O R I G I N A L P A P E R
Feng-Yun Wang Æ Soofin Cheng Æ Chen-Hung Chung Ben-Zu Wan
Y
2
O
3
and MgO co-doped ceria based electrolytes
Received: 2 January 2005 / Revised: 13 January 2005 / Accepted: 6 June 2005 / Published online: 29 July 2005 Springer-Verlag 2005
Abstract Electrolytes of Ce1-x-yYxMgyO2-0.5x-ywere
pre-pared with citrate method and were characterized by inductively coupled plasma-atomic emission spectrome-try, energy dispersive spectromespectrome-try, powder X-ray dif-fraction, and impedance spectroscopy. The effect of composition on the structure, conductivity, and stability of the electrolytes were investigated. When 0£ x £ about 0.2 and 0£ y £ about 0.05, the electrolytes were all single phase materials of ceria-based solid solution. However, when y> about 0.05, the electrolytes became two-phase materials, Y3+ and Mg2+ co-doped ceria-based solid solution and free MgO. The sample with nominal com-position of Ce0.815Y0.065Mg0.12O2-d showed ionic
con-ductivity at 973 K close to or even a little higher than that of similarly prepared Ce0.9Gd0.1O1.95, but had lower cost
of raw materials and a little better stability in reducing atmosphere. The existing of free MgO improved the sta-bility of the electrolytes in reducing atmosphere, but too much free MgO reduced the conductivity.
Keywords Solid electrolyte Æ Co-doped ceria Æ MgO Æ Y2O3Æ Conductivity
Introduction
Doped ceria systems have a long history of investiga-tion as potential materials for oxide ion conducting
electrolytes. While the materials have found applica-tions as oxygen sensor [1], oxygen concentrator cell [2] etc., the most attractive application would be as the electrolytes of intermediate temperature solid oxide fuel cells (ITSOFC) [3]. Although a wide variety of dopants have been shown to be effective in increasing oxide ionic conductivity of doped ceria, alkaline earth and rare earth cations, especially Gd3+ and Sm3+ are considered preferred dopants. The doped ceria with either Gd3+ or Sm3+ shows ionic conductivity much higher than any reported yttria-stabilized zirconia (YSZ) electrolyte in intermediate temperature (IT) range (773973 K). However, at low oxygen partial pressures with associated anodic conditions, the Ce4+ ion of the electrolytes can be partially reduced to Ce3+ [4]. As a result, electronic conduction is developed, which lessens the power output of fuel cells. To solve this problem and/or further improve the ionic con-ductivity, some co-doped ceria were studied. For example, Ce0.8Gd0.2-xPrxO2-d was found a little better
than Ce0.8Gd0.2O2-d in both anti-reduction and ionic
conductivity at 973 K [5]. Ce0.685Gd0.274Mg0.041O2-d
was found a little better than Ce0.685Gd0.315O2-d in
ionic conductivity at 996 K [6]. Although some pro-gresses have been made, the problem remains.
In addition to this problem, doped ceria being re-ported so far are still expensive. No matter singly doped or multiply doped ceria, the dopants were usually chosen from either rare earth cations or alkaline earth cations, or both of them, and the total content of the rare earth cations (including Ce4+) was usually more than 90 mol%. Since rare earth materials are expensive, the cost of raw materials of the doped ceria being reported so far are still high.
Therefore, it is necessary to study ceria-based elec-trolytes for better stability in reduction atmosphere, lower cost of raw materials, and even higher ionic con-ductivity. One possible way might be co-doping ceria with cheaper dopants, such as Mg2+and Y3+ or mak-ing composite materials with the co-doped ceria and free MgO. Although Mg2+ and Y3+, among other alkaline
F.-Y. Wang Æ S. Cheng (&) Æ C.-H. Chung
Department of Chemistry, National Taiwan University, Taipei, 106, Taiwan
E-mail: [email protected] Tel.: +886-2-23638017 Fax: +886-2-23636359 F.-Y. Wang (&)
Department of Chemistry, Foshan University, Foshan, 528000, P.R.China
E-mail: [email protected] B.-Z. Wan
Department of Chemical Engineering,
National Taiwan University, Taipei, 106, Taiwan DOI 10.1007/s10008-005-0030-x
earth cations and rare earth cations, have the size most close to that of Ce4+ion, and both MgO and Y2O3are
relatively cheap and stable, there is still a lack of sys-tematic study on Y3+and Mg2+ co-doped ceria mate-rials and their composite matemate-rials with free MgO.
In this work, Y3+ and Mg2+co-doped ceria and its composite materials with MgO were prepared and characterized. The effect of composition on the struc-ture, conductivity, and stability of the electrolytes were investigated.
Materials and methods Sample preparation
The starting materials were the nitrate salts of reagent grade (Acros) and used as purchased. The aqueous solutions of each metal ions of Y3+, Ce4+, and Mg2+ were prepared by dissolving the nitrate salt in distilled water and diluting them to desired concentrations. An aqueous solution of citric acid (CA) and polyethylene glycol (PEG) in a weight ratio of CA/PEG=60 was also prepared and termed as CP solution. According to the composition (Ce1-x-yYxMgyO2-d, wherein 0£ x £ 0.765,
0£ y £ 0.80) of the electrolyte samples, different vol-umes of metal ion solutions were taken and mixed in a beaker. Then, CP solution was added in the beaker until the molar number of citric acid was equal to the total number of the metal ions in the beaker. The mixed solution was evaporated under constant stirring at 353 K until the solution became gelled. The gel was dried at 378 K, ground and calcined in air at 973 K for 4 h, and then ground again to form calcined powder. The powder was uniaxially pressed under 750 MPa into green pellets using a stainless steel die with 13 mm diameter. The green pellets were then sintered in air at 1,773 K for 14 h with a heating rate of 1 K/min to form dense pellets (simplified as pellet hereafter).
For comparison, the pellet, having composition of Ce0.9Gd0.1O1.95(CGO), was also prepared by a process
analogous to the process described above.
Sample characterization
The crystal phases of the powder and the pellet (ground to powder again for measurement) samples were iden-tified at room temperature using a PANalytical X-ray diffractometer (XRD) (45 kV, 40 mA, Cu K a
radia-tion). The compositions of the pellet samples were analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-AES) (Jarrell-Ash, ICAP9000) and an energy dispersive spectrometer (EDS) (Hitachi S-2400 scanning electron microscope, Kevex Delta class 80000 EDS). Scanning electron microscopy (SEM) analysis of the cross-section of the pellets was conducted on a Hitachi S-2400 electron microscope.
Conductivity measurement
For conductivity measurement, Ag paste was brushed onto both sides of the pellet, then sintered at 1,073 K for 30 min to form Ag electrodes. Pt leads were attached to the electrodes using Ag paste and were sintered again at 1,073 K for 30 min. Impedance was measured using the two-probe method with an Autolab Impedance Ana-lyzer (30 mV, 1 MHz–0.01 Hz). The measurements were taken at constant temperature within 473–973 K and in air (25 sccm). Prior to data collection at each tempera-ture, the system was kept at the constant temperature and in given gas flow for 30 min. The conductivity data were obtained from the impedance plots by using the same method as that described in details in the literature [7]. In the following context, the word ‘‘conductivity’’ is used to represent the total conductivity (St) of grain and
grain boundary.
The stability of electrolytes in 10% H2/N2was tested
at 973 K by monitoring the change of conductivity with time in the given atmosphere.
It has been documented that the main contribution of the conductivity of ceria-based compounds in air is oxide ionic conductivity (>99.5%) and that from elec-tronic conductivity is negligible [3,7]. In this study, the conductivity measured in air was treated as the oxide ionic conductivity only.
Results and discussion Compositions
The compositions of pellet samples were analyzed with EDS and ICP. For ICP analysis, the samples were ground to powder form and dissolved in the mixed solution of hydrofluoric acid and nitric acid. The results of two typical samples were listed in Table1. The compositions analyzed by both techniques were close to
Table 1 Chemical compositions
of the pellet samples Sample Ce:Gd:Y:Mg atomic ratio
Designed EDS ICP
Ce0.90Gd0.10O1.95 0.9:0.1:0:0 0.907:0.093:0:0 0.906:0.094:0:0
the nominal values, indicating that the effect of multi-step preparation, which included the high temperature sintering, on the compositions of the pellet samples is negligible. Therefore, it is reliable to use the nominal composition data of the pellet samples directly.
Crystal structure
For all the electrolyte samples of this work, it was found that calcined samples showed the same XRD patterns as the sintered samples of the same compositions, except that the XRD peaks were less sharp for the former than for the latter. This implies that the sintering process just
caused the increase of crystal size and the increase of pellet compaction.
Figure1 shows the XRD patterns of pellet samples with the nominal composition of Ce0.935-yY0.065
M-gyO1.9675-y, wherein 0 £ y £ 0.80. Here, quartz was used
as inner standard. It seems that all the samples have the same XRD patterns as pure ceria. However, when the XRD patterns were amplified to a sufficient extent, a minor peak of free MgO at about 43 was observed for the samples with y>0.05. This result indicates that the samples with y<0.05 are single-phase materials of ceria-based solid solution; whereas, the samples with y>0.05 are two phase materials, Y and Mg codoped ceria solid solution and free MgO. Similarly, it has been reported in
Fig. 1 XRD patterns of the pellet samples with nominal composition of Ce 0.935-yY0.065MgyO1.9675-y, wherein
0£ y £ 0.80, with quartz as inner standard and Cu as target
Fig. 2 Dependence of the lattice constant of the ceria-based solid solution phase on the total Mg content (y) of the pellet samples with nominal composition of Ce 0.935-yY0.065MgyO1.9675-y
the literature [3] that the solubility of MgO in ceria was about 2 mol%.
Figure2shows the dependence of lattice constant of ceria-based solid solution phase on the total Mg content (y) of the pellet samples with the nominal composition of Ce0.935-yY0.065MgyO1.9675-y, wherein 0 £ y £ 0.80. With y
increasing, the lattice constant decreased weakly in the range of y<0.5, but dramatically in the range of y>0.5. It has been reported [3] that the lattice constant of Mg doped CeO2 hardly changed with Mg content in the
range of 0–50 mol%, and that the lattice constant of Y doped CeO2apparently decreased with the increase of Y
content. Therefore, for our work, the increase of y in the range of y<0.5 did not apparently affect the lattice constant just because Mg has a very little solubility in CeO2. However, when y>0.5, the large increase of y
caused a large increase of Y/Ce ratio, and in turn caused an apparent decrease of the lattice constant.
In order to investigate the effect of Y content (x) and at the same time to avoid the effect of oxygen content, a set of samples were prepared with the nominal compo-sition of Ce0.618-0.5xYxMg0.382-0.5xO1.618, wherein
0£ x £ 0.765. Figure3 shows the XRD patterns of these pellet samples with quartz as inner standard. It seems that all the samples had the XRD patterns similar to that of pure ceria. However, when the XRD patterns were amplified to a sufficient extent, free MgO peak (at about 43) was observed for the samples with x=0–0.2, but no Y2O3 peak was observed for the samples with
x=0.765. This result indicates that the samples with x=0–0.2 are two-phase materials of free MgO and ceria based solid solution. Since the solubility of Y2O3 in
CeO2has been reported to be less than 58 mol% [3], the
sample with x=0.765 might also be a two-phase mate-rial of Y2O3and ceria-based solid solution.
Conductivities
Effect of total Mg content
Figure4 shows the effect of total Mg content (y) on the conductivities of the samples with nominal com-position of Ce0.935-yY0.065MgyO1.9675-y, wherein
0£ y £ 0.35, in air and at 873973 K. The conduc-tivity changed with y like a mountain and at y= about 0.12 reached the maximum of 0.014 S cm 1for 973 K, higher than the value of CGO (0.006 S cm 1), which is
Fig. 3 XRD patterns of the pellet samples with nominal compo-sition of Ce0.6175-0.5xYxMg0.3825-0.5xO1.6175, wherein x=00.765,
with quartz as inner standard, and Cu as target
Fig. 4 The effect of total Mg content (y) on the conductivities of the pellet samples with nominal composition of Ce 0.935-yY0.065MgyO1.9675-y, in air and
between the values in reference [9] (0.0001 S cm 1) and reference [10] (0.013 S cm 1). Similarly, in the study [11] on Mg doped ceria electrolytes, it was reported that the conductivity at 1,073 K reached its maximum at about 10 mol% of total Mg content. Figure 5shows the SEM micrographs of representative pellet samples with different total Mg content (y). Apparently, the grain sizes of all the samples are smaller than 3 lm, and decrease with the total Mg content (y) increasing. It has been reported [7] that when the mean grain size of Ce0.8Gd0.2O2-d is smaller that 3 lm, both the grain
and grain boundary conductivities, especially the latter, increase with grain size decreasing. Analogously, for our results, the decrease of grain size with the increase of total Mg content might cause the increase of total conductivity. On the other hand, the large increase of total Mg content would lead to the large increase of free MgO, which would lead to the decrease of total conductivity because free MgO has much lower
con-ductivity than doped CeO2. These two opposite effects
of total Mg content on conductivity might cause the maximum conductivity emerging at y= about 0.12 (see Fig.4).
Effect of Y dopant
Figure6 shows the conductivities in air of the two-phase pellet samples with nominal compositions of Ce0.585Y0.065Mg0.35O1.618 and Ce0.65Mg0.35O1.65. For
comparison, the data of CGO were also included. It can be seen that the existing of dopant Y3+ was vital for the samples to have high conductivity, and that the conductivity of the Y-containing sample was close to that of CGO. Similarly, it had been reported that the conductivity of Ce0.9Y0.1O1.95 at 1,023 K in air
was close to that of CGO [12], and that the conduc-tivity of Ce0.6Mg0.4O1.6 was close to that of pure CeO2
[11].
Fig. 5 SEM micrographs of pellet samples with nominal composition of Ce 0.935-yY0.065MgyO1.9675-y, a y=0, b
Effect of Y content
In order to investigate the effect of Y content (x) and at the same time to avoid the effect of oxygen content, the conductivities of the pellet samples with the nominal composition of Ce0.618-0.5xYxMg0.382-0.5xO1.618, wherein
0 £ x £ 0.765, were measured in air and at different temperatures. This experiment was repeated two times, and the relative deviation of conductivity was found less than 5%. The average results were shown in Fig.7. It can be seen that the sample with x=0.065 was the best, it had lowest activation energy of conduction and the highest conductivity, close to that of CGO. Similarly, it
has been reported that the maximum conductivity of Y3+doped ceria electrolyte at 460 K emerging at the Y content of about 7 mol% [3].
Stability in 10% H2/N2
Any changes of an electrolyte sample in structure, composition, valence state etc. will be reflected in its changes in conductivity, therefore, the stability of an electrolyte can be tested by monitoring its conductivity as a function of run time under given conditions. Fig-ure8shows the total conductivities (St) of pellet samples
(Ce0.815Y0.065Mg0.12O2-a, CGO, and Ce0.935Y0.065O2-b) in
10% H2/N2and at 973 K as a function of run time. The
Fig. 6 The effect of temperature on the conductivities of different pellet samples in air with nominal composition of (square) Ce0.65Mg0.35O1.65, (circle) Ce0.585Y0.065Mg0.35O1.618, and (triangle)
CGO
Fig. 7 The effect of temperature on the
conductivities of CGO and the pellet samples in air with nominal composition of Ce 0.618-0.5xYxMg0.382-0.5xO1.618,
wherein 0£ x £ 0.765
Fig. 8 Changes of total conductivities (St) of different pellet
total conductivities of the samples all jumped up when the gas was switched from air to 10% H2/N2, then the
conductivity quickly approached constant for the first and CGO samples, but decreased considerably with time for the third sample. This result suggests that the exist-ing of MgO benefited the stability of the electrolyte. The same experiment was repeated two times and the relative deviation of conductivity was found less than 8%.
For the sample of Ce0.935Y0.065O2-bin Fig. 8,
reduc-tion process proceeded with time continuously. In the first reduction period, a few Ce3+ were formed in the ceria lattice, which caused not only the dramatic increase of electronic conductivity, but also a little increase of oxide ion vacancy, and possibly, as a result, a little in-crease of ionic conductivity. Therefore, the total con-ductivity increased rapidly and soon reached its maximum. However, with further reduction going on, Ce2O3 phase might be formed gradually, which caused
the decrease of total conductivity because Ce2O3 has
much lower conductivity than doped ceria [8].
For the sample of Ce0.815Y0.065Mg0.12O2-a in Fig.8,
MgO exists in two different states. One is as dopant in the ceria lattice, which could not only increase the number of oxide ion vacancy, and consequently increase the ionic conductivity, but also improve the thermody-namic stability of the doped ceria because MgO is much more stable than rare earth oxides especially in reducing environment. Another is free MgO, which is distributed on the crystal surface of the doped ceria. These fine free MgO could, like a cover, decrease the contact area be-tween doped ceria particle surface and reducing gas, and therefore, slow down the reducing process of the doped ceria. In the first period of the reducing process, a few Ce3+ were formed in the ceria lattice, which caused a rapid increase of total conductivity, similar to that of Ce0.935Y0.065O2-b. However, in the rest period of the
reducing process, the co-effect of both MgO might cause the following phenomena. When the sample was reduced to a certain degree before the formation of Ce2O3phase,
the system almost approached thermodynamic equilib-rium. Therefore, the further reduction of Ce4+ ap-proached stop, and the total conductivity of the sample approached constant.
Since MgO is much cheaper than the rare earth oxi-des, higher content of MgO in the electrolytes also lowers the cost of raw materials of the electrolytes.
Conclusions
Electrolytes with nominal composition of Ce1-x-yYx
M-g
yO2-0.5x-y were prepared and characterized. When
0£ x £ 0.2 and 0 £ y £ about 0.05, the electrolytes were all single phase materials of ceria-based solid solution. However, when y> about 0.05, the electrolytes became two-phase materials, Y3+ and Mg2+ co-doped ceria-based solid solution and free MgO. The sample with nominal composition of Ce0.815Y0.065Mg0.12O2-d
showed ionic conductivity (0.014 S cm 1) at 973 K close to that of similarly prepared Ce0.9Gd0.1O1.95(CGO), but
had lower cost of raw materials and a little higher sta-bility in reducing atmosphere. The existing of free MgO seemed to improve the stability of the electrolytes in reducing atmosphere, but too much free MgO would reduce the conductivity.
Acknowledgements The financial support from CTCI Foundation, Taiwan is gratefully acknowledged. The authors also like to thank all the members in Prof. Ben-Zu Wan’s laboratory, Department of Chemical Engineering, National Taiwan University, for experi-mental help and valuable discussion.
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