- Email: [email protected]
Silver modified cathode for a micro-tubular, single-chamber solid oxide fuel cell
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Silver modified cathode for a micro-tubular, single-chamber solid oxide fuel cell Naveed Akhtar*, Kevin Kendall Department of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK
article info
abstract
Article history:
Micro-tubular, solid oxide fuel cells consisting of nickel, yttria-stabilized zirconia (NieYSZ)
Received 30 July 2010
anode, yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium cobaltite fer-
Received in revised form
riteegadolinium doped ceria (LSCFeGDC) cathode have been developed and operated
26 September 2010
under single-chamber conditions, utilizing methane/air mixture. The cell performance was
Accepted 11 October 2010
compared with a silver modified cathode by the addition of 10wt% silver-paste in
Available online 16 November 2010
LSCFeGDC cathode. The cells with and without silver addition yielded maximum power densities of 118.75 mW cm2 and 61.53 mW cm2 at 700 C, respectively. The results
Keywords:
demonstrate that silver is a good candidate for enhancing the oxidation reduction kinetics
Solid oxide fuel cell
via improved adsorption, desorption, dissociation and subsequent diffusion. However, long
Single-chamber
term performance of the silver modified cathode is not guaranteed under single-chamber
Silver
conditions.
Structural
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Micro-tubular
1.
Introduction
In recent years there has been an increasing interest in the research and development of single-chamber, solid oxide fuel cells (SC-SOFCs) [1e4]. Though, SC-SOFCs are an ideal candidate for portable type applications where normally low power densities are demanded, they offer compact design due to elimination of gas-separating channels and are very simple to fabricate [2,3]. Furthermore, gas-tight sealing is not required because the reactants are already pre-mixed. Besides these advantages, they still have to suffer many challenges such as, low fuel-utilization/electrical efficiencies, high catalytic/electro-catalytic activity and selectivity of the electrodes for their respective reactions (e.g. anode must maintain high activity for partial oxidation followed by electrochemical oxidation of
the produced species such as carbon monoxide and hydrogen; cathode must be inert towards fuel combustion). Moreover, all of the components (i.e. the anode, cathode, electrolyte and interconnects) must be durable enough to work properly in a dual-atmospheric environment. Not only this, the extremely low power densities of SC-SOFCs put further restrictions on their use, as such, at present they are suitable for miniaturized portable power generations, only [3,5e12]. The purpose of this study is to focus on one of the above listed challenges, i.e. to suggest possible ways to improve the power densities of SC-SOFCs. In this respect, cathode side improvement was opted due to the well-established reason that the cathode overpotential accounts for the major power loss in a solid oxide fuel cell (SOFC) operation [13]. The state of the art, conventional cathode (i.e. lanthanum strontium
* Corresponding author. Present address: Department of Chemical Engineering and Chemistry, Technical University Eindhoven, 5600 MB, Eindhoven, The Netherlands. E-mail address: [email protected] (N. Akhtar). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.043
774
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
manganate, LSM) is limited to high-temperature applications (>800 C), at which an acceptable performance can be realized. Also, being as an electronic conductor, the active region in LSM electrode is limited to the cathodeeelectrolyte interface. In recent years, there has been a strong motivation to reduce the operating temperature of SOFCs, and therefore “Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs)” are gaining more and more interest. For IT-SOFC applications, mixed ionic electronic conductors (MIEC) are considered to be an ideal candidate because of their extended triple phase boundaries (TPBs). Among various MIECs for cathode applications, lanthanum strontium cobaltite ferriteegadolinium doped ceria (LSCFeGDC) composite cathode has been widely used by many researchers [7,10,13]. It should be noted that the addition of GDC into LSCF to form a composite cathode is beneficial in reducing the polarization resistance of pure LSCF, especially at low temperatures (<650 C) [13]. In order to further promote the oxygen surface exchange kinetics, various additives have been previously reported such as palladium (Pd), platinum (Pt) and silver (Ag) etc. [1,14]. Uchida et al. [15] reported that the addition of small nmsize Pt catalyst in lanthanum strontium cobaltite (LSC) based cathode of a conventional SOFC improved the power density compared to non-Pt, LSC cathode. Sasaki et al. [16] investigated the cermetting effect of LSCF with Pt in order to develop a reduced temperature SOFC. They reported that the obtained cathode activity was far from satisfactory, mainly due to reduced activation overpotential of the modified cathode. With an objective of enhancing the catalytic activity of the LSMeYSZ cathode and thus to improve the electrochemical performance of a conventional SOFC, Hannappel et al. [17] added Pt, Pd and Ag to the cathode. They found that the addition of Pt did not result in any positive affect on the electrochemical performance. With respect to Pd containing cells, the only improvement in the electrochemical performance was seen with Pd on activated carbon. They attributed this performance enhancement due to enhanced cathode surface reactions. Cells prepared with Ag powder and Agoxide showed an improved electrochemical performance compared to Ag-free cells sintered at the same temperature. This shows that Ag is an electro-catalytically active element. Hwang et al. [18] studied the effect of Pt addition in LSCF cathode and found to be very effective in reducing the polarization resistance of the LSCF cathode. Liang et al. [19] prepared a novel nano-structured PdeYSZ cathode for conventional IT-SOFC. Nano-sized Pd particles were impregnated onto the porous YSZ structure. Their results show that both the polarization resistance and the activation energy of such cathode are significantly lower than those for conventional perovskite-based cathodes. The addition of silver in LSCF [20], LSCFeGDC [21] and LSCFeSDC [22] cathodes has also been investigated and an improved performance has been reported. It was found that the main role of silver in performance enhancement is to improve the oxygen exchange reaction activity. As inferred from above, the addition of Pd, Pt and Ag in the conventional SOFC cathodes show a similar effect in improving the electrochemical performance by reducing the polarization resistance and activation energy. The use of such noble metals in SC-SOFC may result in different outcome. For
example, Pd and Pt have been reported to be active for methane oxidation [14]. The use of such additives may enhance parasitic loss on the cathode side of an SC-SOFC by consuming methane, which will result in lower electrical efficiency. The silver, on the other hand, is less reactive for methane oxidation and also cheaper than Pd and Pt. Therefore, Ag is opted as an additive for cathode in this study in order to investigate its role in performance enhancement. In this study two different composite cathodes, i.e. LSCFeGDC and LSCFeGDCeAg have been employed and their performance has been studied under single-chamber conditions.
2.
Experimental
The micro-tubular cells consist of NieYSZ/YSZ, anode/electrolyte layers were purchased from Adaptive Materials Inc. (AMI), USA. The LSCFeGDC composite cathode paste was purchased from Fuel Cell Materials, USA. Two different cells with different cathode layers (1st cell: LSCFeGDC, 2nd cell: LSCFeGDCeAg (10 wt %)) of approximately same thicknesses (50 micron) were paint brush and sintered at 1000 C (1st cell) and 850 C (2nd cell), respectively. The sintering temperature for cell-2 has to be kept lower because of silver densification, agglomeration and vaporization at temperatures higher than this [14]. Silver-paste and current collecting wires (99.99% silver, Scientific Wire, UK) were used for both electrodes. The cells were operated using methane/air mixture of 25/ 60 mL min1. The geometry dimensions have been described in our earlier publication [1]. Fig. 1 shows the experimental setup used in this study.
3.
Results and discussion
Fig. 2a and b shows the scanning electron microscope (SEM) photograph of two sintered cells containing different cathodes before operation. As can be seen all three layers (i.e. the anode, electrolyte and cathode) are well adhered in both of the cells. As can be seen from Fig. 2a, the dense microstructure of LSCFeGDC cathode is due to higher sintering temperature (1000 C). However, a slightly greater porosity has been observed in case of LSCFeGDCeAg cathode (Fig. 2b) because of lower sintering temperature (850 C). Fig. 2c and d shows the magnified view of Fig. 2a and b, respectively. Fig. 3a shows the IeV and IeP characteristics of LSCFeGDC cathode based cell. As can be seen, the cell performance improves with increase in temperature until 700 C, afterwards, the open circuit voltage (OCV) drops toe0.8V. This drop in OCV can be associated with lower Gibbs free energy with increasing temperature and methane parasitic combustion at the cathode consuming more oxygen at higher temperatures. The reduced oxygen partial pressure at this higher temperature may result in lower OCV and thereby in performance. Nevertheless, the highest power density of 61.53 mW cm2 is obtained at 700 C as indicated by red curve in Fig. 3a. In Fig. 3b, the IeV and IeP characteristic curves for LSCFeGDCeAg cathode based cell have been shown. As can be seen, addition of silver greatly improves the power density and
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
775
Fig. 1 e Experimental setup.
the maximum power density is found to be 118.4 mW cm2, almost twice than that obtained without silver added cathode at the same operating temperature of 700 C. This result clearly indicates that the addition of silver in the cathode improves the oxygen exchange kinetics and thereby lowering the cathode overpotential.
Although, the addition of silver in the cathode improves the cell performance, it will worth to conduct a short-term degradation test in order to comment on its stability. Therefore, the cell performance was tested over a period ofe10 h for both of the above mentioned cathodes. The degradation test was run at 700 C and 0.5V and the corresponding results are
Fig. 2 e SEM photograph with anode/electrolyte/cathode assembly of (a) NieYSZ/YSZ/LSCFeGDC based cathode; (b) cell with NieYSZ/YSZ/LSCFeGDCeAg based cathode before operation; (c) magnified view of Fig. 2a; (d) magnified view of Fig. 2b.
776
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
Fig. 3 e (a) IeV and IeP curves for (a) LSCF-GDC cathode based cell; (b) LSCFeGDCeAg cathode based cell.
shown in Fig. 4. As can be seen, the cell with LSCDeGDC cathode had fairly stable performance, however, the cell having LSCFeGDCeAg cathode was degraded at an average rate of w5.16%/hr. In order to investigate the cause of the degradation of silver modified cathode, a scanning electron microscop (SEM) analysis has been carried out for both of the cells. Fig. 5a and b shows the post-SEM analysis of with and without silver modified cathodes, respectively, after 10 h of continuous operation at 700 C. As can be seen from Fig. 5a that the
Fig. 5 e SEM photograph with anode/electrolyte/cathode assembly of (a) NieYSZ/YSZ/LSCFeGDC; (b) NieYSZ/YSZ/ LSCFeGDCeAg; (c) magnified view of marked region in Fig. 5b after 10 h of continuous operation.
Fig. 4 e A short-term degradation test for LSCFeGDC and LSCFeGDCeAg cathode based cells (at 700 C and 0.5V).
microstructure of LSCFeGDC cathode is relatively unchanged. However, the presence of silver in LSCFeGDCeAg cathode caused severe inter-diffusion/melting of Ag, thereby blocking the cathode-electrolyte interface (see marked region of Fig. 5b as magnified in Fig. 5c). This could probably result in poor performance because of extensive interfacial polarization and barrier against oxygen ion diffusion through the cathodeeelectrolyte boundary. Although the operating temperature (700 C) is fairly lower than the silver melting point
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
777
references
Fig. 6 e EDX photograph confirming the presence of silver near the electrolyte region for LSCFeGDCeAg cathode.
(960 C), the local heat production in case of SC-SOFC operation has significantly contributed to this effect. In order to confirm the presence of silver that could be covering the electrolyte, an energy dispersive X-ray (EDX) analysis has been performed. A spectrum of the electrolyte segment near the active region is analysed and the corresponding elemental distribution has been shown in Fig. 6. As can be seen, the peaks of silver have been observed indicating the presence of silver near the electrolyte region. Therefore, it can be concluded that the silver could have been contributed to the degradation of LSCFeGDCeAg cathode.
4.
Conclusions
This study shows that silver is a good catalyst for improving oxygen adsorption, desorption, dissociation and diffusion characteristics which could help in lowering the cathode overpotential, thereby greatly enhancing the power densities of solid oxide fuel cells. However, silver has a major drawback due to its vaporization, melting and inter-diffusion during high-temperature fuel cell operation. Not only this, this drawback is more pronounced in single-chamber, solid oxide fuel cell operation because of cell temperatures higher than the operating furnace temperatures.
Acknowledgments The authors would like to thank E.ON-UK and EPSRC-UK for funding Mr. Naveed Akhtar through Dorothy Hodgkin Postgraduate Award (DHPA) scheme.
[1] Akhtar N, Decent SP, Loghin D, Kendall K. Mixed-reactant, micro-tubular solid oxide fuel cell: an experimental study. J Power Sources 2009;193:39e48. [2] Hibino T, Ushiki K, Kuwahara Y. New concept for simplifying SOFC system. Solid State Ionics 1996;91:69e74. [3] Napporn TW, Bedard XJ, Morin F, Meunier M. Operating conditions of a single-chamber SOFC. J Electrochem Soc 2004; 151:2088e94. [4] Akhtar N, Decent SP, Kendall K. Cell temperature measurements in micro-tubular, single-chamber, solid oxide fuel cells (MT-SC-SOFCs). J Power Sources doi:10.1016/j. jpowsour.2009.04.078. [5] Bedard XJ, Napporn TW, Roberge R, Meunier M. Performance and ageing of an anode-supported SOFC operated in single-chamber conditions. J Power Sources 2006;153:108e13. [6] Yano M, Tomita A, Sano M, Hibino T. Recent advances in single-chamber solid oxide fuel cells: a review. Solid State Ionics 2007;177:3351e9. [7] Suzuki T, Jasinski P, Anderson HU, Dogan F. Role of composite cathodes in single-chamber SOFC. J Electrochem Soc 2004;151:1678e82. [8] Shao Z, Mederos J, Chueh WC, Haile SM. High power-density single-chamber fuel cells operated on methane. J Power Sources 2006;162:589e96. [9] Riess I. Significance of impeded reactions in solid state electrochemistry-Conspicuous examples. Solid State Ionics 2006;177:1591e6. [10] Suzuki T, Jasinski P, Anderson HU, Dogan F. Single-chamber electrolyte supported SOFC module. Electrochem Solid State Lett 2004;7:391e3. [11] Stefan IC, Jacobson CP, Visco SJ, Jonghe LCD. Singlechamber fuel cells: flow geometry, rate and composition considerations. Electrochem Solid State Lett 2004;7: 198e200. [12] Tomita A, Hirabayashi D, Hibino T, Nagao M, Sano M. Singlechamber SOFCs with a Ce0.9Gd0.1O1.95 electrolyte film for lowtemperature operation. Electrochem Solid State Lett 2005;8: 63e5. [13] Suzuki T, Jasinski P, Petrovsky V, Anderson HU, Dogan F. Performance of a porous electrolyte in single-chamber SOFCs. J Electrochem Soc 2005;152:527e31. [14] Sasaki K, Hosoda K, Lan TN, Yasumoto K, Wang S, Dokiya M. AgeZr(Sc)O2 cermet cathode for reduced temperature SOFCs. Solid State Ionics 2004;174:97e102. [15] Uchida H, Arisaka S, Watanabe M. High performance electrodes for medium-temperature solid oxide fuel cells: activation of La(Sr)CoO3 cathode with highly dispersed Pt metal electrocatalysts. Solid State Ionics 2000;135: 347e51. [16] Sasaki K, Tamura J, Hosoda H, Lan TN, Yasumoto K, Dokiya M. Pteperovskite cermet cathode for reducedtemperature SOFCs. Solid State Ionics 2002;148:551e5. [17] Haanappel VAC, Rutenbeck D, Mai A, Uhlenbruck S, Sebold D, Wesemeyer H, et al. The influence of noble-metal-containing cathodes on the electrochemical performance of anode-supported SOFCs. J Power Sources 2004;130:119e28. [18] Hwang HJ, Moon JW, Lee S, Lee EA. Electrochemical performance of LSCF-based composite cathodes for intermediate temperature SOFCs. J Power Sources 2005;145:243e8. [19] Liang F, Chen J, Cheng J, Jiang SP, He T, Pu J, et al. Novel nanostructured Pd þ yttrium doped ZrO2 cathodes for intermediate temperature solid oxide fuel cells. Electrochem Commun 2008;10:42e6.
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[20] Simner SP, Anderson MD, Coleman JE, Stevenson JW. Performance of a novel La(Sr)Fe(Co)O3eAg SOFC cathode. J Power Sources 2006;161:115e22. [21] Wang S, Kato T, Nagata S, Honda T, Kaneko T, Iwashita N, et al. Performance of a La0.6Sr0.4Co0.8Fe0.2O3eCe0.8Gd0.2O1.9eAg
cathode for ceria electrolyte SOFCs. Solid State Ionics 2002; 146:203e10. [22] Zhang J, Ji Y, Gao H, He T, Liu J. Composite cathode La0.6Sr0.4Co0. 2Fe0.8O3eSm0.1Ce0.9O1.95eAg for intermediate-temperature solid oxide fuel cells. J Alloys Compd 2005;395:322e5.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Silver modified cathode for a micro-tubular, single-chamber solid oxide fuel cell Naveed Akhtar*, Kevin Kendall Department of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK
article info
abstract
Article history:
Micro-tubular, solid oxide fuel cells consisting of nickel, yttria-stabilized zirconia (NieYSZ)
Received 30 July 2010
anode, yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium cobaltite fer-
Received in revised form
riteegadolinium doped ceria (LSCFeGDC) cathode have been developed and operated
26 September 2010
under single-chamber conditions, utilizing methane/air mixture. The cell performance was
Accepted 11 October 2010
compared with a silver modified cathode by the addition of 10wt% silver-paste in
Available online 16 November 2010
LSCFeGDC cathode. The cells with and without silver addition yielded maximum power densities of 118.75 mW cm2 and 61.53 mW cm2 at 700 C, respectively. The results
Keywords:
demonstrate that silver is a good candidate for enhancing the oxidation reduction kinetics
Solid oxide fuel cell
via improved adsorption, desorption, dissociation and subsequent diffusion. However, long
Single-chamber
term performance of the silver modified cathode is not guaranteed under single-chamber
Silver
conditions.
Structural
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Micro-tubular
1.
Introduction
In recent years there has been an increasing interest in the research and development of single-chamber, solid oxide fuel cells (SC-SOFCs) [1e4]. Though, SC-SOFCs are an ideal candidate for portable type applications where normally low power densities are demanded, they offer compact design due to elimination of gas-separating channels and are very simple to fabricate [2,3]. Furthermore, gas-tight sealing is not required because the reactants are already pre-mixed. Besides these advantages, they still have to suffer many challenges such as, low fuel-utilization/electrical efficiencies, high catalytic/electro-catalytic activity and selectivity of the electrodes for their respective reactions (e.g. anode must maintain high activity for partial oxidation followed by electrochemical oxidation of
the produced species such as carbon monoxide and hydrogen; cathode must be inert towards fuel combustion). Moreover, all of the components (i.e. the anode, cathode, electrolyte and interconnects) must be durable enough to work properly in a dual-atmospheric environment. Not only this, the extremely low power densities of SC-SOFCs put further restrictions on their use, as such, at present they are suitable for miniaturized portable power generations, only [3,5e12]. The purpose of this study is to focus on one of the above listed challenges, i.e. to suggest possible ways to improve the power densities of SC-SOFCs. In this respect, cathode side improvement was opted due to the well-established reason that the cathode overpotential accounts for the major power loss in a solid oxide fuel cell (SOFC) operation [13]. The state of the art, conventional cathode (i.e. lanthanum strontium
* Corresponding author. Present address: Department of Chemical Engineering and Chemistry, Technical University Eindhoven, 5600 MB, Eindhoven, The Netherlands. E-mail address: [email protected] (N. Akhtar). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.043
774
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
manganate, LSM) is limited to high-temperature applications (>800 C), at which an acceptable performance can be realized. Also, being as an electronic conductor, the active region in LSM electrode is limited to the cathodeeelectrolyte interface. In recent years, there has been a strong motivation to reduce the operating temperature of SOFCs, and therefore “Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs)” are gaining more and more interest. For IT-SOFC applications, mixed ionic electronic conductors (MIEC) are considered to be an ideal candidate because of their extended triple phase boundaries (TPBs). Among various MIECs for cathode applications, lanthanum strontium cobaltite ferriteegadolinium doped ceria (LSCFeGDC) composite cathode has been widely used by many researchers [7,10,13]. It should be noted that the addition of GDC into LSCF to form a composite cathode is beneficial in reducing the polarization resistance of pure LSCF, especially at low temperatures (<650 C) [13]. In order to further promote the oxygen surface exchange kinetics, various additives have been previously reported such as palladium (Pd), platinum (Pt) and silver (Ag) etc. [1,14]. Uchida et al. [15] reported that the addition of small nmsize Pt catalyst in lanthanum strontium cobaltite (LSC) based cathode of a conventional SOFC improved the power density compared to non-Pt, LSC cathode. Sasaki et al. [16] investigated the cermetting effect of LSCF with Pt in order to develop a reduced temperature SOFC. They reported that the obtained cathode activity was far from satisfactory, mainly due to reduced activation overpotential of the modified cathode. With an objective of enhancing the catalytic activity of the LSMeYSZ cathode and thus to improve the electrochemical performance of a conventional SOFC, Hannappel et al. [17] added Pt, Pd and Ag to the cathode. They found that the addition of Pt did not result in any positive affect on the electrochemical performance. With respect to Pd containing cells, the only improvement in the electrochemical performance was seen with Pd on activated carbon. They attributed this performance enhancement due to enhanced cathode surface reactions. Cells prepared with Ag powder and Agoxide showed an improved electrochemical performance compared to Ag-free cells sintered at the same temperature. This shows that Ag is an electro-catalytically active element. Hwang et al. [18] studied the effect of Pt addition in LSCF cathode and found to be very effective in reducing the polarization resistance of the LSCF cathode. Liang et al. [19] prepared a novel nano-structured PdeYSZ cathode for conventional IT-SOFC. Nano-sized Pd particles were impregnated onto the porous YSZ structure. Their results show that both the polarization resistance and the activation energy of such cathode are significantly lower than those for conventional perovskite-based cathodes. The addition of silver in LSCF [20], LSCFeGDC [21] and LSCFeSDC [22] cathodes has also been investigated and an improved performance has been reported. It was found that the main role of silver in performance enhancement is to improve the oxygen exchange reaction activity. As inferred from above, the addition of Pd, Pt and Ag in the conventional SOFC cathodes show a similar effect in improving the electrochemical performance by reducing the polarization resistance and activation energy. The use of such noble metals in SC-SOFC may result in different outcome. For
example, Pd and Pt have been reported to be active for methane oxidation [14]. The use of such additives may enhance parasitic loss on the cathode side of an SC-SOFC by consuming methane, which will result in lower electrical efficiency. The silver, on the other hand, is less reactive for methane oxidation and also cheaper than Pd and Pt. Therefore, Ag is opted as an additive for cathode in this study in order to investigate its role in performance enhancement. In this study two different composite cathodes, i.e. LSCFeGDC and LSCFeGDCeAg have been employed and their performance has been studied under single-chamber conditions.
2.
Experimental
The micro-tubular cells consist of NieYSZ/YSZ, anode/electrolyte layers were purchased from Adaptive Materials Inc. (AMI), USA. The LSCFeGDC composite cathode paste was purchased from Fuel Cell Materials, USA. Two different cells with different cathode layers (1st cell: LSCFeGDC, 2nd cell: LSCFeGDCeAg (10 wt %)) of approximately same thicknesses (50 micron) were paint brush and sintered at 1000 C (1st cell) and 850 C (2nd cell), respectively. The sintering temperature for cell-2 has to be kept lower because of silver densification, agglomeration and vaporization at temperatures higher than this [14]. Silver-paste and current collecting wires (99.99% silver, Scientific Wire, UK) were used for both electrodes. The cells were operated using methane/air mixture of 25/ 60 mL min1. The geometry dimensions have been described in our earlier publication [1]. Fig. 1 shows the experimental setup used in this study.
3.
Results and discussion
Fig. 2a and b shows the scanning electron microscope (SEM) photograph of two sintered cells containing different cathodes before operation. As can be seen all three layers (i.e. the anode, electrolyte and cathode) are well adhered in both of the cells. As can be seen from Fig. 2a, the dense microstructure of LSCFeGDC cathode is due to higher sintering temperature (1000 C). However, a slightly greater porosity has been observed in case of LSCFeGDCeAg cathode (Fig. 2b) because of lower sintering temperature (850 C). Fig. 2c and d shows the magnified view of Fig. 2a and b, respectively. Fig. 3a shows the IeV and IeP characteristics of LSCFeGDC cathode based cell. As can be seen, the cell performance improves with increase in temperature until 700 C, afterwards, the open circuit voltage (OCV) drops toe0.8V. This drop in OCV can be associated with lower Gibbs free energy with increasing temperature and methane parasitic combustion at the cathode consuming more oxygen at higher temperatures. The reduced oxygen partial pressure at this higher temperature may result in lower OCV and thereby in performance. Nevertheless, the highest power density of 61.53 mW cm2 is obtained at 700 C as indicated by red curve in Fig. 3a. In Fig. 3b, the IeV and IeP characteristic curves for LSCFeGDCeAg cathode based cell have been shown. As can be seen, addition of silver greatly improves the power density and
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
775
Fig. 1 e Experimental setup.
the maximum power density is found to be 118.4 mW cm2, almost twice than that obtained without silver added cathode at the same operating temperature of 700 C. This result clearly indicates that the addition of silver in the cathode improves the oxygen exchange kinetics and thereby lowering the cathode overpotential.
Although, the addition of silver in the cathode improves the cell performance, it will worth to conduct a short-term degradation test in order to comment on its stability. Therefore, the cell performance was tested over a period ofe10 h for both of the above mentioned cathodes. The degradation test was run at 700 C and 0.5V and the corresponding results are
Fig. 2 e SEM photograph with anode/electrolyte/cathode assembly of (a) NieYSZ/YSZ/LSCFeGDC based cathode; (b) cell with NieYSZ/YSZ/LSCFeGDCeAg based cathode before operation; (c) magnified view of Fig. 2a; (d) magnified view of Fig. 2b.
776
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
Fig. 3 e (a) IeV and IeP curves for (a) LSCF-GDC cathode based cell; (b) LSCFeGDCeAg cathode based cell.
shown in Fig. 4. As can be seen, the cell with LSCDeGDC cathode had fairly stable performance, however, the cell having LSCFeGDCeAg cathode was degraded at an average rate of w5.16%/hr. In order to investigate the cause of the degradation of silver modified cathode, a scanning electron microscop (SEM) analysis has been carried out for both of the cells. Fig. 5a and b shows the post-SEM analysis of with and without silver modified cathodes, respectively, after 10 h of continuous operation at 700 C. As can be seen from Fig. 5a that the
Fig. 5 e SEM photograph with anode/electrolyte/cathode assembly of (a) NieYSZ/YSZ/LSCFeGDC; (b) NieYSZ/YSZ/ LSCFeGDCeAg; (c) magnified view of marked region in Fig. 5b after 10 h of continuous operation.
Fig. 4 e A short-term degradation test for LSCFeGDC and LSCFeGDCeAg cathode based cells (at 700 C and 0.5V).
microstructure of LSCFeGDC cathode is relatively unchanged. However, the presence of silver in LSCFeGDCeAg cathode caused severe inter-diffusion/melting of Ag, thereby blocking the cathode-electrolyte interface (see marked region of Fig. 5b as magnified in Fig. 5c). This could probably result in poor performance because of extensive interfacial polarization and barrier against oxygen ion diffusion through the cathodeeelectrolyte boundary. Although the operating temperature (700 C) is fairly lower than the silver melting point
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 6 ( 2 0 1 1 ) 7 7 3 e7 7 8
777
references
Fig. 6 e EDX photograph confirming the presence of silver near the electrolyte region for LSCFeGDCeAg cathode.
(960 C), the local heat production in case of SC-SOFC operation has significantly contributed to this effect. In order to confirm the presence of silver that could be covering the electrolyte, an energy dispersive X-ray (EDX) analysis has been performed. A spectrum of the electrolyte segment near the active region is analysed and the corresponding elemental distribution has been shown in Fig. 6. As can be seen, the peaks of silver have been observed indicating the presence of silver near the electrolyte region. Therefore, it can be concluded that the silver could have been contributed to the degradation of LSCFeGDCeAg cathode.
4.
Conclusions
This study shows that silver is a good catalyst for improving oxygen adsorption, desorption, dissociation and diffusion characteristics which could help in lowering the cathode overpotential, thereby greatly enhancing the power densities of solid oxide fuel cells. However, silver has a major drawback due to its vaporization, melting and inter-diffusion during high-temperature fuel cell operation. Not only this, this drawback is more pronounced in single-chamber, solid oxide fuel cell operation because of cell temperatures higher than the operating furnace temperatures.
Acknowledgments The authors would like to thank E.ON-UK and EPSRC-UK for funding Mr. Naveed Akhtar through Dorothy Hodgkin Postgraduate Award (DHPA) scheme.
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