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Development of solid oxide fuel cell materials for intermediate-to-low temperature operation
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Development of solid oxide fuel cell materials for intermediate-to-low temperature operation Jianbing Huang a,b, Fucheng Xie a, Cheng Wang a, Zongqiang Mao a,* a
Division of New Energy and Material Chemistry, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China b State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China
article info
abstract
Article history:
The commercialization of solid oxide fuel cell (SOFC) needs the development of functional
Received 17 November 2010
materials for intermediate-to-low temperature (400e700 C, ILT) operation. Recently, we
Received in revised form
have successfully developed new electrolyte materials for ILT-SOFCs, including
3 April 2011
Ce0.8Sm0.2O1.9 (SDC), BaCe0.8Sm0.2O2.9 (BCSO) and SDC-carbonate composites. Compared
Accepted 6 April 2011
with the state-of-the-art yttria-stabilized zirconia (YSZ), these materials exhibit much
Available online 14 May 2011
higher ionic conductivity at ILT range. Especially, SDC-carbonate composites show an ionic conductivity of 102 to 1 Scm1 between 400 and 600 C in fuel cell environment. Some new cathode materials were investigated for above electrolyte materials and showed promising
Keywords: Solid oxide fuel cell (SOFC)
performance. Alternative anode materials were developed to directly utilize alcohol fuels.
Intermediate-to-low
A dry-pressing and co-firing process was employed to fabricate thin SDC and BCSO elec-
temperature
(ILT)
trolyte membranes as well as thick SDC-carbonate composite electrolyte with acceptable
Electrolyte
density on anode substrate. Many efforts have also been made on fabrication of larger-size
Ce0.8Sm0.2O1.9 (SDC)
planar cells and exploitation of reliable sealing materials.
BaCe0.8Sm0.2O2.9 (BCSO)
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
SDC-carbonate composite
1.
Introduction
Fuel cells are currently attracting tremendous interest because of their great potential for power generation to meet the demands of diversified applications in a highly efficient and environmentally benign way. Among various types of fuel cells, generally classified by the electrolyte, solid oxide fuel cells (SOFCs) using an oxide ceramic electrolyte offer significant advantages for residential and auxiliary power units, as well as for larger industrial power applications: highest energy conversion efficiency with heat recovery or combined power generation, multi-fuel capability, and simplicity of system design by internal reforming and modular construction [1].
reserved.
Conventional SOFCs use yttria-stabilized zirconia (YSZ) as the electrolyte. To achieve sufficiently high ionic conductivity (>101 Scm1) and high efficiency, YSZ-based SOFCs are usually operated at high temperature around 1000 C [2]. However, such a high operation temperature poses some disadvantages on commercialization, e.g. long-term stability of the cell components, materials and manufacturing cost, etc. By lowering the operation temperature, a wider range of cheaper materials can be used to construct the fuel cell stack and system. Lower temperature operation also affords more rapid start-up, improved durability, and higher robustness as well as simplified system requirements. Therefore, recent research efforts have been increasingly focusing on the
* Corresponding author. Tel.: þ86 10 62780537; fax: þ86 10 62771150. E-mail addresses: [email protected] (J. Huang), [email protected] (Z. Mao). 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.2011.04.030
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2.
ILT-SOFCs based on SDC electrolyte
It is well known that fluorite structure CeO2 doped with alkaline earth oxide or rare earth oxide possess higher oxygen ion conductivity than YSZ, particularly at low temperatures [4]. Despite their favorable ionic transport properties, doped CeO2 had not, until quite recently, been considered a realistic candidate for fuel cell applications due to their high electronic conductivity. The partial reduction of Ce4þ to Ce3þ in CeO2 under anodic reducing conditions will give n-type electronic conductivity which causes internal short circuit current in a cell, and on the other hand, generates nonstoichiometry and expansion of the lattice which can lead to mechanical failure. Fortunately, the electronic conduction of doped CeO2 electrolyte can be suppressed by decreasing the operation temperature. It is for this reason that doped CeO2-based SOFCs are usually considered to operate at temperatures below 600 C. However, due to the ionic conductivity limit, doped CeO2 electrolyte must be fabricated into a thin and dense ceramic membrane in order to obtain considerable cell output below 600 C. Thus, the sintering behavior of doped CeO2 should be investigated to optimize densification parameters of electrolyte membrane. Ce0.9Gd0.1O1.95 (GDC) and Ce0.8Sm0.2O1.9 (SDC) are the most widely studied electrolytes for SOFCs operating at 500e600 C because they have the highest ionic conductivity and proper thermal expansion coefficient compared with other cell components. But it is difficult to obtain a dense GDC or SDC ceramic by conventional solid-state reaction techniques at sintering temperatures below 1650 C in air [5]. Sintering at too high a temperature requires high energy cost and does not allow co-firing of electrolyte with electrode materials. To facilitate the densification of electrolyte membrane, fine SDC powder was prepared by an oxalate co-precipitation method, and its sintering behavior was investigated by Gao et al. [6]. The results show that SDC powder prepared by this method was realized densification when pressed uniaxially at 200e400 MPa and then sintered at 1350e1400 C for 4 h. The conductivities of the sintered SDC pellet were 0.013 and 0.021 Scm1 at 550 and 600 C, respectively, and the activation
energy for ionic conduction is 0.62 eV. It is expected that anode-supported SOFCs based on SDC electrolyte can be well operated at 500e600 C. To improve the overall performance of ILT-SOFCs, new cathode materials with high electrocatalytic properties and good structural/chemical stability are required. K2NiF4-type oxides are promising candidates as cathode material for ILTSOFCs because of their interesting transport and catalytic properties [7]. In our lab, La2Ni1-xMxO4þd (M ¼ Co, Fe; x ¼ 0e1) series oxides were prepared by a glycineenitrate process and their electrical properties as cathode material for SDC electrolyte were evaluated. Fig. 1 shows the electrical conductivities of La2Ni1xMxO4þd (M ¼ Co, Fe; x ¼ 0e1) cathode materials measured in air. At 600 C, the electrical conductivity decreased in the order La2CoO4þd > La2NiO4þd > La2Ni0.2Co0.8 O4þd > La2Ni0.8Co0.2O4þd > La2Ni0.6Co0.4O4þd > La2Ni0.2Fe0.8O4þd > La2FeO4þd. Among all compositions, La2NiO4þd displayed the lowest activation energy for hoping of small polarons at low temperatures. Fig. 2 shows the area specific resistance (ASR) for each cathode material on SDC electrolyte. It is evident that the ASRs of all compositions were greater than 1 Ucm2 below 600 C. The composition La2Ni0.2Co0.8O4þd showed the smallest ASR value, e.g. 0.71 Ucm2 at 650 C and 1.08 Ucm2 at 600 C. Other compositions La2Ni0.6Co0.4O4þd and La2Ni0.8Co0.2O4þd also showed acceptable ASR values as cathode material for SDC electrolyte. Moreover, composite cathode materials based on stabilized bismuth oxides and silver were studied for SDC electrolytes. Bismuth oxides exhibit high oxygen ion conductivity and silver provides high electronic conductivity, thus the composites are excellent mixed ioniceelectronic conductors (MIECs). For the composite materials consisting of Bi1.14Sr0.43O2.14 (SSB) and Ag, the ASR value was as low as 0.29 Ucm2 at 650 C when the content of Ag2O was 70 wt.% [8]. The substitution of SSB with La0.3Bi1.7O3 (LSB) would further lower the ASR of composite material to 0.18 Ucm2 at 650 C when the fraction of Ag2O was 50 vol.% [9]. It indicates that such composite materials are potential cathodes for ILTSOFCs based on SDC electrolytes. o
t ( C) 800700 600 500
400
300
200
100
13 12
La2CoO4+δ
11
La2NiO4+δ
10
La2Ni08Co0.2O4+δ
La2Ni0.2Co0.8O4+δ
-1
development of intermediate-to-low temperature (400e700 C, ILT) SOFCs. One effective approach to develop ILT-SOFCs is by reducing the thickness of YSZ electrolyte and improving electrode materials. However, there are certain theoretical and practical limitations to the reduction in electrolyte thickness [3]. Another approach is replacing YSZ by alternative electrolytes with higher ionic conductivity at ILT range. In recent years, we have successfully developed new electrolyte materials for ILTSOFCs, including oxygen ion conducting Ce0.8Sm0.2O1.9 (SDC), proton conducting BaCe0.8Sm0.2O2.9 (BCSO) and co-ion (oxygen ion and proton) conducting SDC-carbonate composites. All these materials show much higher ionic conductivity than YSZ at ILT range. Corresponding electrode materials as well as cell fabrication techniques have also been investigated and excellent cell performances have been achieved in ILT operation. In this paper, major results we have obtained with these materials and fabrication techniques are briefly reviewed.
ln(σT) (Scm K)
878
La2Ni0.6Co0.4O4+δ
9
La2Ni0.2Fe0.8O4+δ La2FeO4+δ
8 7 6 5 4 1.0
1.2
1.4
1.6
1.8
2.0
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2.8
-1
1000/T (K )
Fig. 1 e Comparison of the temperature dependence of the electrical conductivity of La2Ni1LxMxO4Dd (M [ Co, Fe; x [ 0e1) in air.
879
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3.
ILT-SOFCs based on BSCO electrolyte
o
700 C o 650 C o 600 C
1.0
0.35 0.30 -2
0.8
Power density (Wcm )
A dry-pressing and co-firing process was applied to fabricate anode-supported electrolyte bi-layer (Ni-SDC/SDC). La2Ni0.2Co0.8O4þd cathode was prepared by screen-printing. A maximum power density of 400 mW cm2 was obtained for a cell with 30 mm thick SDC electrolyte at 600 C. To develop larger-size planar ILT-SOFCs, NiOeSDC anode was prepared by tape-casting and then SDC electrolyte was screen-printed onto the anode substrate, finally the anode/electrolyte bi-layer were formed by co-sintering. A 60 mm 60 mm planar anodesupported electrolyte bi-layer was successfully fabricated. The cell performance was examined using Sm0.5Sr0.5 CoO3-d (SSC)eSDC composite cathode screen-printed on a disc of the bi-layer and a peak power density of 500 mW cm2 was demonstrated at 600 C. In our lab, a new sealing material based on BaOeAl2O3eB2O3eSrOeNiOeLa2O3 glass system was also developed for SDC electrolyte ILT-SOFCs [10]. The coefficient thermal expansion (CTE) of this sealant is 11.84 106 K1 from room temperature to 550 C, which is close to that of SDC electrolyte (11.97 106 K1). Using this sealant, an opencircuit voltage (OCV) ranging from 0.92 V to 0.96 V was achieved on a cell based on SDC electrolyte operating at 600 C. Current effort aims to construct a planar anode-supported ILTSOFC stack based on SDC electrolyte.
Cell voltage (V)
Fig. 2 e Area specific resistance (ASR) of La2Ni1LxMxO4Dd (M [ Co, Fe; x [ 0e1) cathode materials on SDC electrolyte.
vapor produced at the cathode side, which helps to improve the electromotive force (EMF) and the conversion efficiency of SOFC system, in the meantime, the fuel at the anode remains pure and requires no recirculation. Furthermore, doped BaCeO3 materials show much lower electronic conductivity than doped CeO2, resulting in higher OCV and conversion efficiency. Owing to the low sinterability of doped BaCeO3 and insufficient technology development, electrolyte-supported configuration was always adopted with the thickness of electrolyte about 0.5 mm, and the cell output performances were too low to be considered for practical application below 700 C. To improve the performance of SOFC based on doped BaCeO3, the configuration optimization must be made. In our lab, dense BaCe0.8Sm0.2O2.9 (BCSO) thin film was successfully fabricated on porous NiOeBCSO anode substrate by dry-pressing and cofiring process [12]. Then Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF)eBSCO composite cathode was screen-printed onto BSCO electrolyte to complete the cell. Fig. 3 illustrates the I-V and I-P characteristics of the single cell based on a 50 mm thick BSCO electrolyte. The OCVs of 1.049 V at 600 C and 1.032 V at 700 C were achieved, indicating negligible electronic conductivity and gas permeation. Maximum power densities of 132 and 340 m Wcm2 were obtained at 600 and 700 C, respectively. For the first time, stability test was carried out with 75 mm BCSO electrolyte at 650 C at the operating voltage of 0.7 V and current density about 0.12 A cm2, shown in Fig. 4. There was no obvious decline for both voltage and current density for 1000 min, implying the stability of the BCSO electrolyte in H2/ O2 operation. However, the practical application of such type of SOFC is still limited by the chemical stability of doped BaCeO3 electrolyte in atmospheres containing CO2 and H2O. The partial substitution of Zr for Ce will improve the chemical stability of BaCeO3 in CO2 and H2O atmospheres, but it also reduces the electrical conductivity and makes the densification process more difficult [13]. Hence, the composition optimization of doped BaCeO3 and the development of easier sintering process are desirable. To improve the cell performances in ILT
0.25 0.6
0.20 0.15
0.4
0.10 0.2 0.05
Doped BaCeO3 materials have drawn special attention as the substitute electrolyte materials for ILT-SOFCs for their high proton conductivity over the wide rang of 300e1000 C and their activation energies for proton conduction are lower than those for oxygen ion conduction in YSZ and doped CeO2. The use of a protonic conductor as electrolyte of SOFC has some advantages compared with that of the oxygen ion conductor [11]. For instance, the proton conduction leads to the water
0.0 0.0
0.00 0.2
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Current density (Acm )
Fig. 3 e Cell voltage and power density as a function of current density for the cell Ni-BSCOjBSCO (50 mm) jBSCFeBSCO, using humidified H2 (w3% H2O) as fuel and O2 as oxidant.
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Fig. 4 e Short-term stability of a cell with 75 mm BSCO electrolyte and BSCFeBSCO cathode using humidified H2 as fuel and O2 as oxidant.
operation, it is also necessary to develop compatible cathode materials with mixed protoniceelectronic conductivity.
4. ILT-SOFCs based on SDC-carbonate composite electrolyte So far, various single-phase oxide materials have been extensively studied as electrolyte for ILT-SOFCs and made much progress. However, current SOFCs based on these electrolytes can not meet demands for ILT operation because of a basic requirement on ionic conductivity of 101 Scm1 at 600 C. It was found that CeO2-based composite materials composed of a doped CeO2 matrix phase and a dispersed phase such as carbonates, sulphates, halides and hydrates show mixed ionic transport properties in fuel cell operation and the ionic conductivity can reach 101 Scm1 below 600 C [14]. Among these composites, CeO2-carbonate composites are the most commonly used electrolyte materials, which have demonstrated with the best performances in many LTSOFC applications [15,16]. In our group, the development of LT-SOFCs based on novel SDC-carbonate composite electrolytes was initiated at 2002, and we have successfully optimized the composite electrolyte material systems and improved the cell performances in low temperature operation. In order to elucidate the ionic transport phenomena responsible of the enhanced conductivity, we analyzed thoroughly the structural and electrochemical properties of various compositions of SDC-carbonate composites with different experimental conditions [17e19]. It was confirmed by XRD and SEM analyses that the composites were two-phase materials in which the SDC phase showed fluorite structure and the salt phase were amorphous and highly distributed among the SDC. DTA analysis further pointed out that the SDC phase did not alter the macroscopic melting points of the salt phase. FTeIR analysis was also performed on SDC-(0.53Li/ 0.47Na)2CO3 composites with different salt contents and shown in Fig. 5. The infrared adsorption spectra of the composites exhibited infrared absorption bands related to the
presence of bidentate carbonate with p(CO3) (864 cm1) and v(CO3) (1450 cm1) vibrations and both of them split into two bands with the increase of salt content. The weak band at about 3400 cm1 was characteristic of OH group on the surface of CeO2, but it became unapparent with the increase of salt content resulting from the displacement of OH group by CO3 group. A band at 1087 cm1 could be assigned to OeO vibration of O 2 species on the surface of CeO2, which shifted from that of pure CeO2 (1126 cm1) to a lower band. These results revealed that interfacial interaction influenced the species distribution on the surfaces of the SDC phase and the salt phase. The a.c. conductivities of SDC-carbonate composite electrolytes were studied by impedance spectrum in different experimental conditions. It found that a jump in conductivity measured in air atmosphere occurred at certain temperature for the composites, which was distinct from the conductivity behavior of SDC. The jump temperature was mainly dependent on the type of carbonates, and it was normally 20e50 C lower than the eutectic melting points of binary carbonates. We assume that the defects (cationic vacancies) accumulate in the interfacial regions where the space charge zones are formed. When the melting of carbonates arises in sublattice level at a certain temperature caused by interfacial interaction, the mobility of the defects in the interfacial regions increases greatly, leading to enhanced conductivity of the composites. With increasing temperature, the melting of the carbonates extends from sublattice to bulk, and the carbonates become highly mobile and dominate the total conductivity of the composites. The conductivity behaviors of the composites also associated to the salt content, which could be interpreted as a result of the percolation threshold to form consecutive interfaces. The temperature dependence of conductivity in different gas atmosphere was investigated on the SDC-carbonate composite. As shown in Fig. 6, it is evident that the conductivity measured in H2 atmosphere is much higher than that in air or in N2 atmosphere due to the electronic conductivity introduced by SDC. In humidified gas
Absorbance (a.u.)
880
f
3440.8
1504.41434.9 883.3 860.2
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Wavenumber (cm )
Fig. 5 e FTeIR spectra of the SDC-x(0.53Li/0.47Na)2CO3 composites, (a) x [ 10 wt.%, (b) x [ 15 wt.%, (c) x [ 20 wt.%, (d) x [ 25 wt.%, (e) x [ 30 wt.%, (f) x [ 35 wt.%.
881
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0.5
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Ni-Zn based anode Ni-Sn based anode Ni-Fe based anode
o
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Power density (W cm )
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Fig. 6 e Temperature dependence of the electrical conductivity of SDC-20 wt.% (053/0.47Na)2CO3 composite in different gas atmospheres.
atmosphere, proton conduction is facile since water vapor can react with oxygen vacancy in SDC to form interstitial proton [20] and then proton transport via cationic vacancy at the interfaces after thermally activated at certain temperature lower than the melting point of the salts. The d.c. conductivity of SDC-carbonate composite electrolytes can be obtained through direct measurements of the fuel cell I-V curves subtracting the influence of the electrodes and electrode/electrolyte interfaces as described by Zhu et al. [21]. Fig. 7 presents the d.c conductivities derived from the ohmic polarization in I-V curves for SDC-30 wt.%(0.67Li/ 0.33Na)2CO3 composite. It is clear that the d.c. conductivity behaviors are not consistent with the a.c. conductivity behaviors, implying that the conduction mechanism of the composite electrolyte in fuel cell is different from that in air. In H2/air fuel cell, only the source ions proton and oxygen ion can be conducted continuously, alkali metal ion conduction will
Fig. 7 e Comparison of the a.c. conductivities obtained from a.c. impedance spectrum and the d.c conductivities derived from the ohmic polarization in I-V curves for SDC-30 wt.%(0.67Li/0.33Na)2CO3 composite.
Fig. 8 e Electrochemical performances of direct ethanol SOFCs based on SDC-30 wt.% (0.53Li/0.47Na)2CO3 composite electrolyte using various anode materials at 600 C.
be blocked and carbonate ion conduction can be neglected due to extremely low CO2 concentration in air. However, the carbonate ion conduction may play an important role in the condition similar to the operation of molten carbonate fuel cell (MCFC) [22]. For both oxygen ion and carbonate ion conductions, water is formed at the anode side of the fuel cell. But for proton conduction, water is formed at the cathode side. According to water formation at both electrode sides during H2/air fuel cell operation, we discovered for the first time that the composite electrolytes could behave as co-ion (oxygen ion/proton) conductors or proton conductors depending on the salt content and composition [23]. Therefore, the composite electrolyte materials show superior ionic transport properties to single-phase oxide electrolyte materials which are more suitable for low temperature operation. Compatible electrode materials with high electrocatalytic activity at low temperatures have been developed for SDCecarbonate composite electrolytes [24]. Lithiated nickel oxide is selected as cathode material, since it is commonly
Fig. 9 e Short-term stability of a V20 mm single cell based on SDC-30 wt.% (0.67Li/0.33Na)2CO3 composite electrolyte fabricated by an integrated hot-pressing technique in H2/air operation.
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Table 1 e Advantages and disadvantages of possible electrolyte candidates for ILT-SOFCs developed in our lab. Electrolytes Advantages
Disadvantages
SDC Much higher oxygen ion conductivity (102 Scm1 at 600 C) and lower activation energy for ionic transport han those of YSZ Excellent phase stability in fuel cell environment and high temperature Good compatibility with cathode materials Electronic conduction at low pO2 in fuel gas atmosphere results in low OCV and mechanical instability
BSCO
SDC-carbonate composites
Remarkable proton conductivity (102 Scm1 at 600 C) and low activation energy for proton conduction Negligible electronic conductivity
Extremely high ionic (oxygen ion and proton) conductivity at low temperature (102e1 Scm1 at 400e600 C) Negligible electronic conductivity Availability and low price of carbonates Good compatibility with cathode materials
Chemical instability in H2O and CO2 environment at low temperature High synthesis temperature for pure phase oxide powders Lack of compatible cathode materials
Potential instability due to the volatilization loss of carbonates when operated above their melting points Gas crossover due to not fully densified composite electrolytes when operated far below the melting points of carbonates
used in MCFC adopting similar composition of alkaline carbonate as electrolyte. We first made the highest fuel cell performance using this cathode material. A typical fuel cell based on a 0.3 mm thick SDC-30 wt.%(0.67Li/0.33Na)2CO3 composite electrolyte was fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode [25]. With H2 as fuel and air as oxidant, a maximum power density of 1100 mW cm2 and 650 mW cm2 was achieved at 600 C and 500 C, respectively. Even at 400 C, the cell delivered a maximum power density more than 330 mW cm2. Single cells based on SDC-(0.53Li/0.47Na)2CO3 composite electrolytes, nickel oxide as anode and lithiated nickel oxide as cathode, were also fabricated by dry-pressing process [23]. The optimal performances of 1085 mW cm2 at 600 C and 690 mW cm2 at 500 C were achieved for the composite electrolytes containing 25 and 20 wt.% carbonates, respectively. These performances are much higher that the best performances ever reported for the fuel cells based on a thin doped CeO2 electrolyte at low temperature [26]. For the composite electrolyte fuel cells, the OCVs could reach 1.0 V or even higher value at 600 C, indicating that the electronic conductivity can be suppressed by the presence of salt phase in the composite electrolytes. Alternative anode materials, e.g. NieZn, NieSn and NieFe binary systems were developed for LT-SOFCs based on SDC-carbonate composite electrolytes. These anode materials exhibited excellent cell performances in H2 operation. For instance, a cell based on SDC30 wt.%(0.53Li/0.47Na)2CO3 composite electrolyte using NieSn based anode and lithiated nickel oxide cathode showed peak power density of 935 mW cm2 at 600 C and 713 mW cm2 at 500 C. In direct alcohol operation, these anode materials also showed good electrocatalytic activities. Fig. 8 shows the electrochemical performances of direct ethanol SOFCs using various anode materials. The maximum power densities at 600 C were 353, 306 and 223 mW cm2 for the cells using NieZn, NieSn and NieFe based anode materials, respectively. The cell performances in direct alcohol operation can be further improved by optimization of anode composition and microstructure as well as cell fabrication technique. In our group, an integrated hot-pressing technique was developed to fabricate larger-size single cells based on SDC-
carbonate composite electrolytes. This fabrication process involved the integration of metal mesh into single cell and the optimization of anode microstructure. The mechanical strength and roughness of integrated cells were significantly improved. A V20 mm single cell based on SDC-30 wt.%(0.67Li/ 0.33Na)2CO3 composite electrolyte was fabricated by this technique, using nickel oxide as anode and lithiated nickel oxide as cathode [27]. Fig. 9 shows the stability test of the cell at 550 C. It can be seen that a nearly steady output power of about 1.5 W for 2100 min was realized with an operating voltage of about 0.63 V, indicating good stability of the composite electrolyte in H2/air operation. The cell was tested between 500 and 600 C for 110 h and showed good stability, but it still requires assessment of the long-term stability caused by volatilization of the salts in fuel cell operation and the change of cell microstructure. To assure the materials stability, it is suggested that such fuel cells be operated below the melting point of the salts. The fabrication of larger-size (e.g. 60 mm 60 mm) single cells by a modified hotpressing technique is under progress. Based on them, planar LT-SOFC stacks with innovative sealing material will be assembled in the future.
5.
Summary and view
ILT-SOFCs using three kinds of new electrolyte materials and related electrode materials have been successfully developed in our lab. Table 1 gives an overview of the advantages and disadvantages of the different electrolyte materials. Singlephase oxides such as oxygen ion conducting doped CeO2 and proton conducting doped BaCeO3 are candidates as electrolyte for ILT operation, but their ionic conductivities need to be further improved. Doping with multiple elements may be effective to improve the ionic conductivity of these oxide electrolytes, but it is restricted by lattice structure and dopant concentration. Enhanced conductivity with several magnitudes is possibly realized by composite effect through interfacial conduction. Two-phase nanocomposites based on oxygen ion or proton conducting oxides are potential electrolyte materials for LT-SOFCs. Doped CeO2-carbonate
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composites have shown mixed oxygen ion and proton conduction or pure proton conduction with enhanced conductivity at ILT. Another kind of prospective electrolyte materials is doped CeO2eBaCeO3 nanocomposites. These material systems represent a new research field that could be promising for LT-SOFC application. Mechanism understanding is the key for designing new composite electrolyte materials, and it relies on the development of advanced electrochemical characterization techniques. For practical application, cost-effective fabrication processes should be developed to prepare and scale-up the composite electrolytes.
Acknowledgments This work was financially supported by the National Basic Research Program of China (No. 2007CB209705), the National Natural Science Foundation of China (No. 50902083), EC FP6 NANOCOFC (No. SSA 032308), China Postdoctoral Science Foundation funded project (No. 20090450378) and the fundamental research funds for the central universities (No. 08142002).
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[11] Yang L, Zuo CD, Wang SZ, Peng Z, Liu ML. A novel composite cathode for low-temperature SOFCs based on oxide proton conductors. Adv Mater 2008;20(17):3280e3. [12] Peng RR, Wu Y, Yang LZ, Mao ZQ. Electrochemical properties of intermediate- temperature SOFCs based on proton conducting Sm-doped BaCeO3 electrolyte thin film. Solid State Ionics 2006;177(3e4):389e93. [13] Kreuer KD. Proton-conducting oxides. Annu Rev Mater Res 2003;33:333e59. [14] Zhu B. Next generation fuel cell R&D. Int J Energy Res 2006; 30(11):895e903. [15] 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(1e2):47e53. [16] Zhu B. Functional ceria-slat-composite materials for advanced ITSOFC applications. J Power Sources 2003;114(1): 1e9. [17] Huang JB, Mao ZQ, Yang LZ, Peng RR. SDC-carbonate composite electrolytes for low-temperature SOFCs. Electrochem Solid State Lett 2005;8(9):A437e40. [18] Huang JB, Gao Z, Mao ZQ. Effect of salt composition on the electrical properties of samaria-doped ceria/carbonate composite electrolytes for low-temperature SOFCs. Int J Hydrogen Energy 2010;35(9):4270e5. [19] Gao Z, Huang JB, Mao ZQ, Wang C, Liu ZX. Preparation and characterization of nanocrystalline Ce0.8Sm0.2O1.9 for low temperature solid oxide fuel cells based on composite electrolyte. Int J Hydrogen Energy 2010;35(2):731e7. [20] Steele BCH. Appraisal of Ce1-yGdyO2-y/2 electrolytes for ITSOFC operation at 500 C. Solid State Ionics 2000;129(1e4): 95e110. [21] Zhu B, Liu XR, Zhou P, Zhu ZG, Zhu W, Zhou SF. Costeffective yttrium doped ceria-based composite ceramic materials for intermediate temperature solid oxide fuel cell applications. J Mater Sci Lett 2001;20(7):591e4. [22] Xia C, Li Y, Tian Y, Liu QH, Zhao YC, Jia LJ, et al. A high performance composite ionic conducting electrolyte for intermediate temperature fuel cell and evidence for ternary ionic conduction. J Power Sources 2008;188(1):156e62. [23] Huang JB, Mao ZQ, Liu ZX, Wang C. Development of novel low-temperature SOFCs with co-ionic conducting SDCcarbonate composite electrolytes. Electrochem Commun 2007;9(10):2601e5. [24] Huang JB, Gao RF, Mao ZQ, Feng JY. Investigation of La2NiO4þd-based cathodes for SDC-carbonate composite electrolyte intermediate temperature fuel cells. Int J Hydrogen Energy 2010;35(7):2657e62. [25] Huang JB, Mao ZQ, Liu ZX, Wang C. Performance of fuel cells with proton-conducting ceria-based composite electrolyte and nickel-based electrodes. J Power Sources 2008;175(1): 238e43. [26] Shao ZP, Haile SM. A high-performance cathode for the next generation of solid-oxide fuel cells. Nature 2004;431(7005): 170e3. [27] Huang JB, Yang LZ, Gao RF, Mao ZQ, Wang C. A high performance ceramic fuel cell with samarium doped ceriacarbonate composite electrolyte at low temperatures. Electrochem Commun 2006;8(5):785e9.
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Development of solid oxide fuel cell materials for intermediate-to-low temperature operation Jianbing Huang a,b, Fucheng Xie a, Cheng Wang a, Zongqiang Mao a,* a
Division of New Energy and Material Chemistry, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China b State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China
article info
abstract
Article history:
The commercialization of solid oxide fuel cell (SOFC) needs the development of functional
Received 17 November 2010
materials for intermediate-to-low temperature (400e700 C, ILT) operation. Recently, we
Received in revised form
have successfully developed new electrolyte materials for ILT-SOFCs, including
3 April 2011
Ce0.8Sm0.2O1.9 (SDC), BaCe0.8Sm0.2O2.9 (BCSO) and SDC-carbonate composites. Compared
Accepted 6 April 2011
with the state-of-the-art yttria-stabilized zirconia (YSZ), these materials exhibit much
Available online 14 May 2011
higher ionic conductivity at ILT range. Especially, SDC-carbonate composites show an ionic conductivity of 102 to 1 Scm1 between 400 and 600 C in fuel cell environment. Some new cathode materials were investigated for above electrolyte materials and showed promising
Keywords: Solid oxide fuel cell (SOFC)
performance. Alternative anode materials were developed to directly utilize alcohol fuels.
Intermediate-to-low
A dry-pressing and co-firing process was employed to fabricate thin SDC and BCSO elec-
temperature
(ILT)
trolyte membranes as well as thick SDC-carbonate composite electrolyte with acceptable
Electrolyte
density on anode substrate. Many efforts have also been made on fabrication of larger-size
Ce0.8Sm0.2O1.9 (SDC)
planar cells and exploitation of reliable sealing materials.
BaCe0.8Sm0.2O2.9 (BCSO)
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
SDC-carbonate composite
1.
Introduction
Fuel cells are currently attracting tremendous interest because of their great potential for power generation to meet the demands of diversified applications in a highly efficient and environmentally benign way. Among various types of fuel cells, generally classified by the electrolyte, solid oxide fuel cells (SOFCs) using an oxide ceramic electrolyte offer significant advantages for residential and auxiliary power units, as well as for larger industrial power applications: highest energy conversion efficiency with heat recovery or combined power generation, multi-fuel capability, and simplicity of system design by internal reforming and modular construction [1].
reserved.
Conventional SOFCs use yttria-stabilized zirconia (YSZ) as the electrolyte. To achieve sufficiently high ionic conductivity (>101 Scm1) and high efficiency, YSZ-based SOFCs are usually operated at high temperature around 1000 C [2]. However, such a high operation temperature poses some disadvantages on commercialization, e.g. long-term stability of the cell components, materials and manufacturing cost, etc. By lowering the operation temperature, a wider range of cheaper materials can be used to construct the fuel cell stack and system. Lower temperature operation also affords more rapid start-up, improved durability, and higher robustness as well as simplified system requirements. Therefore, recent research efforts have been increasingly focusing on the
* Corresponding author. Tel.: þ86 10 62780537; fax: þ86 10 62771150. E-mail addresses: [email protected] (J. Huang), [email protected] (Z. Mao). 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.2011.04.030
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 ) 8 7 7 e8 8 3
2.
ILT-SOFCs based on SDC electrolyte
It is well known that fluorite structure CeO2 doped with alkaline earth oxide or rare earth oxide possess higher oxygen ion conductivity than YSZ, particularly at low temperatures [4]. Despite their favorable ionic transport properties, doped CeO2 had not, until quite recently, been considered a realistic candidate for fuel cell applications due to their high electronic conductivity. The partial reduction of Ce4þ to Ce3þ in CeO2 under anodic reducing conditions will give n-type electronic conductivity which causes internal short circuit current in a cell, and on the other hand, generates nonstoichiometry and expansion of the lattice which can lead to mechanical failure. Fortunately, the electronic conduction of doped CeO2 electrolyte can be suppressed by decreasing the operation temperature. It is for this reason that doped CeO2-based SOFCs are usually considered to operate at temperatures below 600 C. However, due to the ionic conductivity limit, doped CeO2 electrolyte must be fabricated into a thin and dense ceramic membrane in order to obtain considerable cell output below 600 C. Thus, the sintering behavior of doped CeO2 should be investigated to optimize densification parameters of electrolyte membrane. Ce0.9Gd0.1O1.95 (GDC) and Ce0.8Sm0.2O1.9 (SDC) are the most widely studied electrolytes for SOFCs operating at 500e600 C because they have the highest ionic conductivity and proper thermal expansion coefficient compared with other cell components. But it is difficult to obtain a dense GDC or SDC ceramic by conventional solid-state reaction techniques at sintering temperatures below 1650 C in air [5]. Sintering at too high a temperature requires high energy cost and does not allow co-firing of electrolyte with electrode materials. To facilitate the densification of electrolyte membrane, fine SDC powder was prepared by an oxalate co-precipitation method, and its sintering behavior was investigated by Gao et al. [6]. The results show that SDC powder prepared by this method was realized densification when pressed uniaxially at 200e400 MPa and then sintered at 1350e1400 C for 4 h. The conductivities of the sintered SDC pellet were 0.013 and 0.021 Scm1 at 550 and 600 C, respectively, and the activation
energy for ionic conduction is 0.62 eV. It is expected that anode-supported SOFCs based on SDC electrolyte can be well operated at 500e600 C. To improve the overall performance of ILT-SOFCs, new cathode materials with high electrocatalytic properties and good structural/chemical stability are required. K2NiF4-type oxides are promising candidates as cathode material for ILTSOFCs because of their interesting transport and catalytic properties [7]. In our lab, La2Ni1-xMxO4þd (M ¼ Co, Fe; x ¼ 0e1) series oxides were prepared by a glycineenitrate process and their electrical properties as cathode material for SDC electrolyte were evaluated. Fig. 1 shows the electrical conductivities of La2Ni1xMxO4þd (M ¼ Co, Fe; x ¼ 0e1) cathode materials measured in air. At 600 C, the electrical conductivity decreased in the order La2CoO4þd > La2NiO4þd > La2Ni0.2Co0.8 O4þd > La2Ni0.8Co0.2O4þd > La2Ni0.6Co0.4O4þd > La2Ni0.2Fe0.8O4þd > La2FeO4þd. Among all compositions, La2NiO4þd displayed the lowest activation energy for hoping of small polarons at low temperatures. Fig. 2 shows the area specific resistance (ASR) for each cathode material on SDC electrolyte. It is evident that the ASRs of all compositions were greater than 1 Ucm2 below 600 C. The composition La2Ni0.2Co0.8O4þd showed the smallest ASR value, e.g. 0.71 Ucm2 at 650 C and 1.08 Ucm2 at 600 C. Other compositions La2Ni0.6Co0.4O4þd and La2Ni0.8Co0.2O4þd also showed acceptable ASR values as cathode material for SDC electrolyte. Moreover, composite cathode materials based on stabilized bismuth oxides and silver were studied for SDC electrolytes. Bismuth oxides exhibit high oxygen ion conductivity and silver provides high electronic conductivity, thus the composites are excellent mixed ioniceelectronic conductors (MIECs). For the composite materials consisting of Bi1.14Sr0.43O2.14 (SSB) and Ag, the ASR value was as low as 0.29 Ucm2 at 650 C when the content of Ag2O was 70 wt.% [8]. The substitution of SSB with La0.3Bi1.7O3 (LSB) would further lower the ASR of composite material to 0.18 Ucm2 at 650 C when the fraction of Ag2O was 50 vol.% [9]. It indicates that such composite materials are potential cathodes for ILTSOFCs based on SDC electrolytes. o
t ( C) 800700 600 500
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development of intermediate-to-low temperature (400e700 C, ILT) SOFCs. One effective approach to develop ILT-SOFCs is by reducing the thickness of YSZ electrolyte and improving electrode materials. However, there are certain theoretical and practical limitations to the reduction in electrolyte thickness [3]. Another approach is replacing YSZ by alternative electrolytes with higher ionic conductivity at ILT range. In recent years, we have successfully developed new electrolyte materials for ILTSOFCs, including oxygen ion conducting Ce0.8Sm0.2O1.9 (SDC), proton conducting BaCe0.8Sm0.2O2.9 (BCSO) and co-ion (oxygen ion and proton) conducting SDC-carbonate composites. All these materials show much higher ionic conductivity than YSZ at ILT range. Corresponding electrode materials as well as cell fabrication techniques have also been investigated and excellent cell performances have been achieved in ILT operation. In this paper, major results we have obtained with these materials and fabrication techniques are briefly reviewed.
ln(σT) (Scm K)
878
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9
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8 7 6 5 4 1.0
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Fig. 1 e Comparison of the temperature dependence of the electrical conductivity of La2Ni1LxMxO4Dd (M [ Co, Fe; x [ 0e1) in air.
879
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3.
ILT-SOFCs based on BSCO electrolyte
o
700 C o 650 C o 600 C
1.0
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0.8
Power density (Wcm )
A dry-pressing and co-firing process was applied to fabricate anode-supported electrolyte bi-layer (Ni-SDC/SDC). La2Ni0.2Co0.8O4þd cathode was prepared by screen-printing. A maximum power density of 400 mW cm2 was obtained for a cell with 30 mm thick SDC electrolyte at 600 C. To develop larger-size planar ILT-SOFCs, NiOeSDC anode was prepared by tape-casting and then SDC electrolyte was screen-printed onto the anode substrate, finally the anode/electrolyte bi-layer were formed by co-sintering. A 60 mm 60 mm planar anodesupported electrolyte bi-layer was successfully fabricated. The cell performance was examined using Sm0.5Sr0.5 CoO3-d (SSC)eSDC composite cathode screen-printed on a disc of the bi-layer and a peak power density of 500 mW cm2 was demonstrated at 600 C. In our lab, a new sealing material based on BaOeAl2O3eB2O3eSrOeNiOeLa2O3 glass system was also developed for SDC electrolyte ILT-SOFCs [10]. The coefficient thermal expansion (CTE) of this sealant is 11.84 106 K1 from room temperature to 550 C, which is close to that of SDC electrolyte (11.97 106 K1). Using this sealant, an opencircuit voltage (OCV) ranging from 0.92 V to 0.96 V was achieved on a cell based on SDC electrolyte operating at 600 C. Current effort aims to construct a planar anode-supported ILTSOFC stack based on SDC electrolyte.
Cell voltage (V)
Fig. 2 e Area specific resistance (ASR) of La2Ni1LxMxO4Dd (M [ Co, Fe; x [ 0e1) cathode materials on SDC electrolyte.
vapor produced at the cathode side, which helps to improve the electromotive force (EMF) and the conversion efficiency of SOFC system, in the meantime, the fuel at the anode remains pure and requires no recirculation. Furthermore, doped BaCeO3 materials show much lower electronic conductivity than doped CeO2, resulting in higher OCV and conversion efficiency. Owing to the low sinterability of doped BaCeO3 and insufficient technology development, electrolyte-supported configuration was always adopted with the thickness of electrolyte about 0.5 mm, and the cell output performances were too low to be considered for practical application below 700 C. To improve the performance of SOFC based on doped BaCeO3, the configuration optimization must be made. In our lab, dense BaCe0.8Sm0.2O2.9 (BCSO) thin film was successfully fabricated on porous NiOeBCSO anode substrate by dry-pressing and cofiring process [12]. Then Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF)eBSCO composite cathode was screen-printed onto BSCO electrolyte to complete the cell. Fig. 3 illustrates the I-V and I-P characteristics of the single cell based on a 50 mm thick BSCO electrolyte. The OCVs of 1.049 V at 600 C and 1.032 V at 700 C were achieved, indicating negligible electronic conductivity and gas permeation. Maximum power densities of 132 and 340 m Wcm2 were obtained at 600 and 700 C, respectively. For the first time, stability test was carried out with 75 mm BCSO electrolyte at 650 C at the operating voltage of 0.7 V and current density about 0.12 A cm2, shown in Fig. 4. There was no obvious decline for both voltage and current density for 1000 min, implying the stability of the BCSO electrolyte in H2/ O2 operation. However, the practical application of such type of SOFC is still limited by the chemical stability of doped BaCeO3 electrolyte in atmospheres containing CO2 and H2O. The partial substitution of Zr for Ce will improve the chemical stability of BaCeO3 in CO2 and H2O atmospheres, but it also reduces the electrical conductivity and makes the densification process more difficult [13]. Hence, the composition optimization of doped BaCeO3 and the development of easier sintering process are desirable. To improve the cell performances in ILT
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Doped BaCeO3 materials have drawn special attention as the substitute electrolyte materials for ILT-SOFCs for their high proton conductivity over the wide rang of 300e1000 C and their activation energies for proton conduction are lower than those for oxygen ion conduction in YSZ and doped CeO2. The use of a protonic conductor as electrolyte of SOFC has some advantages compared with that of the oxygen ion conductor [11]. For instance, the proton conduction leads to the water
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Fig. 3 e Cell voltage and power density as a function of current density for the cell Ni-BSCOjBSCO (50 mm) jBSCFeBSCO, using humidified H2 (w3% H2O) as fuel and O2 as oxidant.
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 ) 8 7 7 e8 8 3
Fig. 4 e Short-term stability of a cell with 75 mm BSCO electrolyte and BSCFeBSCO cathode using humidified H2 as fuel and O2 as oxidant.
operation, it is also necessary to develop compatible cathode materials with mixed protoniceelectronic conductivity.
4. ILT-SOFCs based on SDC-carbonate composite electrolyte So far, various single-phase oxide materials have been extensively studied as electrolyte for ILT-SOFCs and made much progress. However, current SOFCs based on these electrolytes can not meet demands for ILT operation because of a basic requirement on ionic conductivity of 101 Scm1 at 600 C. It was found that CeO2-based composite materials composed of a doped CeO2 matrix phase and a dispersed phase such as carbonates, sulphates, halides and hydrates show mixed ionic transport properties in fuel cell operation and the ionic conductivity can reach 101 Scm1 below 600 C [14]. Among these composites, CeO2-carbonate composites are the most commonly used electrolyte materials, which have demonstrated with the best performances in many LTSOFC applications [15,16]. In our group, the development of LT-SOFCs based on novel SDC-carbonate composite electrolytes was initiated at 2002, and we have successfully optimized the composite electrolyte material systems and improved the cell performances in low temperature operation. In order to elucidate the ionic transport phenomena responsible of the enhanced conductivity, we analyzed thoroughly the structural and electrochemical properties of various compositions of SDC-carbonate composites with different experimental conditions [17e19]. It was confirmed by XRD and SEM analyses that the composites were two-phase materials in which the SDC phase showed fluorite structure and the salt phase were amorphous and highly distributed among the SDC. DTA analysis further pointed out that the SDC phase did not alter the macroscopic melting points of the salt phase. FTeIR analysis was also performed on SDC-(0.53Li/ 0.47Na)2CO3 composites with different salt contents and shown in Fig. 5. The infrared adsorption spectra of the composites exhibited infrared absorption bands related to the
presence of bidentate carbonate with p(CO3) (864 cm1) and v(CO3) (1450 cm1) vibrations and both of them split into two bands with the increase of salt content. The weak band at about 3400 cm1 was characteristic of OH group on the surface of CeO2, but it became unapparent with the increase of salt content resulting from the displacement of OH group by CO3 group. A band at 1087 cm1 could be assigned to OeO vibration of O 2 species on the surface of CeO2, which shifted from that of pure CeO2 (1126 cm1) to a lower band. These results revealed that interfacial interaction influenced the species distribution on the surfaces of the SDC phase and the salt phase. The a.c. conductivities of SDC-carbonate composite electrolytes were studied by impedance spectrum in different experimental conditions. It found that a jump in conductivity measured in air atmosphere occurred at certain temperature for the composites, which was distinct from the conductivity behavior of SDC. The jump temperature was mainly dependent on the type of carbonates, and it was normally 20e50 C lower than the eutectic melting points of binary carbonates. We assume that the defects (cationic vacancies) accumulate in the interfacial regions where the space charge zones are formed. When the melting of carbonates arises in sublattice level at a certain temperature caused by interfacial interaction, the mobility of the defects in the interfacial regions increases greatly, leading to enhanced conductivity of the composites. With increasing temperature, the melting of the carbonates extends from sublattice to bulk, and the carbonates become highly mobile and dominate the total conductivity of the composites. The conductivity behaviors of the composites also associated to the salt content, which could be interpreted as a result of the percolation threshold to form consecutive interfaces. The temperature dependence of conductivity in different gas atmosphere was investigated on the SDC-carbonate composite. As shown in Fig. 6, it is evident that the conductivity measured in H2 atmosphere is much higher than that in air or in N2 atmosphere due to the electronic conductivity introduced by SDC. In humidified gas
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Fig. 5 e FTeIR spectra of the SDC-x(0.53Li/0.47Na)2CO3 composites, (a) x [ 10 wt.%, (b) x [ 15 wt.%, (c) x [ 20 wt.%, (d) x [ 25 wt.%, (e) x [ 30 wt.%, (f) x [ 35 wt.%.
881
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Fig. 6 e Temperature dependence of the electrical conductivity of SDC-20 wt.% (053/0.47Na)2CO3 composite in different gas atmospheres.
atmosphere, proton conduction is facile since water vapor can react with oxygen vacancy in SDC to form interstitial proton [20] and then proton transport via cationic vacancy at the interfaces after thermally activated at certain temperature lower than the melting point of the salts. The d.c. conductivity of SDC-carbonate composite electrolytes can be obtained through direct measurements of the fuel cell I-V curves subtracting the influence of the electrodes and electrode/electrolyte interfaces as described by Zhu et al. [21]. Fig. 7 presents the d.c conductivities derived from the ohmic polarization in I-V curves for SDC-30 wt.%(0.67Li/ 0.33Na)2CO3 composite. It is clear that the d.c. conductivity behaviors are not consistent with the a.c. conductivity behaviors, implying that the conduction mechanism of the composite electrolyte in fuel cell is different from that in air. In H2/air fuel cell, only the source ions proton and oxygen ion can be conducted continuously, alkali metal ion conduction will
Fig. 7 e Comparison of the a.c. conductivities obtained from a.c. impedance spectrum and the d.c conductivities derived from the ohmic polarization in I-V curves for SDC-30 wt.%(0.67Li/0.33Na)2CO3 composite.
Fig. 8 e Electrochemical performances of direct ethanol SOFCs based on SDC-30 wt.% (0.53Li/0.47Na)2CO3 composite electrolyte using various anode materials at 600 C.
be blocked and carbonate ion conduction can be neglected due to extremely low CO2 concentration in air. However, the carbonate ion conduction may play an important role in the condition similar to the operation of molten carbonate fuel cell (MCFC) [22]. For both oxygen ion and carbonate ion conductions, water is formed at the anode side of the fuel cell. But for proton conduction, water is formed at the cathode side. According to water formation at both electrode sides during H2/air fuel cell operation, we discovered for the first time that the composite electrolytes could behave as co-ion (oxygen ion/proton) conductors or proton conductors depending on the salt content and composition [23]. Therefore, the composite electrolyte materials show superior ionic transport properties to single-phase oxide electrolyte materials which are more suitable for low temperature operation. Compatible electrode materials with high electrocatalytic activity at low temperatures have been developed for SDCecarbonate composite electrolytes [24]. Lithiated nickel oxide is selected as cathode material, since it is commonly
Fig. 9 e Short-term stability of a V20 mm single cell based on SDC-30 wt.% (0.67Li/0.33Na)2CO3 composite electrolyte fabricated by an integrated hot-pressing technique in H2/air operation.
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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 ) 8 7 7 e8 8 3
Table 1 e Advantages and disadvantages of possible electrolyte candidates for ILT-SOFCs developed in our lab. Electrolytes Advantages
Disadvantages
SDC Much higher oxygen ion conductivity (102 Scm1 at 600 C) and lower activation energy for ionic transport han those of YSZ Excellent phase stability in fuel cell environment and high temperature Good compatibility with cathode materials Electronic conduction at low pO2 in fuel gas atmosphere results in low OCV and mechanical instability
BSCO
SDC-carbonate composites
Remarkable proton conductivity (102 Scm1 at 600 C) and low activation energy for proton conduction Negligible electronic conductivity
Extremely high ionic (oxygen ion and proton) conductivity at low temperature (102e1 Scm1 at 400e600 C) Negligible electronic conductivity Availability and low price of carbonates Good compatibility with cathode materials
Chemical instability in H2O and CO2 environment at low temperature High synthesis temperature for pure phase oxide powders Lack of compatible cathode materials
Potential instability due to the volatilization loss of carbonates when operated above their melting points Gas crossover due to not fully densified composite electrolytes when operated far below the melting points of carbonates
used in MCFC adopting similar composition of alkaline carbonate as electrolyte. We first made the highest fuel cell performance using this cathode material. A typical fuel cell based on a 0.3 mm thick SDC-30 wt.%(0.67Li/0.33Na)2CO3 composite electrolyte was fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode [25]. With H2 as fuel and air as oxidant, a maximum power density of 1100 mW cm2 and 650 mW cm2 was achieved at 600 C and 500 C, respectively. Even at 400 C, the cell delivered a maximum power density more than 330 mW cm2. Single cells based on SDC-(0.53Li/0.47Na)2CO3 composite electrolytes, nickel oxide as anode and lithiated nickel oxide as cathode, were also fabricated by dry-pressing process [23]. The optimal performances of 1085 mW cm2 at 600 C and 690 mW cm2 at 500 C were achieved for the composite electrolytes containing 25 and 20 wt.% carbonates, respectively. These performances are much higher that the best performances ever reported for the fuel cells based on a thin doped CeO2 electrolyte at low temperature [26]. For the composite electrolyte fuel cells, the OCVs could reach 1.0 V or even higher value at 600 C, indicating that the electronic conductivity can be suppressed by the presence of salt phase in the composite electrolytes. Alternative anode materials, e.g. NieZn, NieSn and NieFe binary systems were developed for LT-SOFCs based on SDC-carbonate composite electrolytes. These anode materials exhibited excellent cell performances in H2 operation. For instance, a cell based on SDC30 wt.%(0.53Li/0.47Na)2CO3 composite electrolyte using NieSn based anode and lithiated nickel oxide cathode showed peak power density of 935 mW cm2 at 600 C and 713 mW cm2 at 500 C. In direct alcohol operation, these anode materials also showed good electrocatalytic activities. Fig. 8 shows the electrochemical performances of direct ethanol SOFCs using various anode materials. The maximum power densities at 600 C were 353, 306 and 223 mW cm2 for the cells using NieZn, NieSn and NieFe based anode materials, respectively. The cell performances in direct alcohol operation can be further improved by optimization of anode composition and microstructure as well as cell fabrication technique. In our group, an integrated hot-pressing technique was developed to fabricate larger-size single cells based on SDC-
carbonate composite electrolytes. This fabrication process involved the integration of metal mesh into single cell and the optimization of anode microstructure. The mechanical strength and roughness of integrated cells were significantly improved. A V20 mm single cell based on SDC-30 wt.%(0.67Li/ 0.33Na)2CO3 composite electrolyte was fabricated by this technique, using nickel oxide as anode and lithiated nickel oxide as cathode [27]. Fig. 9 shows the stability test of the cell at 550 C. It can be seen that a nearly steady output power of about 1.5 W for 2100 min was realized with an operating voltage of about 0.63 V, indicating good stability of the composite electrolyte in H2/air operation. The cell was tested between 500 and 600 C for 110 h and showed good stability, but it still requires assessment of the long-term stability caused by volatilization of the salts in fuel cell operation and the change of cell microstructure. To assure the materials stability, it is suggested that such fuel cells be operated below the melting point of the salts. The fabrication of larger-size (e.g. 60 mm 60 mm) single cells by a modified hotpressing technique is under progress. Based on them, planar LT-SOFC stacks with innovative sealing material will be assembled in the future.
5.
Summary and view
ILT-SOFCs using three kinds of new electrolyte materials and related electrode materials have been successfully developed in our lab. Table 1 gives an overview of the advantages and disadvantages of the different electrolyte materials. Singlephase oxides such as oxygen ion conducting doped CeO2 and proton conducting doped BaCeO3 are candidates as electrolyte for ILT operation, but their ionic conductivities need to be further improved. Doping with multiple elements may be effective to improve the ionic conductivity of these oxide electrolytes, but it is restricted by lattice structure and dopant concentration. Enhanced conductivity with several magnitudes is possibly realized by composite effect through interfacial conduction. Two-phase nanocomposites based on oxygen ion or proton conducting oxides are potential electrolyte materials for LT-SOFCs. Doped CeO2-carbonate
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 ) 8 7 7 e8 8 3
composites have shown mixed oxygen ion and proton conduction or pure proton conduction with enhanced conductivity at ILT. Another kind of prospective electrolyte materials is doped CeO2eBaCeO3 nanocomposites. These material systems represent a new research field that could be promising for LT-SOFC application. Mechanism understanding is the key for designing new composite electrolyte materials, and it relies on the development of advanced electrochemical characterization techniques. For practical application, cost-effective fabrication processes should be developed to prepare and scale-up the composite electrolytes.
Acknowledgments This work was financially supported by the National Basic Research Program of China (No. 2007CB209705), the National Natural Science Foundation of China (No. 50902083), EC FP6 NANOCOFC (No. SSA 032308), China Postdoctoral Science Foundation funded project (No. 20090450378) and the fundamental research funds for the central universities (No. 08142002).
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