Facile conversion of layered ruddlesden-popper-related structure y 2O3-doped Sr2CeO4 into fast oxide ion-conducting fluorite-type y2O3-doped CeO2

Article abstract of DOI:10.1021/ic801729x

The present work shows a new solid- and gas-phase reaction technique for the preparation of a fast oxide-ion-conducting Y₂O₃-doped Ce_1-x-Y_xO_2-δ (x = 0.1, 0.2) (YCO), which involves the reaction of lay

Full text of DOI:10.1021/ic801729x

Inorg. Chem. 2009, 48, 257-266
Facile Conversion of Layered Ruddlesden-Popper-Related Structure
Y2O3-Doped Sr2CeO4 into Fast Oxide Ion-Conducting Fluorite-Type
Y2O3-Doped CeO2
Ryan Georg Gerlach, Surinderjit Singh Bhella, and Venkataraman Thangadurai*
UniVersity of Calgary, Department of Chemistry, 2500 UniVersity DriVe NW,
Calgary, Alberta, T2N 1N4, Canada
Received September 8, 2008
The present work shows a new solid- and gas-phase reaction technique for the preparation of a fast oxide-ion-
conducting Y2O3-doped Ce1-xYxO2-δ (x ) 0.1, 0.2) (YCO), which involves the reaction of layered (Ruddlesden-Popper
K2NiF4-type) structure Y2O3-doped Sr2CeO4 (YSCO) with CO2 at an elevated temperature and subsequent acid-
washing. A powder X-ray diffraction study revealed the formation of a single-phase cubic fluorite-type YCO for the
CO2-reacted and subsequent acid-washed product. Energy dispersive X-ray analysis showed the absence of Sr in
the CO2-treated and subsequent acid-washed product, confirming the transformation of layered YSCO into YCO.
The cubic lattice constant was found to decrease with increasing Y content in YCO, which is consistent with the
other YCO samples reported in the literature. The scanning electron microscopy study showed smaller-sized particles
for the product obtained after CO2- and acid-washed YCO samples, while the high-temperature sintered YCO and
the precursor YSCO exhibit larger-sized particles. The bulk ionic conductivity of the present CO2-capture-method-
prepared YCO exhibits about one and half orders of magnitude higher electrical conductivity than that of the
undoped CeO2 and was found to be comparable to those of ceramic- and wet-chemical-method synthesized rare-
earth-doped CeO2.
Introduction
fluorite-type Y2O3-doped ZrO2 (YSZ) oxide ion electrolyte,
which can be operated in a temperature range of about
800-1000 °C.2,4,5 The YSZ-based SOFC exhibits several
disadvantages that are attributed to the high temperature of
operation. They are detrimental as mechanical stress is
induced because of different thermal expansion coefficients
of the electrolyte and electrodes, interfacial diffusion of Sr
and La from the cathode to the electrolyte, a lack of
appropriate sealing materials, and a high cost of bipolar
separators.2,3 Therefore, there is a great deal of interest in
developing new materials with desired physical, chemical,
and mechanical properties for the development of advanced
intermediate temperature (IT) SOFCs (500-750 °C).9-11
A number of inorganic crystal structures that include
perovskites, fluorites, pyrochlores, perovskite-related brown-
millerites, layered perovskite (K2NiF4-related), and Aurivilius
Oxide ion electrolytes are ionic conductors in which the
current is carried by the migration of oxide ions through the
oxide ion vacancies.1 They find applications in several solid-
state ionic devices such as solid oxide fuel cells (SOFCs),
electrolysis, gas sensors (e.g., O2, H2, CH4, etc.), and oxygen
pumps.2-8 Presently, SOFC technology has generated im-
mense interests due to its high efficiency and clean conver-
sion of energy and potential to be powered by a wide variety
of fuels (e.g., H2, NH3, hydrocarbons, CO). The current
SOFC commercialization is mainly based on the conventional
* Author to whom correspondence should be addressed. Phone: 001 403
210 8649. E-mail: vthangad@ucalgary.ca.
(1) Goodenough, J. B. Annu. ReV. Mater. Res. 2003, 33, 91–128.
(2) Minh, N. Q. J. Am. Ceram. Soc. 1993, 76, 563–588.
(3) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5–39.
(4) Singhal, S. C. Solid State Ionics 2002, 152-153, 405–410.
(5) Ormerod, R. M. Chem. Soc. ReV. 2003, 32, 17–28.
(6) Weber, A.; Ivers-Tiffee, E. J. Power Sources 2004, 127, 273–283.
(7) Yano, M.; Tomita, A.; Sano, M.; Hibino, T. Solid State Ionics 2007,
177, 3351–3359.
(9) Kendrick, E.; Kendrick, J.; Knight, K. S.; Islam, M. S.; Slater, P. R.
Nat. Mater. 2007, 6, 871–875.
(10) Kuang, X.; Green, M. A.; Niu, H.; Zajdel, P.; Dickinson, C.; Claride,
J. B.; Jantsky, L.; Rosseinsky, M. J. Nat. Mater. 2008, 7, 498–504.
(11) Brett, D. J. L.; Atkinson, A.; Brandon, N. P.; Skinner, S. J. Chem.
Soc. ReV. 2008, 37, 1568–1578.
(8) Ni, M.; Leung, M. K. H.; Leung, D. Y. C. Int. J. Hydrogen Energy
2008, 33, 2337–2354.
10.1021/ic801729x CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 1, 2009 257
Published on Web 12/05/2008

Gerlach et al.
phases and apatites are being considered for the development
of fast oxide ion electrolytes with superior functional
properties for applications in IT-SOFCs.12-21 Among them,
the fluorite-type structured rare-earth (RE)-doped CeO2
materials have been investigated as a potential electrolyte
for IT-SOFCs.22-26 For example, Y-, Sm-, and Gd-doped
CeO2 show a bulk oxide ion conductivity of about 10-2 S/cm
above 500 °C, which is about 15 times higher than that of
YSZ at 500 °C and about 4 times higher than that above
750 °C.22 There are several synthesis methods, including
conventional ceramic (solid-state) and soft-chemical methods
such as glycine-nitrate,27 coprecipitation,28 and polymeriza-
tion,29 that are employed to prepare doped ceria electrolytes.
These preparation methods play a significant role in deter-
mining the microstructure and particle size distributions,
which influence the electrical conductivity,30-33 and particle-
size-dependent conductivity was explained using the space
charge model.34,35
type structure precursors, we have prepared novel meta-stable
In-doped CeO2 and Ca+Sm-doped CeO2 at 800 °C using
the new CO2 capture method.38 It is pertinent to mention
here that our attempt to prepare single-phase In-doped CeO2
using a ceramic (solid-state) method at elevated temperatures
was unsuccessful.26,38 Unlike BaCeO3,37a the Ba-containing
double perovskite-like Ba3Ca1+xNb2-xO
9-δ (BCN)37c and Ta-
doped BCN exhibit excellent chemical stability in 100% CO2
at 800 °C and in boiling water for a long period of time.39
With our own continuing interest in understanding the
thermodynamic stability of alkaline-earth-containing metal
oxides possessing three-dimensional perovksite and layered
(two-dimensional) perovskite-related structures in CO2, we
have investigated the chemical and structural stability of a
new layered-structure Y2O3-doped Sr2CeO4 (YSCO). Our
work clearly demonstrates the formation of fluorite-type
oxide-ion-conducting YCO from the layered perovskite-
related YSCO by reaction with CO2 at an elevated temper-
ature and subsequent acid-washing. The parent Sr2CeO4 is
a well-known blue-color-emitting phosphore, and its crystal
structure and thermodynamic stability have been character-
ized.40-43 Also, the present work shows a path for a new
precursor route synthesis of doped CeO2 materials by
designing appropriate precursor compounds that are reactive
in CO2 at elevated temperatures.
Recently, we have re-examined the chemical stability of
Y-doped BaCeO3 in CO2 at a temperature range of 600-1000
°C.36 Our investigation was found to be consistent with the
literature37 on the formation of BaCO3 and fluorite-type
Y2O3-doped CeO2 (YCO). During the course of our studies
toward a further understanding of the formation of fluorite-
type doped CeO2 oxide ion electrolytes from the perovskite-
Experimental Section
(12) Estell, T. H.; Flengas, S. N. Chem. ReV. 1970, 70, 339–376.
(13) Kendall, K. R.; Navas, C.; Thomas, J. K.; Zur Loye, H. C. Solid State
Ionics 1995, 82, 215–223.
Preparation and Characterization. Sr2Ce0.9Y0.1O3.95 and
Sr2Ce0.8Y0.2O3.9 were prepared by a conventional ceramic (solid-
state) method using stoichiometric amounts of CeO2 (>99.9% Alfa
Aesar), Y2O3 (>99.9% Alfa Aesar; dried at 800 °C in the air and
cooled to room temperature in a desiccator), and SrCO3 (99% Alfa
Aesar) at elevated temperatures in the air. The desired amount of
2-propanol was added to the starting materials in a zirconia bowl
and was ball-milled (Pulverisette, Fritsch, Germany) for about 2 h
at 150 rotations per minute using zirconia balls. The sample was
then taken out of the ball mill, dried in ambient air, and placed in
a clean alumina crucible and heated at 800 °C for 12 h. The resultant
product was then reground and sintered at 1350 °C for 24 h with
a heating and cooling rate of 10 °C per min. In the final stage of
heat treatment, the reaction products were pressed into pellets by
isostatic pressure. The sintered pellets were crushed into powder
for phase characterization employing a powder X-ray diffractometer
(PXRD; Bruker D8 powder X-ray diffractometer (Cu KR, 40 kV,
40 mA)).
(14) Kendall, R.; Navas, C.; Thomas, J. K.; Zur Loye, H. C. Chem. Mater.
1996, 8, 642–649.
(15) Inaba, H.; Tagawa, H. Solid State Ionics 1996, 83, 1–16.
(16) Shuk, P.; Wiemho¨fer, H.-D.; Guth, U.¨; Go¨pel, W.; Greenblatt, M.
Solid State Ionics 1996, 89, 179–196.
(17) Boivin, J. C.; Mairesse, G. Chem. Mater. 1998, 10, 2870–2888.
(18) Kharton, V. V.; Marques, F. M. B.; Atkinson, A. Solid State Ionics
2004, 174, 135–145.
(19) Wincewicz, K. C.; Cooper, J. S. J. Power Sources 2005, 140, 280–
296.
(20) Fergus, J. W. J. Power Sources 2006, 162, 30–40.
(21) Kendrick, E.; Islam, M. S.; Slater, P. R. J. Mater. Chem. 2007, 17,
3104–3111.
(22) Steele, B. C. H. Solid State Ionics 2000, 129, 95–110.
(23) Park, S.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265–266.
(24) Hibino, T.; Hashimoto, A.; Inoue, T.; Tokuno, J. I.; Yoshida, S. I.;
Sano, M. Science 2000, 288, 2031–2033.
(25) Brandon, N. P.; Blake, A.; Corcoran, D.; Cumming, D.; Duckett, A.;
El-Koury, K.; Haigh, D.; Kidd, C.; Leah, R.; Lewis, G.; Matthews,
C.; Maynard, N.; Oishi, N.; McColm, T.; Trezona, R.; Selcuk, A.;
Schmidt, M.; Verdugo, L. J. Fuel Cell Sci. Technol. 2004, 1, 61–65.
(26) Pearce, M. C.; Thangadurai, V. Asia-Pacific J. Chem. Eng. 2008, in
press.
(27) Xia, C.; Liu, M. Solid State Ionics 2002, 152-153, 423–430.
(28) Ding, D.; Liu, B.; Zhu, Z.; Zhou, S.; Xia, C. Solid State Ionics 2008,
179, 896–899.
(29) (a) Biswas, M.; Prabhakaran, K.; Gokhale, N. M.; Sharma, S. C. Mater.
Res. Bull. 2007, 42, 609–617. (b) Lu, C.; Worrell, W. L.; Gorte, R. J.;
Vohsb, J. M. J. J. Electrochem. Soc. 2003, 150, A354–A358.
(30) Kosacki, I.; Suzuki, T.; Petrovsky, V.; Anderson, H. U. Solid State
Ionics 2000, 136-137, 1225–1233.
(31) Tschope, A.; Sommer, E.; Birringer, R. Solid State Ionics 2001, 139,
255–265.
(32) Suzuki, T.; Kosacki, I.; Anderson, H. U. Solid State Ionics 2002, 151,
111–121.
(37) (a) Gopalan, S.; Virkar, A. V. J. Electochem. Soc. 1993, 140, 1060–
1065. (b) Tanner, C. W.; Virkar, A. V. J. Electrochem. Soc. 1996,
143, 1386–1389. (c) Bhide, S. V.; Virkar, A. V. J. Electrochem. Soc.
1999, 146, 4386–4392. (d) Ryu, K. H.; Haile, S. M. Solid State Ionics
1999, 125, 355–367. (e) Azad, A. K.; Irvine, J. T. S. Solid State Ionics
2007, 178, 635–64. (f) Zakowsky, N.; Williamson, S.; Irvine, J. T. S.
Solid State Ionics 2005, 176, 3019–3026. (g) Matsumoto, H.; Ka-
wasaki, Y.; Ito, N.; Enoki, M.; Ishihara, T. Electrochem. Solid-State
Lett. 2007, 10, B77-B80.
(38) Trobec, F.; Thangadurai, V. Inorg. Chem. 2008, 47, 8972–8984.
(39) Bhella, S. S.; Thangadurai, V. J. Power Sources 2008, DOI: 10.1016/
j.jpowsour.2008.09.110.
(33) Surble, S.; Baldinozzi, G.; Dolle, M.; Gosset, D.; Petot, C.; Petot-
Ervas, G. Ionics 2008, 14, 33–36.
(34) Tschope, A. Solid State Ionics 2001, 139, 267–280.
(35) Tschope, A.; Kilassonia, S.; Briringer, R. Solid State Ionics 2004, 173,
57–61.
(36) Sneha, S. B.; Thangadurai, V. J. Solid State Chem. 2007, 180, 2661–
266.
(40) Danielson, E.; Devenney, M.; Giaquinta, D. M.; Golden, J. H.;
Haushalter, R. C.; McFarland, E. W.; Poojary, D. M.; Reaves, C. M.;
Weinberg, W. H.; Wu, X. D. J. Mol. Struct. 1998, 470, 229–235.
(41) Hirai, T.; Kawamura, Y. J. Phys. Chem. B 2004, 108, 12763–12769.
(42) Shirsat, A. N.; Kaimal, K. N. G.; Bharadwaj, S. R.; Das, D. Therm.
Chim. Acta 2006, 447, 101–105.
(43) Lu, C. H.; Chen, C. T. J. Sol-Gel. Sci. Technol. 2007, 43, 179–185.
258 Inorganic Chemistry, Vol. 48, No. 1, 2009

Facile ConWersion Y2O3-Doped Sr2CeO4 into Y2O3-Doped CeO2
Figure 1. PXRD patterns of (a) Sr2Ce0.9Y0.1O3.95, (b) Sr2Ce0.8Y0.2O3.9 and (c) parent Sr2CeO4.40 Parts d-f show the corresponding selected area PXRD. We
see that all of the observed diffraction peaks match well with the expected Sr2CeO4 structure.
A known quantity (about 25 g) of YSCO powder was reacted in
100% CO2 at 600-800 °C for 12 h. After the reaction, the sample
was cooled to room temperature (RT) while passing CO2. The
weight gain in the sample was measured using an analytical balance.
Then, the resultant product was washed thoroughly using a 1:5 HCl-
to-water mixture. Acid was added until stopped CO2 evolved, and
the sample was filtered, washed thoroughly with water, and dried
in an ambient atmosphere at RT. The resultant powder sample was
characterized using PXRD, scanning electron microscopy (SEM),
and energy dispersive X-ray analysis (EDX; Philips XL 30). The
lattice constant was determined from PXRD data by least-squares
refinement.
Electrical Conductivity Study. Electrical conductivity measure-
ments of the CO2-capture-method-prepared YCO pellets (∼0.15
Figure 2. Comparison of crystal structure of Ruddlesdon-Popper (RP)
cm in thickness and ∼1 cm in diameter) of the acid-washed powder
phase K2NiF4-related layered Sr2TiO4 (a) with structure of Sr2CeO4 (b).
The balls represent Sr atoms. Parts c and d show the sharing of the single
(sintered at 1350 °C for 24 h in an ambient atmosphere) were
performed in the air, N2, and Ar using Pt electrodes (Pt paste
octahedral layers in Sr2TiO4 and Sr2CeO4, respectively. The octahedral units
are edge-shared in Sr2CeO4,40 while they are corner-connected in
obtained from Heraeus Inc., LP A88-11S, Germany, cured at 800
°C in the air for 1 h to remove the organic binders) and employing
AC impedance and a gain-phase analyzer (SI model no. 1260; 0.01
Hz to 10 MHz). A two-probe electrochemical cell was used. The
applied AC amplitude was 100 mV. Before each electrical
measurement, the sample was equilibrated at a constant temperature
for at least 1 h. The measurements were made for two subsequent
heating and cooling runs.
Sr2TiO4.44,45
included in Figure 1, and it was found that the doped sample
shows a similar PXRD pattern.40,43 Also, we see that Y2O3-
doped Sr2CeO4 samples show a slight shift in the peak
position compared to the parent compound. There were no
major diffraction peaks observed due to potential impurity
in the SrO-CeO2-Y2O3 system. It is very interesting to note
that the crystal structure of Sr2CeO4 has a certain similarity
to that of a well-known Ruddlesden-Popper-type (RP)
layered structure Sr2TiO4 (K2NiF4-like).44,45 In Figure 2, we
compare the crystal structures of the parent Sr2CeO4 and
Results and Discussion
Phase and Microstructure. The PXRD study showed the
formation of a single-phase Sr2CeO4 structure for all of the
investigated YSCO samples. Figure 1 shows the PXRD
patterns of Sr2Ce0.9Y0.1O3.95 and Sr2Ce0.8Y0.2O3.9. For com-
parison, the PXRD data of the parent Sr2CeO4 are also
(44) Ruddlesden, S. N.; Popper, P. Acta Crystallogr. 1957, 10, 538–539.
(45) Ruddlesden, S. N.; Popper, P. Acta Crystallogr. 1958, 11, 54–55.
Inorganic Chemistry, Vol. 48, No. 1, 2009 259

Gerlach et al.
Figure 3. PXRD showing the conversion of K2NiF4-related layered
structured Y-doped Sr2CeO4 into fluorite-type structure 10 mol % Y-doped
CeO2 (a) as-prepared Sr2Ce0.9Y0.1O3.95, (b) sample “a” heated to 800 °C in
CO2 for 12 h, (c) sample “b” washed with dilute acid and subsequently
dried in ambient air, and (d) sample “c” sintered at 1350 °C for 24 h in the
air. The main diffraction peaks due to SrCO346-49 are marked as “x” in b.
Figure 4. PXRD showing the conversion of K2NiF4-related layered
Y-doped Sr2CeO4 into fluorite-type structure 20 mol % Y-doped CeO2 (a)
as-prepared Sr2Ce0.8Y0.2O3.9, (b) sample “a” heated to 800 °C in CO2 for
12 h, (c) sample “b” washed with dilute acid and subsequently dried in
ambient air, and (d) sample “c” sintered at 1350 °C for 24 h in the air. The
46-49
main diffraction peaks due to SrCO3
are marked as “x” in b.
Sr2TiO4. The lattice parameters are related as aSr CeO
≈
2
4
ꢀ2aSr TiO ; bSr CeO ≈ ꢀ2aSr TiO , and cSr CeO ≈ cSr TiO . Both
2
4
2
4
2
4
2
4
2
4
Sr2CeO4 and Sr2TiO4 contain MO6 (M ) Ce, Ti) octahedral
layers, which are separated by an alkaline layer.40,43-45
However, the main difference is that in Sr2CeO4 the adjacent
MO6 octahedral units are connected via an edge, while in
Sr2TiO4, they are corner-sharing. The Ce-Ce distance in
Sr2CeO4 is 3.5 Å,40,43 which is about 0.5 Å lower than the
Ti-Ti distance in the RP phase Sr2TiO4.44,45 Alkali and
proton-containing layered perovskite-type metal oxides,
including RP and the Dion-Jacobson phase, exhibit very
interesting soft-chemical properties.46-48 Accordingly, our
future work will be directed to the substitution of alkali ions
and protons and the trivalent cation for Sr in Sr2CeO4, which
is likely to show similar technologically relevant chemical
and physical properties.
Figure 5. Comparison of the experimental lattice constants of Y2O3-doped
CeO2 with theoretical lattice parameter calculated using eq 2.54 The observed
lattice constant falls within the range reported for the same composition
prepared by ceramic or soft-chemical methods.54,55
Figure 3 shows the PXRD patterns for the conversion of
10 mol % YSCO into the corresponding YCO, which can
be expressed using the chemical reaction
consistent with proposed reaction 1. About 80% of CO2
capture per ceramic gram of material was observed experi-
mentally at 800 °C. This variation in the CO2 capturing value
may be attributed to the desorption of CO2 and also may be
due to potential decomposition of the carbonates at elevated
temperatures. It is also mentioned here that the CO2 capturing
ability of these materials was found to vary with the amount
of the YSCO compound placed into the CO2 reactor and
with the temperature of the reaction. Samples were reheated
in CO2 to ensure the completion of decomposition reaction
1. Interestingly, the PXRD study of the acid-washed sample
shows a single-phase fluorite-type structure with a cell
constant of 5.412(1) Å for the x ) 1 member of YCO (Figure
3c). The formation of 20 mol % Y2O3-doped CeO2 from the
corresponding YSCO is shown in Figure 4 (a ) 5.400(3) Å
for the acid-washed sample for x ) 0.2 member).
Sr2(Ce, Y)O4-δ 2CO , 800 °C8 2SrCO3 +
2
(Ce, Y)O2-δ Dilute HCl8 (Ce, Y)O2-δ (1)
-2SrCO3
The exposure to CO2 at elevated temperatures shows a
complex diffraction pattern due to the presence of Sr-
CO349-52 and the fluorite-type CeO2 (Figure 3b).26,36,38 The
weight gain after the CO2 reaction was found to be close to
about 2 mol of CO2 per formula of Sr2CeO4, which is
(46) Gopalakrishnan, J. Chem. Mater. 1995, 1265–1275.
(47) Bhuvanesh, N. S. P.; Gopalakrishnan, J. J. Mater. Chem. 1997, 7,
2297–2306.
(48) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2002, 14, 1455–1471.
(49) Guth, U.; Barwisch, F.; Wulff, H.; Mobius, H. H. Cryst. Res. Technol.
1986, 21, 431–435.
(50) Cao, M.; Wu, X.; He, X.; Hu, C. Langmuir 2005, 21, 6093–6096.
(51) Yu, J.; Guo, H.; Cheng, B. J. Solid State Chem. 2006, 179, 800–803.
(52) Shi, L.; Du, F. Mater. Lett. 2007, 61, 3262–3264.
Figure 5 shows the theoretical lattice constant for
Ce1-xYxO2-δ (CYO) together with the literature data.54,55 The
260 Inorganic Chemistry, Vol. 48, No. 1, 2009

Facile ConWersion Y2O3-Doped Sr2CeO4 into Y2O3-Doped CeO2
structure employing the CO2 capturing method.36,38 In the
present work, we have used the two-dimensional layered-
structured YSCO as a precursor to prepare fast oxide-ion-
conducting YCO. Furthermore, the lattice constant of YCO falls
within the range reported for Y2O3-doped CeO2 prepared by
other methods (Figure 5), suggesting that the Y2O3-doped
fluorite compound can be obtained just by the CO2 reaction at
elevated temperatures.54,55 We speculate that one could prepare
novel fluorite-type structure materials by designing appropriate
precursor compounds that are reactive in CO2 and form
carbonate at elevated temperatures.
The SEM investigation on powdered samples was carried
out to understand the particle size distributions as well as the
morphology. Figures 8 and 9 show typical SEM images
corresponding to the PXRD results shown in Figures 3 and 4,
respectively. A distinct difference was found in the particle size
and morphology of the as-prepared precursor, CO2-treated, acid-
washed product, and the sintered samples. Doped Sr2CeO4
precursor samples showed large-sized particles compared to
those of CO2 treated materials. As expected, the CO2-reacted
and subsequent acid-washed product shows much smaller-sized
particles. The sintered fluorite compounds show a significant
growth in the particle size, which may be due to particle growth
at elevated temperatures. This observation is consistent with
our previous study.36
Table 1. Compound, Lattice Constant, and Electrical Properties of
CYO Prepared Using CO2 Capture Method
compound,
cell constant
capacitance
T (°C)
σ(bulk) (S/cm)
(F) × 10-10
Ce0.9Y0.1O1.95, 5.421(2) Å
248
281
308
328
351
377
411
250
304
308
350
356
400
3.30 × 10-6
1.04 × 10-5
1.61 × 10-5
3.09 × 10-5
9.10 × 10-5
1.73 × 10-4
2.85 × 10-4
1.14 × 10-5
6.17 × 10-5
6.98 × 10-5
2.31 × 10-4
3.25 × 10-4
6.67 × 10-4
9.1
7.2
6.83
8.2
9.95
4.77
1.9
7.87
8.41
7.66
7.9
7.02
9.1
Ce0.8Y0.2O1.9, 5.411(2) Å
expected lattice constant was calculated according to the
following equation:54
4
a ) [xrY + (1 - x)rCe + (1 - 0.25x)rOx
+
3+
4+
(CN8)
(CN8)
o
√
3
0.25xr ] × 0.9971 (2)
••
Vo
where x is the Y content (0 e x e 0.4) in Ce1-xYxO2-δ. The
ionic radius values were taken from the literature, where the
Y+ ionic size is rY )1.02 Å, the Ce4+ ionic size is (rCe
)
3+
4+
(CN8)
(CN8)
) 0.97 Å, the O2- ionic size is (rO ) ) 1.38 Å, and the oxygen
x
o
54b
••
vacancy radius is (rV ) ) 1.164 Å.
Expression 2 was
o
Electrical Conductivity. AC impedance studies on CO2-
capture-prepared YCO were found to be similar to those of
materials synthesized by other methods. A bulk, grain
boundary and electrode contributions in the frequency range
of 10-2 to 107 Hz were observed. Figure 10 shows typical
AC impedance plots of 10 and 20 mol % YCO obtained in
the air. These impedance plots were highly reproducible
during the heating and cooling runs. Similar AC impedance
plots were reported for the present CO2-capture- and ceramic-
method-synthesized doped CeO2.26,36,38 A clear intercept or
minimum due to bulk contributions in the high-frequency
rangewasobserved,especiallyatlowtemperatures(∼200-450
°C). The capacitance value obtained from the high-frequency
are using the relationship ωRC ) 1 (where ω ) 2πf, f is the
frequency, and R is the resistance) was found to be in the
range of 10-10 F values at the temperature range 250-400
°C (Table 1). This value is typical of the bulk contribution
to the ionic conduction.56,57 We observed a large overlap
between the grain-boundary and electrode effects at low
frequencies over the investigated temperature range (Figure
10). Hence, we are unable to meaningfully separate the bulk,
grain-boundary, and electrode contributions over the entire
temperature range of investigation. Thus, we have uniformly
taken the bulk resistance from the high-frequency minimum/
intercept along the Z′ axis of the impedance plots to calculate
the electrical conductivity data.
developed originally by Hong and Virkar54a to compute the
lattice constant of several metal-ion-doped CeO2’s. The theoreti-
cal lattice constant was found to slightly decrease with increas-
ing Y content in YCO.54 Taking Shannon’s ionic radii into
consideration (the ionic radius for 8-fold oxygen-coordinated
Ce4+ ion is 0.97 Å; Y3+ is 1.02 Å),53 one would envisage that
the lattice constant should increase with increasing Y content
in YCO. But, an opposite trend was observed which may be
attributed to the formation of oxide ion vacancies.54 Also, the
lattice constant obtained for YCO prepared from Sr2CeO4 was
found to be consistent with the literature trend (Figure 5).54,55
The lattice parameter of the sintered (at 1350 °C) samples
showed a slight increase (∼0.01 Å) in the lattice constant (Table
1 and Figures 3d and 4d).
The EDX analysis confirms further conversion of the layered
structured 10 mol % Y2O3-doped Sr2CeO4 compound into the
corresponding YCO (Figure 6). EDX clearly shows the absence
of Sr in the CO2-treated and subsequent acid-washed (Figure
6b) and sintered (Figure 6c) samples. Similar EDX data were
observed for the x ) 0.2 member sintered sample (Figure 6d).
The relative intensity due to Ce and Y was also found to be
vary in the x ) 0.1 and 0.2 members of CYO. The transforma-
tion of layered-structured Sr2CeO4 into fluorite-type CeO2 is
shown schematically in Figure 7. It must be mentioned that
we have recently shown the preparation of fast oxide-ion-
conducting Y-, Sm-, Sm+Ca-, and In-doped CeO2 from the
corresponding three-dimensional Ba-containing perovskite-like
Figure 11 shows the Arrhenius plot for the bulk ionic
conductivity of CO2-capture-method-prepared YCO in the
(53) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767.
(54) (a) Hong, S. J.; Virkar, A. J. Am. Ceram. Soc. 1995, 78, 433–439. (b)
Zhang, T. S.; Ma, J.; Huang, H. T.; Hing, P.; Xia, Z. T.; Chan, S. H.;
Kilner, J. A. Solid State Sci. 2005, 5, 1505–1511.
(55) Li, J. G.; Ikegami, T.; Wang, Y.; Mori, T. J. Solid State Chem. 2002,
168, 52–59.
(56) Bauerle, J. E. J. Phys. Chem. Solids 1969, 30, 2657–2670.
(57) (a) Irvine, J. T. S.; Sinclair, D. C.; West, A. R. AdV. Mater. 1990, 2,
132–138. (b) Zhao, H.; Feng, S. Chem. Mater. 1999, 11, 958–964.
(c) Losilla, E.; Aranda, M. A. G.; Bruque, S.; Sanz, J.; Paris, M. A.;
Campo, J.; West, A. R. Chem. Mater. 2000, 12, 2134–2142.
Inorganic Chemistry, Vol. 48, No. 1, 2009 261

Gerlach et al.
Figure 6. Energy dispersive X-ray analysis (EDX) of (a) as-prepared Sr2Ce0.9Y0.1O3.95, (b) sample “a” heated at 800 °C in CO2 for 12 h and washed with dilute
acid and dried in ambient air, and (c) sample “b” sintered at 1350 °C for 24 h in the air. In d, we show the EDX data of the Ce0.8Y0.2O1.9 sintered sample.
Typical impedance plots obtained for 20 mol % Y2O3-
doped CeO2 at 700 and 800 °C in three different atmospheres
are shown in Figure 12. As expected, the bulk resistance
(high-frequency intercept) of doped YCO was found to be
the same in the investigated temperature range of 600-800
°C in the air, N2, and Ar. The electrode contribution was
found to increase with decreasing oxygen partial pressures.
Figure 13 compares the bulk ionic conductivity data of
Ce0.8Y0.2O1.9 with that of the doped CeO2 reported in the
Figure 7. A schematic representation showing the transformation of (a)
Sr2CeO4 into (b) fluorite-type structure CeO2 under CO2 reaction at an
elevated temperature.
literature.15 The electrical conductivity data of the presently
prepared CO2-capture-method YCO exhibit comparable
electrical conductivity values to those in the literature,15
especially at low temperatures (e 600 °C).
present work. The electrical conductivity data obtained during
the first cycle heating and cooling and subsequent runs follow
the same line, suggesting the equilibrium behavior of the sample.
Very interestingly, the electrical conductivity of CO2-capture-
method-prepared doped samples was found to be about one
and half orders of magnitude higher than that of undoped CeO2.
The bulk ionic conductivity of the x ) 0.2 member was found
to be slightly higher than that of the x ) 0.1 member. The
activation energy for the electrical conductivity was found to
be 0.88 eV for the x ) 0.1 member and 0.82 eV for the x )
0.2 member of CYO, which is comparable to that of the RE-
doped CeO2 reported in the literature.27,58-61
Conclusions
The present study deals with the preparation of novel
fluorite-type structure materials by designing appropriate
precursor compounds that are reactive in CO2. We have
(58) Mogensen, M.; Sammes, N.; Tompsett, G. Solid State Ionics 2000,
129, 63–94.
(59) Huang, W.; Shuk, P.; Greenblatt, M. Solid State Ionics 1997, 100,
23–27.
(60) Huang, W.; Shuk, P.; Greenblatt, M. Chem. Mater. 1997, 9, 2240–
2245.
(61) Huang, K.; Feng, M.; Goodenough, J. B. J. Am. Ceram. Soc. 1998,
81, 357–362.
262 Inorganic Chemistry, Vol. 48, No. 1, 2009

Facile ConWersion Y2O3-Doped Sr2CeO4 into Y2O3-Doped CeO2
Figure 8. Scanning electron microscopy (SEM) images of (a) as-prepared Sr2Ce0.9Y0.1O3.95, (b) sample “a” heated to 800 °C in CO2 for 12 h, (c) sample
“b” washed with dilute acid and subsequently dried in ambient air, and (d) sample “c” sintered at 1350 °C for 24 h in the air. The left-hand side shows the
diagram at 5 µm, and the right-hand side shows the corresponding samples ((e)–(h)) at a higher scale, 20 µm.
Inorganic Chemistry, Vol. 48, No. 1, 2009 263

Gerlach et al.
Figure 9. Scanning electron microscopy (SEM) images of (a) as-prepared Sr2Ce0.8Y0.2O3.9, (b) sample “a” heated to 800 °C in CO2 for 12 h, (c) sample “b”
washed with dilute acid and subsequently dried in ambient air, and (d) sample “c” sintered at 1350 °C for 24 h in the air. The left-hand side shows the
diagram at 5 µm, the and right-hand side shows the corresponding samples ((e)–(h)) at a higher scale, 20 µm.
shown the formation of Y2O3-doped CeO2 (YCO) from
the corresponding Y2O3-doped layered-structured Sr2CeO4
and CO2 at 800 °C and subsequent to acid washing. The
amount of CO2 gained was found to be consistent with 2
264 Inorganic Chemistry, Vol. 48, No. 1, 2009

Facile ConWersion Y2O3-Doped Sr2CeO4 into Y2O3-Doped CeO2
Figure 10. AC impedance plots of CO2 capture method synthesized Ce0.9Y0.1O1.95 at (a) 280 °C and (b) 400 °C in the air. Parts c and d show the ac
impedance data for Ce0.8Y0.2O1.9 at 308 and 750 °C in the air, respectively. Insets in a-c show the expanded view.
Figure 11. Arrhenius plots for the electrical conductivity of Ce0.9Y0.1O1.95
Figure 12. AC impedance plots of CO2 capture method synthesized
and Ce0.8Y0.2O1.9 prepared by the CO2 capturing method. For comparison,
Ce0.8Y0.2O1.9 at 700 and 800 °C in the air and N2 and Ar in the frequency
the electrical conductivities of parent CeO226 and 10 mol % Y-doped CYO36
range 10-2 to 107 Hz. Inset shows the expanded view.
are included.
mol of CO2 per formula unit of the layered structure
cerates. The PXRD investigations show that the lattice
parameter decreases with increasing temperature, which
is expected on the basis of the ionic radii trend. Also, the
lattice constant value was found to be comparable to that
of ceramic- and chemical-method-synthesized YCO. EDX
investigations further confirmed the PXRD results. The
SEM characterization reveals much smaller-sized particles
Inorganic Chemistry, Vol. 48, No. 1, 2009 265

Gerlach et al.
for the CO2-treated and acid-washed samples compared
to the precursor and sintered product. The capacitance
value determined using the high-frequency part was found
to be on the order of 10-10 F. The bulk ionic conductivity
of CO2-capture-method-prepared YCO exhibits about one
and half orders higher electrical conductivity than that of
the undoped CeO2. The activation energy for bulk ionic
conductivity of YCO was found to be comparable to those
of ceramic-method-prepared YCO samples with a similar
chemical composition.
Acknowledgment. This research was supported partially
by funding to the NSERC Solid Oxide Fuel Cells Canada
Strategic Research Network and the Discovery Grants from
the Natural Science and Engineering Research Council
(NSERC). We also would like to thank the Canada Founda-
tion for Innovation (CFI) for support through the Leaders
Opportunity Fund (LOF) funding.
Figure 13. Comparison of the electrical conductivity of Ce0.8Y0.2O1.9
prepared by CO2 capture method using Sr2Ce0.8Y0.2O3.9 with the
conventional YSZ,11 parent CeO2,26 and rare-earth-doped CeO215 reported
in the literature.
IC801729X
266 Inorganic Chemistry, Vol. 48, No. 1, 2009