM, Quanta 200, FEI corporation, Holand). And the phases were determined by X-ray diffraction (XRD, X’Pert PRO, PANalytical B.V. Corporation, Holand) with Cu Ka radiation under the conditions of 40 kV and 40 mA. The scanning rate was 15 min1, and the 2q angle ranged from 20 to 80 .
3. 2.
Experimental
2.1.
Cell preparation
NiO powder (average particle size w;1 mm, Chengdu Haite Advanced Materials Co., Ltd, China) with a small amount of carbon black as pore former was firstly ball milled at room temperature for 24 h in an organic mixture of xylene and ethanol with Menhaden fish oil (Richard E. Mistler, Inc, USA) as the dispersant. Soon after addition of the organic binder and plasticizer (Solutia Inc, USA), another ball mill of 24 h was followed. The ball-milled slurry of NiO was degassed in moderate vacuum, and tape cast and dried in air to form flexible green tape with a thickness of about 0.9 mm. Discs with a diameter of 26 mm were cut from the cast tape, onto which the functional anode, electrolyte and cathode were subsequently paste screen printed in sequence. The functional anode was consisted of NiO (average particle size w1 mm, Inco, USA) and 8% yttria-stabilized zirconia (YSZ, TZ-8Y of Tosoh Corporation, Japan) in a mass ratio of 64:36 and the electrolyte was made of the YSZ. The material for the cathode was a proprietary composite based on the YSZ with small amounts of precious metal. Such prepared cells were debindered and fired at 1450 C in air for 2 h. The heating rate was 1 C min1. The sintered dimensions of the YSZ electrolyte and the cathode were 14 and 8 mm, respectively. The active area of the cell was 0.5 cm2.
2.2.
4593
Results and discussion
The sintered NiO support was relatively dense, achieved about 92% of the theoretic density of NiO after being co-fired with other cell components. After 9 h reduction in the reducing gas, the porosity reached around 26%, close to that presented in porous SS430 and HastelloyX supports [11] but slightly lower than the conventionally expected value of 30–40%. The reduction started from the exposed surface of the NiO support and penetrated progressively to the functional anode layer. Fig. 1 shows the XRD pattern of the reduced cell, obtained from the side of the functional anode layer covered by dense electrolyte. It is noted that the predominant phases were YSZ and Ni; and the intensity of diffraction peaks generated from the unreduced NiO was relatively weak after 9 h reduction. This suggests that the NiO support will be converted to Ni and the NiO–YSZ functional anode converted to Ni–YSZ during the start-up of cell operation. The SEM micrograph of the reduced cell is shown in Fig. 2. All the interfaces between the cell components were well bonded, no crack was observed. Although the coefficient of thermal expansion (CTE) of the YSZ (10.9 106 K1 [12]) and Ni (15–18 106 K1 [13]) is somewhat mismatched, the adherence between the Ni–YSZ
Cell characterization and evaluation
The cell was evaluated electrochemically using a home-made setup described in Refs. [1,3], with ceramic felt and mica sheet as the seal of anode and cathode side, respectively. Pt meshes were placed on both the cathode and anode as the current collector, and Pt wires were attached to the mesh for applying the electrical current and measuring the voltage. The electrochemical measurement was carried out at 650, 700, 750 and 800 C, respectively, with humidified H2 as the fuel and air as the oxidant, and the cell impedance was acquired using a potentiostat/galvanostat (Solartron 1480A) coupled with a 1260 frequency response analyzer. The current–voltage–
Fig. 1 – X-ray diffraction pattern of the cell reduced at 750 C in 4 vol.%H2–N2 gas for 9 h.
4594
international journal of hydrogen energy 35 (2010) 4592–4596
Fig. 4 – AC impedance spectra of the Ni-supported cell under open circuit condition over a temperature range between 650 and 800 C.
Fig. 2 – Cross-sectional SEM micrograph of the reduced cell, showing thin electrolyte and electrodes on porous Ni substrate.
functional anode and the Ni support was well maintained. Since the cell was graded in terms of Ni from the substrate to the electrolyte with a functional anode of Ni–YSZ cermet in between, the CTE mismatch between the Ni support and the cell assembly would be minimized even for large size cells. The yielding of the ‘‘soft’’ Ni is expected to be able to compensate the strain caused by the CTE mismatch. It can also be seen from Fig. 2 that the YSZ electrolyte was dense with only few observed isolated holes; and both the electrolyte and the functional anode were thin, in between 5 and 10 mm.
Fig. 3 – I–V and I–P curves of metal-supported cell tested at various temperatures between 650 and 800 C with humidified H2 as the fuel and air as the oxidant.
Fig. 3 presents the current–voltage (I–V) and current–power (I–P) curves of the Ni-supported cell in the temperature range between 700 and 800 C. The open circuit voltage (OCV) is above 1.0 V; minor gas leakage might have occur during the cell test so that the OCV was not as high as expected. The maximum power density of the cell was in the range of 0.23– 0.80 W/cm2 at temperatures between 650 and 800 C, which is significantly higher than that reported in Refs. [14–16] where similar cell component materials were used. Fig. 4 is the AC impedance spectra of the Ni-supported cell under open circuit condition at various temperatures between 650 and 800 C; and accordingly the ohmic resistance of the cell Ro and the electrode polarization resistance Rp can be obtained. The Ro is represented by the low frequency intercept of the spectrum at the real axis, and Rp is determined by the difference between the high and low frequency intercepts. The temperature dependence of the resistances is shown in Fig. 5. The Ro decreased from 0.45 to 0.22 U cm2 and the RP decreased from 3.09 to 0.28 U cm2 as temperature increased from 650 to 800 C. The ohmic resistance is mainly contributed by the YSZ used in the cell, which has a lower conductivity at low temperatures.
Fig. 5 – Temperature dependence of ohmic and electrode polarization resistances of the metal-supported cell between 650 and 800 8C.
international journal of hydrogen energy 35 (2010) 4592–4596
4595
Fig. 6 – Effect of pore former on sintering of the NiO support and the YSZ electrolyte printed on the support: (a) without and (b) with 3 wt% addition of the carbon black in the substrate.
It is expected that if electrolyte materials with higher ionic conductivity, such as Gd-doped ceria (GDC) or Sr- and Mgdoped lanthanum gallate (LSGM) are used, the ohmic loss will be greatly decreased, and in turn the cell performance will be enhanced. Brandon et al. [17] have fabricated metalsupported SOFC with GDC as the electrolyte, and a power density of 310 mW cm2 was obtained at 600 C with an Ro as low as 0.08 U cm2. Ishihara et al. [10] achieved a maximum power density of 400 mW/cm2 at 500 C with a metalsupported cell using LSGM electrolyte. Compared with the Rp, the contribution of Ro to the total cell ohmic resistance is less important at low temperatures, and gradually becomes equivalent to the Rp as temperature increases. The Rp contains the polarization resistances from both the anode and cathode, and the cathode polarization is orders of magnitude higher than that of the anode. Therefore, for further improving the cell performance, high conductive electrolyte and low polarization cathode are demanded. From the above results, it is considered that the process that was used to prepare the Ni-supported cell is promising due to its simple nature and low cost. The key challenge to this process is the densification of the electrolyte layer during cofiring, while the electrodes remain porous. Fig. 6 [18] shows the effect of pore former on the sintering of the NiO substrate (originally 26 mm in diameter) and the YSZ electrolyte printed on the substrate (originally 18 mm in diameter) at various temperatures in air for 2 h. Without adding a pore former, as shown in Fig. 6a, the substrate started to shrink at temperature around 700 C and a total shrinkage of 15% was achieved after sintered at 1450 C; whereas the YSZ started to shrink at temperature around 800 C and the shrinkage of the specimen sintered at 1450 C was only 12%. However, with the addition of 3 wt% of carbon black as the pore former of the NiO substrate, both the substrate and electrolyte shrank at almost the same rate as temperature increased and reached a total shrinkage of 21 and 20% at 1450 C (Fig. 6b), respectively. NiO is easier to be sintered than the YSZ, and the densified NiO support at lower temperatures is likely to prevent the YSZ electrolyte layer from being fully sintered. Therefore, it is helpful to use pore formers in the slurry of NiO, such as corn starch or carbon black, to synchronize the shrinkage of the NiO substrate and the YSZ electrolyte during the co-firing. It is also realized that using Ni as the substrate for metalsupported SOFCs may have several other potential issues,
such as the redox capability, the strength and the microstructure stability of the support. The redox capability is always an issue for Ni in SOFCs, no matter it is used in the support or in the anode. The formation of cracks in YSZ electrolyte and micro-cracks in Ni-YSZ anode at the interface between Ni and YSZ was observed as a result of severe redox cycles [19]. As a matter of fact, compared to the conventional Ni–YSZ anode supported cell, pure Ni as a cell support would not cause more significant crack formation in the cell during redox as there are no extra interfaces inside the support. The lack of strength and continuous sintering of the Ni support at the operation temperature should be taken into consideration in future studies.
4.
Conclusions
From this study, the following conclusions can be made: (1) Simple process of tape casting, screen printing and cofiring can be successfully used to fabricate Ni-supported SOFC cells at low cost, achieving a power density between 0.23 and 0.80 W/cm2 in the temperature range of 650–800 C. (2) The coefficient of thermal expansion mismatch between the porous Ni support and the cell assembly is not likely to cause delamination at the interface (at least not in small cells such as the one used in this study). (3) Electrode polarization resistance is the primary component of the total cell resistance at all temperatures, and at high temperatures close to 800 C, the ohmic and polarization losses are equivalent.
Acknowledgements This work was financially supported by the National 863 project under the contract 2006AA03Z227. The authors acknowledge Wei Qu and Cyrille Dece`s-Petit of National Research Council Canada for their assistance in cell testing, and the Analytical and Testing Center of Huazhong University of Science and Technology for SEM and XRD characterizations.
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