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Enhanced ionic conductivity in calcium doped ceria – Carbonate electrolyte: A composite effect
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 ) 1 9 4 0 1 e1 9 4 0 6
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Enhanced ionic conductivity in calcium doped ceria e Carbonate electrolyte: A composite effect Ying Ma a,*, Xiaodi Wang a, Hassan Ahmed Khalifa a, Bin Zhu b, Mamoun Muhammed a a b
Division of Functional Materials, Royal Institute of Technology (KTH), S-16440 Stockholm, Sweden Department of Energy Technology, Royal Institute of Technology (KTH), S-10044 Stockholm, Sweden
article info
abstract
Article history:
Recently, ceria-based nanocomposites, as a proton and oxygen ion conductor, has been
Received 17 May 2011
developed as promising electrolyte candidates for low-temperature solid oxide fuel cells
Received in revised form
(LTSOFCs). Up to now, samarium doped ceria (SDC) was studied as a main oxide for
20 September 2011
nanocomposite electrolyte; while calcium doped ceria (CDC) is considered as a good
Accepted 22 September 2011
alternative from both material performance and economical aspects. Yet the conduction
Available online 5 November 2011
behavior of CDC-based composite has not been reported. In the present study, calcium doped ceria was prepared by oxalate co-precipitation method, and used for the fabrication
Keywords:
of CDC/Na2CO3 composite. The thermal decomposition process, structure and morphology
Nanocomposite electrolyte
of the samples were characterized by TGA, XRD, SEM, etc. The oxygen ion conductivity of
Calcium doped ceria (CDC)
single phase CDC sample was measured by electrochemical impedance spectroscopy (EIS),
Proton conductivity
while the proton and oxygen ion conductivity of CDC/Na2CO3 nanocomposite sample were
Oxygen ion conductivity
determined by four-probe d.c. measurements. The CDC/Na2CO3 samples show significantly
Solid oxide fuel cells (SOFCs)
enhanced overall ionic conductivity compared to that of single phase CDC samples, demonstrating pronounced composite effect. This confirms that the use of nanocomposite as electrolyte can effectively lower the operation temperature of SOFC due to improved ionic conductivity. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As one of the most efficient and environmentally benign energy conversion devices, solid oxide fuel cells (SOFCs) have attracted much attention in recent years [1,2]. Conventional SOFCs with yttria-stabilized zirconia (YSZ) as electrolyte require high operation temperature (800e1000 C), which causes significant problems like material degradation [3,4], as well as other technological complications and economic barrier for wider applications. Therefore, there is a broad interest in reducing the operation temperature of SOFCs, e.g. in the range below 600 C [5]. A key requirement is to achieve
high ionic conductivity of the electrolyte at such lowtemperature. Doped ceria has been considered as a candidate electrolyte suitable in low-temperature range [6,7], besides its wide use as oxygen sensor, and three-way catalyst [8,9], etc. However, doped ceria shows electronic conductivity at lower oxygen partial pressure which results in a significant power loss, thus preventing its further commercialization [10,11]. The development of effective electrolyte materials for low-temperature SOFCs (LTSOFCs) is a grand challenge for the SOFC community. In 2001, Bin Zhu has first suggested that ceria-based composites can serve as promising electrolyte materials for
* Corresponding author. Tel.: þ46 8790 8926; fax: þ46 8790 9072. E-mail address: [email protected] (Y. Ma). 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.2011.09.122
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LTSOFC [12]. Later it has been demonstrated that enhanced ionic conductivity can be achieved in the ceria-based composite systems, resulting in superior fuel cell performances [13,14]. This composite approach was designed and fabricated based on the utilization of the interface as path for ionic conduction. In our previous reports, some new composite electrolytes with unique nanostructures, i.e. coreeshell samarium doped ceria (SDC)/Na2CO3 nanocomposite [15], and SDC nanowires based nanocomposite [16] were developed, where the interface region is dominant in the overall transport in these nanostructured composite materials. Furthermore, in our recent work [17], we demonstrated that the SDC/Na2CO3 nanocomposite electrolyte possesses a unique simultaneous Hþ/O2 conduction property, where the proton conduction mainly accounts for ionic conductivity and fuel cell performance of SDC/Na2CO3 nanocomposite. In order to develop more efficient electrolyte based on the composite approach, we considered calcium doped ceria (CDC) as a good candidate since it shows comparable high conductivity as SDC or gadolinium doped ceria (GDC) [18,19]. Besides, it has been demonstrated that CDC is a good oxygen storage material due to oxygen vacancy created by calcium doping [20]. Furthermore, calcium is much less expensive as compared to costly rare earth elements. Although there are immense published studies on SDC or GDC based composite systems and single phase CDC electrolyte [21,22], reports on CDC-based composite are rather scarce. In the present study, we herein report the preparation of CDC nanomaterials, by oxalate co-precipitation, and CDC/Na2CO3 nanocomposite. Comprehensive characterizations of the samples were conducted by means of TGA, XRD, SEM, etc. The ionic conductivity properties of both single phase CDC and CDC/Na2CO3 nanocomposite was investigated by employing impedance spectroscopy and four-probe d.c. technique.
For the synthesis of CDC composite, Na2CO3 was introduced as secondary phase. The CDC/Na2CO3 composite was prepared by a dry mixing method. The prepared CDC was mixed with Na2CO3 salt with weight ratio of CDC: Na2CO3 ¼ 4:1 according to previous optimized results in the literature. The mixture was well ground in an agate mortar, calcined at 700 C in air for 1h and taken out directly from the furnace to room temperature to form CDC/Na2CO3 composite with Na2CO3 weight content of 20%. Thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FT-IR) experiment was carried out with a heating rate of 5 C/min in synthetic air using TA TGA Q500 interfaced with Nicolet iS10 FT-IR spectrometer from Thermo Scientific. Powder X-ray diffraction (XRD) patterns of the samples were collected using Philips X’pert pro super ˚ ) for phase Diffractometer with Cu Ka radiation (l ¼ 1.5418 A analysis and crystal size calculation. Zeiss Ultra 55 fieldemission scanning electron microscope (FESEM) was used to examine the morphology and microstructure of samples. The CDC/Na2CO3 nanocomposite material was pressed under 300 MPa into pellets, followed by sintering at 680 C for 30 min. The d.c. conductivity of the nanocomposite electrolyte was measured by the four-probe technique following the same process as reported in our previous work [17]. The proton conductivity and oxygen ion conductivity of CDC/ Na2CO3 nanocomposite were determined separately under two different gas atmosphere, 5% H2 and air respectively. As a comparison, pellets of pure CDC sample were cold pressed and sintered at 1400 C for 3 h to reach a relative density of 96.2%, as measured by using Archimedes’ principle. The electrical conductivity of CDC pellet was measured by a.c. impedance method using Solartron 1287 potentiostat and Solartron 1255 Frequency Response Analyzer in the frequency range from 1 mHz to 1 MHz with an applied signal of 20 mV.
2.
3.
Experimental
Ca-doped ceria (CDC) sample with the composition of Ce0.8Ca0.2O2d was selected since it shows the highest oxygen ion conductivity as reported [18]. The Ce0.8Ca0.2O2d sample was prepared by co-precipitation method using ammonium oxalate as the precipitating agent. All the chemicals were used as received without further purification. The stock solution was made by dissolving cerium nitrate hexahydrate (Ce(NO3)3$6H2O, 99.9%, Chempur, Karlsruhe, Germany) and calcium nitrate tetrahydrate (Ca(NO3)2$4H2O, 99.9%, SigmaeAldrich) in distilled water with a molar ratio of Ce3þ:Ca2þ ¼ 4:1, and the total concentration of cation [Ce3þ] þ [Ca2þ] ¼ 0.3 M. Aqueous solutions of ammonium oxalate ((NH4)2C2O4$H2O, 99.5%, Merck) with a concentration of 0.3 M were used as the precipitating agent. In a typical procedure, 50 ml mixed salts solution was added at a rate of 5 ml min1 into 150 ml oxalate solution under vigorous stirring at room temperature where white precipitate formed instantaneously. The resultant suspension was homogenized for 1 h and then filtered. The precipitate cake was washed repeatedly with distilled water and ethanol, followed by drying at 80 C to obtain the CDC oxalate precursor. Finally, the precursor was calcined at 650 C in air for 3 h and converted to the Ce0.8Ca0.2O2d.
Results and discussion
Thermogravimetric analysis (TGA) of as-prepared oxalate precursor was conducted in air, as shown in Fig. 1. It can be seen clearly from the curve that two main weight loss processes take place in the temperature range of (i) room temperature to ca. 150 C and (ii) 150e650 C. There is no mass loss above 650 C.
Fig. 1 e TGA curve and GrameSchmidt plot of as-prepared oxalate precursor.
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 ) 1 9 4 0 1 e1 9 4 0 6
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The initial weight loss of ca. 21% to 150 C is due to loss of water. A weight loss of 46.1% (the amount of water was excluded) was obtained in the temperature range of 150e600 C, which corresponds to the conversion of Ce0.8Ca0.2(C2O4)1.8 to Ce0.8Ca0.2O1.8. The measured weight loss (46.1%) is in a good agreement with the theoretical weight loss of 46.6%. The overall calcination reaction is expressed as in Eq. (1). Ce0:8 Ca0:2 ðC2 O4 Þ1:8 þ0:9O2 /Ce0:8 Ca0:2 O1:8 þ 3:6CO2
(1)
This reaction is confirmed by the corresponding in situ GrameSchmidt plot and FT-IR spectra. The GrameSchmidt plot in Fig. 1 indicates that the thermal decomposition process of oxalate precursor can be divided into three distinct regions: 250e400 C, 400e500 C and 500e650 C, due to different decomposition steps of cerium oxalate and calcium oxalate. From separate GrameSchmidt plot of cerium oxalate and calcium oxalate, it was confirmed that the first region (250e400 C) is a result of the thermal decomposition of cerium oxalate, while the other two regions are due to the thermal decomposition of calcium oxalate. The FT-IR spectra of gases evolved at temperatures corresponding to the peaks on the GrameSchmidt plot are presented in Fig. 2. At 100 C, only H2O is detected. CO2 (2362, 2329 and 668 cm1) starts to release after 250 C, continues up to 330 C and further appears at 470 and 600 C. After 650 C, there is no major reaction is detected. Therefore, the calcination temperature of CDC precursor was determined to be 650 C to form CDC solid solution. Fig. 3 shows that XRD patterns of the sintered specimens of pure CDC and CDC/Na2CO3 composite. All the peaks of CDC specimen were indexed to the cubic fluorite-type structure CeO2 (JCPDS 34-0394) without peaks related to CaO, which means that the Ca2þ ions are completely doped into CeO2 crystal structure. The CDC phase in CDC/Na2CO3 composite sample exhibits the same fluorite-type structure diffraction peaks with broader peak widths due to smaller particle size. However, there is no peak observed which is related to Na2CO3, even as the content of Na2CO3 in the composite is 20%, indicating that Na2CO3 exists as amorphous phase in the nanocomposite, which is identical to our previous study [15]. The average crystallite size of pure CDC sample is estimated to
Fig. 2 e FT-IR spectra of the evolved gases from the oxalate precursor during TGA experiment at different temperature.
Fig. 3 e XRD patterns of sintered specimens of pure CDC and CDC/Na2CO3 composite.
be 32 nm as calculated by Scherrer equation, while the crystallite size of CDC phase in CDC/Na2CO3 composite sample is determined to be 12.4 nm. The much smaller crystallite size of CDC phase in the composite can be attributed to the presence of Na2CO3 which acts as a significant diffusion barrier, and restrains interparticle diffusion and growth of CDC particles [23] The morphology and particle size of pure CDC and CDC/ Na2CO3 composite samples were characterized by SEM and given in Fig. 4. Fig. 4a reveals that the CDC/Na2CO3 nanocomposite consists of CDC particles smaller than 100 nm and show faceted and occasionally irregular shape. A further detailed SEM image (insert of Fig. 4a) clearly indicated that CDC particles are evenly covered with homogeneous Na2CO3 coatings. As we reported earlier [23], amorphous Na2CO3 shell serves as diffusion barrier in the composite structure, the interparticle contact and growth of CDC particles is minimized. Therefore, CDC/Na2CO3 composite exhibits a good nanostructure with grain-boundary can be easily distinguished. In contrast, for the pure CDC sample (Fig. 4b), high temperature calcination leads to excessive agglomeration and grain growth, due to their large volume fraction of highenergy intercrystalline regions without diffusion barrier. Conventional electrochemical impedance spectroscopy (EIS) is useful for studying the conductivity of conventional single phase electrolyte. However, it is not suitable for studying the conductivity of composite systems, where multiple mobile charge carriers contribute to the overall measured conductivity, i.e. O2, Hþ, Naþ, and CO2 3 . Hence the contribution of the specific ions of interest in SOFC operation, i.e. O2 and Hþ, cannot be distinguished from contributions of other ions when studied by EIS. In our recent study [17], we measured the proton and oxygen ion conductivity of SDC/Na2CO3 nanocomposite electrolyte by the four-probe d.c. measurements, which gives an unequivocal conductivity measure of mobile ions under different gas atmospheres over a wide temperature range. Furthermore, from a practical point of view, d.c. measurements are more close to the actual application regime of SOFCs. Therefore, in this present study, we use the four-probe d.c. measurement to determine the O2 and Hþ conductivity of
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Fig. 4 e SEM images of (a) CDC/Na2CO3 composite and (b) pure CDC sample. The inset of (a) is a further detailed observation with a scale bar of 20 nm.
CDC/Na2CO3 nanocomposite. While for pure CDC pellets, the oxygen ion conductivity is studied by EIS method. Fig. 5 shows the total ionic conductivity of CDC/Na2CO3 nanocomposite determined by the four-probe d.c. measurements, compared to the oxygen ion conductivity of pure CDC pellet studied by EIS. It can be seen that the overall ionic conductivity of the CDC/Na2CO3 nanocomposite electrolyte, which is the sum of the proton conductivity and oxygen ion conductivity, is about one to two orders of magnitude higher than that of pure CDC in the temperature range of 200 Ce600 C. The significantly enhanced ionic conductivity of CDC/Na2CO3 electrolyte demonstrates a distinct “composite effect”, which can be ascribed to interfacial interaction. Unlike the conventional superionic conduction/transition that appears in a single phase bulk accompanying a structural change, the superionic conduction in the composites is determined by the interfaces, i.e. a process breaks symmetry of matrix by introducing a structural discontinuity [24]. In contrast with the bulk, where electroneutrality must be obeyed, at the interface a narrow charged zone, the so-called space-charge zone is tolerable and thermodynamically necessary [25]. The defect concentrations in the space-charge
zone are much higher than that in the bulk, thus interface supplies high conductivity pathways for ionic transportation and conduction. Especially in nanocomposite, this interfacial effect is dominant in overall ionic transport. Therefore, ionic conductivity of CDC/Na2CO3 nanocomposite increases significantly compared to the single phase CDC electrolyte. Fig. 6 displays the temperature dependence of conductivity of CDC/Na2CO3 nanocomposite electrolyte under two different gas atmosphere, 5% H2 and air, as determined by the four-probe d.c. measurements. Under hydrogen gas atmosphere, only proton conductivity (sHþ ) makes a significant contribution to the overall measured conductivity, while the conductivity measured under air is mainly resulting from oxygen ion conductivity (sO2 ). It is shown in Fig. 6 that sHþ of CDC/Na2CO3 nanocomposite electrolyte is much higher than sO2 with 1e2 orders of magnitude over the whole temperature range (200e600 C), which resembles the conductivity curves of SDC/Na2CO3 nanocomposite as we reported previously [17]. For example, at 600 C sHþ is approximately 0.05 S/cm while sO2 is 0.004 S/cm. The proton conductivity curve shows a change of the slope at around 400 C, corresponding to activation energy of 0.475 eV above 400 C and 1.059 eV below
Fig. 5 e Overall conductivity comparison of pure CDC oxygen ion conductor and CDC/Na2CO3 nanocomposite electrolyte.
Fig. 6 e Proton and oxygen ion conductivity of CDC/Na2CO3 nanocomposite electrolyte measured by four-probe d.c. technique.
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Table 1 e Conductivity comparison of CDC/Na2CO3 and SDC/Na2CO3 nanocomposites. (The error bars of conductivity values are within the range of the last significant decimal.). 600 C sHþ CDC=Na2 CO3 (20 wt.%) SDC=Na2 CO3 (20 wt.%)
550 C sO2
2
5:2 10 6:0 102
500 C
sHþ 3
4:5 10 4:9 103
400 C. The transition of activation energy is explained to be due to the glass transition of carbonate [17]. On the other hand, the oxygen ion conductivity has a nearly constant slope over the whole temperature range, corresponding to activation energy of 0.949 eV. Thus, different from conventional electrolytes in SOFCs, the CDC/Na2CO3 nanocomposite electrolyte possesses the unique Hþ/O2- dual ion conduction property. From the conductivity data, it is clearly demonstrated that the interface introduced by carbonate in the CDC/Na2CO3 nanocomposite supplies high conductive paths for proton transport, whilst oxygen ions are most probably transported within CDC phase. The dual ion conduction of protons and oxygen ions not only enhances the total ionic conductivity, but also promotes reactions at both electrodes, thus the fuel cell output based on nanocomposite electrolyte can be improved. The transportation of proton in the interface was explained by a “swing model” in our previous work [17]. When protons approach the composite electrolyte from anode, it forms meta-stable hydrogen bonds with oxygen ions from both doped ceria surface and CO2 3 group. When the operating temperature is above the glass transition temperature of amorphous carbonate phase, the bending and stretching vibration of CeO bonds are enhanced, as well as the mobility and rotation of group. These enhanced movements facilitate rapid CO2 3 breaking and forming of hydrogen bonds in the interface region, leading to effective long-range proton transportation driven by proton concentration gradient. In this process, carbonate serves as a “bridge” for protons to move from one hydrogen bond to another. Compared to the ionic conductivity of SDC/Na2CO3 nanocomposite [17], CDC-based nanocomposite shows a slightly lower conductivity values, as shown in Table 1. This can be explained by the “swing model” reported in our previous work [17]. The lower ionic conductivity of CDCbased nanocomposite is because of the lower proton conductivity. In CDC/Na2CO3 composite electrolyte, the Ca2þ forms weaker O-Ca bond than Sm3þ [26], and as the acceptor in hydrogen bond, O from O-Ca will form stronger hydrogen bond. Then, the hydrogen bond formed between proton and oxygen ions from CDC surface is stronger than formed with oxygen ions from CO2 3 group. The meta-hydrogen bond may be dragged by CDC surface, then impede rapid breaking and forming, result in lower conductivity. At any rate, the total ionic conductivity of CDC/Na2CO3 composite is still high enough for SOFCs application. Therefore, CDC/Na2CO3 nanocomposite can be regarded as an alternative potential electrolyte material for LTSOFCs. Future work will be done on application of CDC/Na2CO3 nanocomposite as electrolyte for SOFC single cells.
sO2 2
4:3 10 5:1 102
4.
sHþ 3
2:5 10 3:1 103
sO2 2
3:3 10 3:9 102
1:2 103 1:6 103
Conclusion
In summary, calcium doped ceria was considered to construct nanocomposite electrolyte for LTSOFCs. Calcium doped ceria was prepared by oxalate co-precipitation method, and the CDC/Na2CO3 composite were made by a dry mixing method. The TGA analysis shows that the CDC precursor was completely converted to oxide at temperature above 600 C. XRD and SEM characterizations confirm that CDC solid solution was formed and the amorphous Na2CO3 phase effectively prevents gain growth of CDC. The CDC/Na2CO3 sample shows a significantly enhanced overall ionic conductivity compared to that of single phase CDC sample, demonstrating a distinguished composite effect, which verifies that nanocomposite approach is an effective way to develop electrolyte for LTSOFCs. The good ionic conductivity of CDC/Na2CO3 nanocomposite demonstrates that it can be regarded as a potential electrolyte material for LTSOFCs.
Acknowledgment This work was supported by the Swedish Research Council and the Swedish Agency for International Development Cooperation (SIDA) (Project No. 2005-6355), and partially supported by funding from Functional Materials Division, Royal Institute of Technology (KTH).
references
[1] Jacobson AJ. Materials for solid oxide fuel cells. Chem Mater 2010;22:660e74. [2] Zhang X, Chan SH, Li G, Ho HK, Li J, Feng Z. A review of integration strategies for solid oxide fuel cells. J Power Sources 2010;195:685e702. [3] Singhal SC, Kendall K. High temperature solid oxide fuel cells: fundamentals, design, and applications. Elsevier Science Ltd; 2003. [4] Butz B, Kruse P, Stoermer H, Gerthsen D, Mueller A, Weber A, et al. Correlation between microstructure and degradation in conductivity for cubic Y2O3-doped ZrO2. Solid State Ionics 2006;177:3275e84. [5] Brett DJL, Atkinson A, Brandon NP, Skinner SJ. Intermediate temperature solid oxide fuel cells. Chem Soc Rev 2008;37: 1568e78. [6] Inaba H, Tagawa H. Ceria-based solid electrolytes. Solid State Ionics 1996;83:1e16.
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[7] Mogensen M, Sammes NM, Tompsett GA. Physical, chemical and electrochemical properties of pure and doped ceria. Solid State Ionics 2000;129:63e94. [8] Palmqvist AEC, Zwinkels MFM, Zhang Y, Jara˚s SG, Muhammed M. Reduction of sulfur dioxide by carbon monoxide over doped nanophase cerium oxides. Nanostruct Mater 1998;8:801e13. [9] Palmqvist AEC, Johansson EM, Jara˚s SG, Muhammed M. Total oxidation of methane over doped nanophase cerium oxides. Catal Lett 1998;56:69e75. [10] Reddy KR, Karan K. Sinterability, mechanical, microstructural, and electrical properties of gadoliniumdoped ceria electrolyte for low-temperature solid oxide fuel cells. J Electroceram 2005;15:45e56. [11] Wang S, Kobayashi T, Dokiya M, Hashimoto T. Electrical and ionic conductivity of Gd-doped ceria. J. Electrochem. Soc. 2000;147:3606e9. [12] Zhu B, Liu X, Zhou P, Yang X, Zhu Z, Zhu W. Innovative solid carbonate-ceria composite electrolyte fuel cells. Electrochem Commun 2001;3:566e71. [13] Zhu B, Yang XT, Xu J, Zhu ZG, Ji SJ, Sun MT, et al. Innovative low temperature SOFCs and advanced materials. J Power Sources 2003;118:47. [14] Zhu B. Functional ceria-salt-composite materials for advanced SOFC applications. J Power Sources 2003;114:1e9. [15] Wang X, Ma Y, Raza R, Muhammed M, Zhu B. Novel core-shell SDC/amorphous Na2CO3 nanocomposite electrolyte for lowtemperature SOFCs. Electrochem Commun 2008;10:1617e20. [16] Ma Y, Wang X, Li S, Toprak MS, Zhu B, Muhammed M. Samarium-doped ceria nanowires: novel synthesis and application in low-temperature solid oxide fuel cells. Adv Mater 2010;22:1640e4.
[17] Wang X, Ma Y, Li S, Kashyout A, Zhu B, Muhammed M. Ceriabased nanocomposite with simultaneous proton and oxygen ion conductivity for low-temperature solid oxide fuel cells. J Power Sources 2011;196:2754e8. [18] Banerjee S, Devi PS. Understanding the effect of calcium on the properties of ceria prepared by a mixed fuel process. Solid State Ionics 2008;179:661e9. [19] Arai H, Kunisaki T, Shimizu Y, Seiyama T. Electrical properties of calcia-doped ceria with oxygen ion conduction. Solid State Ionics 1986;20:241e8. [20] de Carolis S, Pascual JL, Pettersson LGM, Baudin M, Wojcik M, Hermansson K, et al. Structure and electronic properties of ca-doped CeO2 and implications on catalytic activity: an experimental and theoretical study. J Phys Chem B 1999;103: 7627e36. [21] Yan M, Mori T, Zou J, Drennan J. Effect of grain growth on densification and conductivity of ca-doped CeO2 electrolyte. J Am Ceram Soc 2009;92:2745e50. [22] Thangadurai V, Kopp P. Chemical synthesis of Ca-doped CeO2dIntermediate temperature oxide ion electrolytes. J Power Sources 2007;168:178e83. [23] Ma Y, Wang X, Raza R, Muhammed M, Zhu B. Thermal stability study of SDC/Na2CO3 nanocomposite electrolyte for low-temperature SOFCs. Int J Hydrogen Energy 2010;35: 2580e5. [24] Maier J. Nanoionics: ion transport and electrochemical storage in confined systems. Nat Mater 2005;4:805e15. [25] Jamnik J, Maier J, Pejovnik S. Interfaces in solid ionic conductors: equilibrium and small signal picture. Solid State Ionics 1995;75:51e8. [26] Brown I. On the geometry of O-H.O hydrogen bonds. Acta Crystallogr A 1976;32:24e31.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Enhanced ionic conductivity in calcium doped ceria e Carbonate electrolyte: A composite effect Ying Ma a,*, Xiaodi Wang a, Hassan Ahmed Khalifa a, Bin Zhu b, Mamoun Muhammed a a b
Division of Functional Materials, Royal Institute of Technology (KTH), S-16440 Stockholm, Sweden Department of Energy Technology, Royal Institute of Technology (KTH), S-10044 Stockholm, Sweden
article info
abstract
Article history:
Recently, ceria-based nanocomposites, as a proton and oxygen ion conductor, has been
Received 17 May 2011
developed as promising electrolyte candidates for low-temperature solid oxide fuel cells
Received in revised form
(LTSOFCs). Up to now, samarium doped ceria (SDC) was studied as a main oxide for
20 September 2011
nanocomposite electrolyte; while calcium doped ceria (CDC) is considered as a good
Accepted 22 September 2011
alternative from both material performance and economical aspects. Yet the conduction
Available online 5 November 2011
behavior of CDC-based composite has not been reported. In the present study, calcium doped ceria was prepared by oxalate co-precipitation method, and used for the fabrication
Keywords:
of CDC/Na2CO3 composite. The thermal decomposition process, structure and morphology
Nanocomposite electrolyte
of the samples were characterized by TGA, XRD, SEM, etc. The oxygen ion conductivity of
Calcium doped ceria (CDC)
single phase CDC sample was measured by electrochemical impedance spectroscopy (EIS),
Proton conductivity
while the proton and oxygen ion conductivity of CDC/Na2CO3 nanocomposite sample were
Oxygen ion conductivity
determined by four-probe d.c. measurements. The CDC/Na2CO3 samples show significantly
Solid oxide fuel cells (SOFCs)
enhanced overall ionic conductivity compared to that of single phase CDC samples, demonstrating pronounced composite effect. This confirms that the use of nanocomposite as electrolyte can effectively lower the operation temperature of SOFC due to improved ionic conductivity. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As one of the most efficient and environmentally benign energy conversion devices, solid oxide fuel cells (SOFCs) have attracted much attention in recent years [1,2]. Conventional SOFCs with yttria-stabilized zirconia (YSZ) as electrolyte require high operation temperature (800e1000 C), which causes significant problems like material degradation [3,4], as well as other technological complications and economic barrier for wider applications. Therefore, there is a broad interest in reducing the operation temperature of SOFCs, e.g. in the range below 600 C [5]. A key requirement is to achieve
high ionic conductivity of the electrolyte at such lowtemperature. Doped ceria has been considered as a candidate electrolyte suitable in low-temperature range [6,7], besides its wide use as oxygen sensor, and three-way catalyst [8,9], etc. However, doped ceria shows electronic conductivity at lower oxygen partial pressure which results in a significant power loss, thus preventing its further commercialization [10,11]. The development of effective electrolyte materials for low-temperature SOFCs (LTSOFCs) is a grand challenge for the SOFC community. In 2001, Bin Zhu has first suggested that ceria-based composites can serve as promising electrolyte materials for
* Corresponding author. Tel.: þ46 8790 8926; fax: þ46 8790 9072. E-mail address: [email protected] (Y. Ma). 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.2011.09.122
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LTSOFC [12]. Later it has been demonstrated that enhanced ionic conductivity can be achieved in the ceria-based composite systems, resulting in superior fuel cell performances [13,14]. This composite approach was designed and fabricated based on the utilization of the interface as path for ionic conduction. In our previous reports, some new composite electrolytes with unique nanostructures, i.e. coreeshell samarium doped ceria (SDC)/Na2CO3 nanocomposite [15], and SDC nanowires based nanocomposite [16] were developed, where the interface region is dominant in the overall transport in these nanostructured composite materials. Furthermore, in our recent work [17], we demonstrated that the SDC/Na2CO3 nanocomposite electrolyte possesses a unique simultaneous Hþ/O2 conduction property, where the proton conduction mainly accounts for ionic conductivity and fuel cell performance of SDC/Na2CO3 nanocomposite. In order to develop more efficient electrolyte based on the composite approach, we considered calcium doped ceria (CDC) as a good candidate since it shows comparable high conductivity as SDC or gadolinium doped ceria (GDC) [18,19]. Besides, it has been demonstrated that CDC is a good oxygen storage material due to oxygen vacancy created by calcium doping [20]. Furthermore, calcium is much less expensive as compared to costly rare earth elements. Although there are immense published studies on SDC or GDC based composite systems and single phase CDC electrolyte [21,22], reports on CDC-based composite are rather scarce. In the present study, we herein report the preparation of CDC nanomaterials, by oxalate co-precipitation, and CDC/Na2CO3 nanocomposite. Comprehensive characterizations of the samples were conducted by means of TGA, XRD, SEM, etc. The ionic conductivity properties of both single phase CDC and CDC/Na2CO3 nanocomposite was investigated by employing impedance spectroscopy and four-probe d.c. technique.
For the synthesis of CDC composite, Na2CO3 was introduced as secondary phase. The CDC/Na2CO3 composite was prepared by a dry mixing method. The prepared CDC was mixed with Na2CO3 salt with weight ratio of CDC: Na2CO3 ¼ 4:1 according to previous optimized results in the literature. The mixture was well ground in an agate mortar, calcined at 700 C in air for 1h and taken out directly from the furnace to room temperature to form CDC/Na2CO3 composite with Na2CO3 weight content of 20%. Thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FT-IR) experiment was carried out with a heating rate of 5 C/min in synthetic air using TA TGA Q500 interfaced with Nicolet iS10 FT-IR spectrometer from Thermo Scientific. Powder X-ray diffraction (XRD) patterns of the samples were collected using Philips X’pert pro super ˚ ) for phase Diffractometer with Cu Ka radiation (l ¼ 1.5418 A analysis and crystal size calculation. Zeiss Ultra 55 fieldemission scanning electron microscope (FESEM) was used to examine the morphology and microstructure of samples. The CDC/Na2CO3 nanocomposite material was pressed under 300 MPa into pellets, followed by sintering at 680 C for 30 min. The d.c. conductivity of the nanocomposite electrolyte was measured by the four-probe technique following the same process as reported in our previous work [17]. The proton conductivity and oxygen ion conductivity of CDC/ Na2CO3 nanocomposite were determined separately under two different gas atmosphere, 5% H2 and air respectively. As a comparison, pellets of pure CDC sample were cold pressed and sintered at 1400 C for 3 h to reach a relative density of 96.2%, as measured by using Archimedes’ principle. The electrical conductivity of CDC pellet was measured by a.c. impedance method using Solartron 1287 potentiostat and Solartron 1255 Frequency Response Analyzer in the frequency range from 1 mHz to 1 MHz with an applied signal of 20 mV.
2.
3.
Experimental
Ca-doped ceria (CDC) sample with the composition of Ce0.8Ca0.2O2d was selected since it shows the highest oxygen ion conductivity as reported [18]. The Ce0.8Ca0.2O2d sample was prepared by co-precipitation method using ammonium oxalate as the precipitating agent. All the chemicals were used as received without further purification. The stock solution was made by dissolving cerium nitrate hexahydrate (Ce(NO3)3$6H2O, 99.9%, Chempur, Karlsruhe, Germany) and calcium nitrate tetrahydrate (Ca(NO3)2$4H2O, 99.9%, SigmaeAldrich) in distilled water with a molar ratio of Ce3þ:Ca2þ ¼ 4:1, and the total concentration of cation [Ce3þ] þ [Ca2þ] ¼ 0.3 M. Aqueous solutions of ammonium oxalate ((NH4)2C2O4$H2O, 99.5%, Merck) with a concentration of 0.3 M were used as the precipitating agent. In a typical procedure, 50 ml mixed salts solution was added at a rate of 5 ml min1 into 150 ml oxalate solution under vigorous stirring at room temperature where white precipitate formed instantaneously. The resultant suspension was homogenized for 1 h and then filtered. The precipitate cake was washed repeatedly with distilled water and ethanol, followed by drying at 80 C to obtain the CDC oxalate precursor. Finally, the precursor was calcined at 650 C in air for 3 h and converted to the Ce0.8Ca0.2O2d.
Results and discussion
Thermogravimetric analysis (TGA) of as-prepared oxalate precursor was conducted in air, as shown in Fig. 1. It can be seen clearly from the curve that two main weight loss processes take place in the temperature range of (i) room temperature to ca. 150 C and (ii) 150e650 C. There is no mass loss above 650 C.
Fig. 1 e TGA curve and GrameSchmidt plot of as-prepared oxalate precursor.
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The initial weight loss of ca. 21% to 150 C is due to loss of water. A weight loss of 46.1% (the amount of water was excluded) was obtained in the temperature range of 150e600 C, which corresponds to the conversion of Ce0.8Ca0.2(C2O4)1.8 to Ce0.8Ca0.2O1.8. The measured weight loss (46.1%) is in a good agreement with the theoretical weight loss of 46.6%. The overall calcination reaction is expressed as in Eq. (1). Ce0:8 Ca0:2 ðC2 O4 Þ1:8 þ0:9O2 /Ce0:8 Ca0:2 O1:8 þ 3:6CO2
(1)
This reaction is confirmed by the corresponding in situ GrameSchmidt plot and FT-IR spectra. The GrameSchmidt plot in Fig. 1 indicates that the thermal decomposition process of oxalate precursor can be divided into three distinct regions: 250e400 C, 400e500 C and 500e650 C, due to different decomposition steps of cerium oxalate and calcium oxalate. From separate GrameSchmidt plot of cerium oxalate and calcium oxalate, it was confirmed that the first region (250e400 C) is a result of the thermal decomposition of cerium oxalate, while the other two regions are due to the thermal decomposition of calcium oxalate. The FT-IR spectra of gases evolved at temperatures corresponding to the peaks on the GrameSchmidt plot are presented in Fig. 2. At 100 C, only H2O is detected. CO2 (2362, 2329 and 668 cm1) starts to release after 250 C, continues up to 330 C and further appears at 470 and 600 C. After 650 C, there is no major reaction is detected. Therefore, the calcination temperature of CDC precursor was determined to be 650 C to form CDC solid solution. Fig. 3 shows that XRD patterns of the sintered specimens of pure CDC and CDC/Na2CO3 composite. All the peaks of CDC specimen were indexed to the cubic fluorite-type structure CeO2 (JCPDS 34-0394) without peaks related to CaO, which means that the Ca2þ ions are completely doped into CeO2 crystal structure. The CDC phase in CDC/Na2CO3 composite sample exhibits the same fluorite-type structure diffraction peaks with broader peak widths due to smaller particle size. However, there is no peak observed which is related to Na2CO3, even as the content of Na2CO3 in the composite is 20%, indicating that Na2CO3 exists as amorphous phase in the nanocomposite, which is identical to our previous study [15]. The average crystallite size of pure CDC sample is estimated to
Fig. 2 e FT-IR spectra of the evolved gases from the oxalate precursor during TGA experiment at different temperature.
Fig. 3 e XRD patterns of sintered specimens of pure CDC and CDC/Na2CO3 composite.
be 32 nm as calculated by Scherrer equation, while the crystallite size of CDC phase in CDC/Na2CO3 composite sample is determined to be 12.4 nm. The much smaller crystallite size of CDC phase in the composite can be attributed to the presence of Na2CO3 which acts as a significant diffusion barrier, and restrains interparticle diffusion and growth of CDC particles [23] The morphology and particle size of pure CDC and CDC/ Na2CO3 composite samples were characterized by SEM and given in Fig. 4. Fig. 4a reveals that the CDC/Na2CO3 nanocomposite consists of CDC particles smaller than 100 nm and show faceted and occasionally irregular shape. A further detailed SEM image (insert of Fig. 4a) clearly indicated that CDC particles are evenly covered with homogeneous Na2CO3 coatings. As we reported earlier [23], amorphous Na2CO3 shell serves as diffusion barrier in the composite structure, the interparticle contact and growth of CDC particles is minimized. Therefore, CDC/Na2CO3 composite exhibits a good nanostructure with grain-boundary can be easily distinguished. In contrast, for the pure CDC sample (Fig. 4b), high temperature calcination leads to excessive agglomeration and grain growth, due to their large volume fraction of highenergy intercrystalline regions without diffusion barrier. Conventional electrochemical impedance spectroscopy (EIS) is useful for studying the conductivity of conventional single phase electrolyte. However, it is not suitable for studying the conductivity of composite systems, where multiple mobile charge carriers contribute to the overall measured conductivity, i.e. O2, Hþ, Naþ, and CO2 3 . Hence the contribution of the specific ions of interest in SOFC operation, i.e. O2 and Hþ, cannot be distinguished from contributions of other ions when studied by EIS. In our recent study [17], we measured the proton and oxygen ion conductivity of SDC/Na2CO3 nanocomposite electrolyte by the four-probe d.c. measurements, which gives an unequivocal conductivity measure of mobile ions under different gas atmospheres over a wide temperature range. Furthermore, from a practical point of view, d.c. measurements are more close to the actual application regime of SOFCs. Therefore, in this present study, we use the four-probe d.c. measurement to determine the O2 and Hþ conductivity of
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Fig. 4 e SEM images of (a) CDC/Na2CO3 composite and (b) pure CDC sample. The inset of (a) is a further detailed observation with a scale bar of 20 nm.
CDC/Na2CO3 nanocomposite. While for pure CDC pellets, the oxygen ion conductivity is studied by EIS method. Fig. 5 shows the total ionic conductivity of CDC/Na2CO3 nanocomposite determined by the four-probe d.c. measurements, compared to the oxygen ion conductivity of pure CDC pellet studied by EIS. It can be seen that the overall ionic conductivity of the CDC/Na2CO3 nanocomposite electrolyte, which is the sum of the proton conductivity and oxygen ion conductivity, is about one to two orders of magnitude higher than that of pure CDC in the temperature range of 200 Ce600 C. The significantly enhanced ionic conductivity of CDC/Na2CO3 electrolyte demonstrates a distinct “composite effect”, which can be ascribed to interfacial interaction. Unlike the conventional superionic conduction/transition that appears in a single phase bulk accompanying a structural change, the superionic conduction in the composites is determined by the interfaces, i.e. a process breaks symmetry of matrix by introducing a structural discontinuity [24]. In contrast with the bulk, where electroneutrality must be obeyed, at the interface a narrow charged zone, the so-called space-charge zone is tolerable and thermodynamically necessary [25]. The defect concentrations in the space-charge
zone are much higher than that in the bulk, thus interface supplies high conductivity pathways for ionic transportation and conduction. Especially in nanocomposite, this interfacial effect is dominant in overall ionic transport. Therefore, ionic conductivity of CDC/Na2CO3 nanocomposite increases significantly compared to the single phase CDC electrolyte. Fig. 6 displays the temperature dependence of conductivity of CDC/Na2CO3 nanocomposite electrolyte under two different gas atmosphere, 5% H2 and air, as determined by the four-probe d.c. measurements. Under hydrogen gas atmosphere, only proton conductivity (sHþ ) makes a significant contribution to the overall measured conductivity, while the conductivity measured under air is mainly resulting from oxygen ion conductivity (sO2 ). It is shown in Fig. 6 that sHþ of CDC/Na2CO3 nanocomposite electrolyte is much higher than sO2 with 1e2 orders of magnitude over the whole temperature range (200e600 C), which resembles the conductivity curves of SDC/Na2CO3 nanocomposite as we reported previously [17]. For example, at 600 C sHþ is approximately 0.05 S/cm while sO2 is 0.004 S/cm. The proton conductivity curve shows a change of the slope at around 400 C, corresponding to activation energy of 0.475 eV above 400 C and 1.059 eV below
Fig. 5 e Overall conductivity comparison of pure CDC oxygen ion conductor and CDC/Na2CO3 nanocomposite electrolyte.
Fig. 6 e Proton and oxygen ion conductivity of CDC/Na2CO3 nanocomposite electrolyte measured by four-probe d.c. technique.
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Table 1 e Conductivity comparison of CDC/Na2CO3 and SDC/Na2CO3 nanocomposites. (The error bars of conductivity values are within the range of the last significant decimal.). 600 C sHþ CDC=Na2 CO3 (20 wt.%) SDC=Na2 CO3 (20 wt.%)
550 C sO2
2
5:2 10 6:0 102
500 C
sHþ 3
4:5 10 4:9 103
400 C. The transition of activation energy is explained to be due to the glass transition of carbonate [17]. On the other hand, the oxygen ion conductivity has a nearly constant slope over the whole temperature range, corresponding to activation energy of 0.949 eV. Thus, different from conventional electrolytes in SOFCs, the CDC/Na2CO3 nanocomposite electrolyte possesses the unique Hþ/O2- dual ion conduction property. From the conductivity data, it is clearly demonstrated that the interface introduced by carbonate in the CDC/Na2CO3 nanocomposite supplies high conductive paths for proton transport, whilst oxygen ions are most probably transported within CDC phase. The dual ion conduction of protons and oxygen ions not only enhances the total ionic conductivity, but also promotes reactions at both electrodes, thus the fuel cell output based on nanocomposite electrolyte can be improved. The transportation of proton in the interface was explained by a “swing model” in our previous work [17]. When protons approach the composite electrolyte from anode, it forms meta-stable hydrogen bonds with oxygen ions from both doped ceria surface and CO2 3 group. When the operating temperature is above the glass transition temperature of amorphous carbonate phase, the bending and stretching vibration of CeO bonds are enhanced, as well as the mobility and rotation of group. These enhanced movements facilitate rapid CO2 3 breaking and forming of hydrogen bonds in the interface region, leading to effective long-range proton transportation driven by proton concentration gradient. In this process, carbonate serves as a “bridge” for protons to move from one hydrogen bond to another. Compared to the ionic conductivity of SDC/Na2CO3 nanocomposite [17], CDC-based nanocomposite shows a slightly lower conductivity values, as shown in Table 1. This can be explained by the “swing model” reported in our previous work [17]. The lower ionic conductivity of CDCbased nanocomposite is because of the lower proton conductivity. In CDC/Na2CO3 composite electrolyte, the Ca2þ forms weaker O-Ca bond than Sm3þ [26], and as the acceptor in hydrogen bond, O from O-Ca will form stronger hydrogen bond. Then, the hydrogen bond formed between proton and oxygen ions from CDC surface is stronger than formed with oxygen ions from CO2 3 group. The meta-hydrogen bond may be dragged by CDC surface, then impede rapid breaking and forming, result in lower conductivity. At any rate, the total ionic conductivity of CDC/Na2CO3 composite is still high enough for SOFCs application. Therefore, CDC/Na2CO3 nanocomposite can be regarded as an alternative potential electrolyte material for LTSOFCs. Future work will be done on application of CDC/Na2CO3 nanocomposite as electrolyte for SOFC single cells.
sO2 2
4:3 10 5:1 102
4.
sHþ 3
2:5 10 3:1 103
sO2 2
3:3 10 3:9 102
1:2 103 1:6 103
Conclusion
In summary, calcium doped ceria was considered to construct nanocomposite electrolyte for LTSOFCs. Calcium doped ceria was prepared by oxalate co-precipitation method, and the CDC/Na2CO3 composite were made by a dry mixing method. The TGA analysis shows that the CDC precursor was completely converted to oxide at temperature above 600 C. XRD and SEM characterizations confirm that CDC solid solution was formed and the amorphous Na2CO3 phase effectively prevents gain growth of CDC. The CDC/Na2CO3 sample shows a significantly enhanced overall ionic conductivity compared to that of single phase CDC sample, demonstrating a distinguished composite effect, which verifies that nanocomposite approach is an effective way to develop electrolyte for LTSOFCs. The good ionic conductivity of CDC/Na2CO3 nanocomposite demonstrates that it can be regarded as a potential electrolyte material for LTSOFCs.
Acknowledgment This work was supported by the Swedish Research Council and the Swedish Agency for International Development Cooperation (SIDA) (Project No. 2005-6355), and partially supported by funding from Functional Materials Division, Royal Institute of Technology (KTH).
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