Chem. Mater. 2009, 21, 2319–2326
2319
Double-Perovskite Anode Materials Sr2MMoO6 (M ) Co, Ni) for
Solid Oxide Fuel Cells
Yun-Hui Huang,*,†,# Gan Liang,‡ Mark Croft,§,| Matti Lehtimaki,⊥ Maarit Karppinen,⊥ and
¨
John B. Goodenough†
Texas Materials Institute, ETC 9.102, The UniVersity of Texas at Austin, Austin, Texas 78712 Department
of Physics, Sam Houston State UniVersity, HuntsVille, Texas 77341, Department of Physics, Rutgers
UniVersity, Piscataway, New Jersey 08854, National Synchrotron Light Source, BrookhaVen National
Laboratory, Upton, New York 11973, Laboratory of Inorganic Chemistry, Department of Chemistry,
Helsinki UniVersity of Technology, FI-02015 TKK, Finland, and State Key Laboratory of Materials
Processing and Die and Mold Technology, School of Materials Science and Engineering, Huazhong
UniVersity of Science and Technology, Wuhan, Hubei 430074, China
ReceiVed December 16, 2008. ReVised Manuscript ReceiVed March 28, 2009
Double-perovskites Sr2MMoO6 (M ) Co, Ni) have been investigated as anode materials for a solid
oxide fuel cell. At room temperature, both Sr2CoMoO6 and Sr2NiMoO6 are tetragonal (I4/m).
X-ray
absorption spectroscopy confirmed the presence of Co2+/Mo6+ and Ni2+/Mo6+ pairs in the oxygenstoichiometric compounds. The samples contain a limited concentration of oxygen vacancies in the reducing
The Review on The University of Texas at San Antonio
Introduction The power of America lies within the heart of its people and the ability to have their voices heard. One of the best ways to accomplish this is through an electoral vote. By voting, the people of America or any Democratic country can control the route of the government and the decisions it makes. To decide if those decisions be new and current with society, or kept traditional is why ...
atmospheres at an anode. Reoxidation is facile below 600 °C; they become antiferromagnetic at low
temperatures TN ) 37 and 80 K for M ) Co and Ni, respectively. As an anode with a 300 µm thick
La0.8Sr0.2Ga0.83Mg0.17O2.815 electrolyte and SrFe0.2Co0.8O3-δ as a cathode, Sr2CoMoO6 exhibited maximum
power densities of 735 mW/cm2 in H2 and 527 mW/cm2 in wet CH4 at 800 °C; Sr2NiMoO6 shows a
notable power output only in dry CH4. The high performance of Sr2CoMoO6 in wet CH4 may be due to
its catalytic effect on steam reforming of methane, but some degradation of the structure that occurred
in CH4 obscures identification of the catalytic reaction processes at the surface. However, the stronger
octahedral-site preference of Ni2+ versus Co2+ can account for the lower performance of the M ) Ni
anode.
Introduction
The solid oxide fuel cell (SOFC) is an electrochemical
device that can be used for either stationary or mobile
generation of electrical energy from a gaseous fuel. The
conventional SOFC, which uses Y2xZr1-2xO2-x (YSZ) as the
electrolyte and a porous Ni-YSZ cermet anode, is commercially viable with pure H2 or syngas as the fuel; but this
anode is fouled by carbon deposition and sulfur poisoning
when operated on natural gas.1-3 Development of an anode
material that can operate on natural gas would provide a
cheaper, more convenient SOFC. For this application, oxides
that are mixed oxide-ion/electron conductors (MIECs) in the
reducing atmosphere at the anode have been under investigation.3,4 The double- perovskite Sr2MgMoO6 is a promising
MIEC with an excellent tolerance to sulfur that gives direct
electrochemical oxidation in dry methane at 800 °C.5,6
Moreover, La-doped Sr2MgMoO6 performs somewhat better
* To whom correspondence should be addressed. E-mail: huangyh@
mail.hust.edu.cn.
†
The University of Texas at Austin.
‡
Sam Houston State University.
§
Rutgers University.
|
Brookhaven National Laboratory.
The Essay on The Importance of Ions
Chemistry The Importance of Ions In chemistry, we attempt to grasp numerous rules and facts so that we can better understand the world around us down to the most basic of levels. Ions are amongst the most important binding agents of the universe and the least significant item on a person’s list of graces at thanksgiving. A key to being able to comprehend how chemistry works is knowing what ions ...
⊥
Helsinki University of Technology.
#
Huazhong University of Science and Technology.
(1)
(2)
(3)
(4)
McIntosh, S.; Gorte, R. J. Chem. ReV. 2004, 104, 4845.
Sun, C. W.; Stimming, U. J. Power Sources 2007, 171, 247.
Goodenough, J. B.; Huang, Y. H. J. Power Sources 2007, 173, 1.
Atkinson, A.; Barnett, S.; Gorte, R. J.; Irvine, J. T. S.; McEvoy, A. J.;
Mogensen, M.; Singhal, S. C.; Vohs, J. Nat. Mater. 2004, 3, 17.
on natural gas.7 These results have prompted a study of other
members of the Sr2MMoO6 family containing a 3d-block
transition-metal M to investigate the role of the M cation.
The ordered double-perovskites A2BB′O6 have alternating
BO6/2 and B′O6/2 corner-shared octahedra. Substitution at A
or B sites can alter the cation valence and oxygen-vacancy
concentration. As is well-known, cation valence and oxygenvacancy concentration play important roles in the physical
and electrochemical properties of the double perovskites.8-11
In Sr2MgMoO6, Mg ions show unchanged divalence; only
the valence of Mo ions changes from +6 to +5 with the
introduction of oxygen vacancies. Co and Ni ions are
multivalent; the evolution of cation valence and oxygenvacancy concentration in Sr2CoMoO6 and Sr2NiMoO6 is
more complicated than that in Sr2MgMoO6. In this paper,
we have systematically explored the valence states and the
(5) Huang, Y. H.; Dass, R. I.; Xing, Z. L.; Goodenough, J. B. Science
2006, 312, 254.
(6) Huang, Y. H.; Dass, R. I.; Denyszyn, J. C.; Goodenough, J. B. J.
Electrochem. Soc. 2006, 153, A1266.
(7) Ji, Y.; Huang, Y. H.; Ying, J. R.; Goodenough, J. B. Electrochem.
Commun. 2007, 9, 1881.
(8) Kobayashi, K. I.; Kimura, T.; Sawada, H.; Terakura, K.; Tokura, Y.
Nature (London) 1998, 395, 677.
(9) Serrate, D.; De Teresa, J. M.; Ibarra, M. R. J. Phys.: Condens. Matter
2007, 19, 023201.
(10) Garcıa-Hernandez, M.; Martınez, J. L.; Martınez-Lope, M. J.; Casais,
´
´
´
´
M. T.; Alonso, J. A. Phys. ReV. Lett. 2001, 86, 2443.
(11) Huang, Y. H.; Karppinen, M.; Yamauchi, H.; Goodenough, J. B. Phys.
The Essay on Errors, Uncertainties and Measurements
Abstract In this experiment, different measuring devices were used, namely the vernier calliper, micrometer calliper, foot rule, and the electronic gram balance. These devices were used to obtain the mean diameter, volume, mass, and the experimental value of density of the sphere of known composition. 1. Introduction Measurement is the process or act of determining the size, length, quantity, etc. ...
ReV. B 2006, 73, 104408.
10.1021/cm8033643 CCC: $40.75
2009 American Chemical Society
Published on Web 05/01/2009
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Chem. Mater., Vol. 21, No. 11, 2009
electrochemical performances of the double-perovskites
Sr2MMoO6 (M ) Co, Ni) as anodes of a SOFC operating
on dry and wet methane and on H2.
Huang et al.
Table 1. Room-Temperature Lattice Parameters, Bond Lengths,
Bond Angles, Density (d), Atom Occupancy (gM), and Degree of
Cationic Ordering (ξ) Obtained by Rietveld Refinement on XRD
Patterns for Sr2MMoO6 (M ) Co, Ni)
M
Experimental Section
Sr2CoMoO6 and Sr2NiMoO6 samples were synthesized via a
sol-gel method with stoichiometric SrCO3 (Alirich, 99%),
(NH4)6Mo7O24 · 4H2O (Fisher Scientific, assay MoO3, 81.5%), and
Co3O4 (Alfa, 99%) or NiO (Alfa, 99%) as the starting materials.
SrCO3 and Co3O4, NiO were first dissolved with diluted nitric acid
and then mixed with (NH4)6Mo7O24 · 4H2O. Ethylenediaminetetraacetic acid was added as a complexant to achieve a clear aqueous
solution. The pH of the solution was adjusted to 9-10 with
ammonia. The solution was evaporated on a hot plate to become a
gel. The gel was first decomposed at 400 °C in air for 6 h and then
calcined at 800 °C in air for 10 h. The calcined powder was
pelletized and finally sintered at 1250 °C in air for 24 h to achieve
a pure phase. Synthesis of other compounds, La0.8Sr0.2Ga0.83Mg0.17O2.815, SrCo0.8Fe0.2O3-δ, and La0.4Ce0.6O2-δ, has been described
elsewhere.6
The phase purity and the lattice parameters of the samples were
checked by X-ray powder diffraction (XRD, Philips X-pert, Cu KR
radiation).
The diffraction profiles were analyzed with a Rietveld
refinement program, RIETAN 2000. Redox behaviors of the
samples were investigated by thermogravimetric analysis (TGA,
Perkin-Elmer Pyris 1 and Netzsch STA 449 C) in air and 5% H2/
Ar gas flows in the temperature range from room temperature to
1000 °C. In these TGA experiments, the amount of sample powder
was ∼15 mg and the heating/cooling rate was either 2 or 10 °C/
The Essay on Determination of Dissolved Oxygen In a Water
INTRODUCTION In an alkaline solution, dissolved oxygen will oxidize manganese(II) to the trivalent state. 8OH-(aq) + 4Mn2+(aq) + 2H2O(l) －－＞ 4Mn(OH)3(s) The analysis is completed by titrating the iodine produced from potassium iodide by manganese(III) hydroxide. 2Mn(OH)3(s) + 2I-(aq) + 6 H+(aq) －－＞ 2Mn2+(aq) + I2(aq) + 6H2O(l) Sodium thiosulphate is used as the titrant. Success of the method is ...
min. Micrographs were taken by a scanning electron microscope
(SEM, Hitachi: S4500).
Magnetization measurements were made
with a superconducting quantum interference device (Quantum
Design: MPMS-XL5).
The conductivity was measured by a standard
dc four-probe method with our own setup. The samples for
conductivity measurement were polished into rectangular bars; Pt
wire and Pt paste were used to make the four probes. Before
measurement, the samples were reduced in 5% H2/Ar in our own
setup at 800 °C for 20 h to ensure formation of oxygen vacancies.
Oxygen partial pressure was monitored with a Thermox CG1000
oxygen analyzer (Ametek).
The Mo L3-edge, Ni K-edge, and Co K-edge X-ray absorption
spectroscopy (XAS) measurements were performed in the fluorescence mode and on powdered samples on beamline X-19A at the
National Synchrotron Light Source, Brookhaven National Laboratory. A double-crystal Si(111) monochromator was used. The XAS
samples were prepared by dusting a fine powder of the samples
onto scotch tape. In the case of the Ni and Co K-edge measurements, the X-ray beam transmitted through the samples allowed
transmission-mode measurements along with edges of simultaneously run standards located on the down-beam side of the sample.
The absolute energy calibration was set to the elemental edge (first
inflection point).
The relative energy scale was maintained to better
than (0.05 eV with the simultaneously run standards. In the case
of the Mo L3-edges, standards were run periodically in the sample
sequence and the energy scale is better than (0.1 eV.
Single SOFC test cells were fabricated by an electrolytesupported technique with 300 µm thick La0.8Sr0.2Ga0.83Mg0.17O2.815
(LSGM) as the electrolyte and SrCo0.8Fe0.2O3-δ (SCF) as the cathode.
The fabrication method of the single cell has been described in
detail in our previous work.6 A thin buffer layer of La0.4Ce0.6O2-δ
(LDC) between the anode and the electrolyte was used to prevent
Co
Ni
space group
a (Å)
b (Å)
c (Å)
V (Å3)
d (g/cm3)
The Essay on The pathway of air in amphibians, birds, fish, and humans.
How are they alike, and different? Is one more efficient than another? Gills/Lungs?HumansIn humans air travels into the mouth, or nose, and into the nasal cavity, followed by pharynx. The pharynx is where food and air cross paths. The pharynx increases the chance of choking, but also allows breathing when exercising and respiration though the mouth, if the nose is closed. Next, epiglottis opens ...
M-O1 (Å)
M-O2 (Å)
〈M-O〉 (Å)
Mo-O1 (Å)
Mo-O2 (Å)
〈Mo-O〉 (Å)
M-O1-Mo (deg)
M-O2-Mo (deg)
gM
ξ
Rp (%)
Rwp (%)
I4/m
5.5726(3)
5.5726(3)
7.9575(5)
247.11(3)
5.723
2.044(×2)
2.039(×4)
2.041
1.935(×2)
1.935(×4)
1.935
180(×2)
165.13(×4)
0.971(9)
0.942
7.45
10.46
I4/m
5.5463(2)
5.5463(2)
7.8933(3)
242.81(1)
5.825
2.003(×2)
1.987(×4)
1.993
1.953(×2)
1.951(×4)
1.952
180(×2)
169.57(×4)
0.979(5)
0.958
5.54
7.23
interdiffusion of ionic species between anode and electrolyte.12 Pt
gauze with a small amount of Pt paste in separate dots was used as
a current collector at both the anode and cathode sides for ensuring
contact. A double-layer sealing design was applied to the single
cells. Before testing, the cells were exposed to 5% H2/Ar for 20 h
at 800 °C to reduce the anode and then purged with fuel gas for
2 h. The performance measurements were performed on an EG&G
potentiostat/galvanostat model 273 with a homemade LabView
program.
Results and Discussion
The XRD patterns show that the samples were doubleperovskite phases. Sr2CoMoO6 and Sr2NiMoO6 both have a
tetragonal structure with space group I4/m. The lattice
parameters, bond lengths, bond angles, and site occupancies
obtained by Rietveld refinement are displayed in Table 1.
The lattice cell volume of M ) Co is larger than that of M
) Ni; the mean bond length 〈Co-O〉 is longer than that of
〈Ni-O〉. The order is consistent with their ionic radii (sixfold
coordination), i.e., Co2+ 0.745 Å (HS), Ni2+ 0.69 Å. The
bond length 〈Mo-O〉 is shorter than that of 〈M-O〉 because
Mo6+ (0.59 Å) and Mo5+ (0.61 Å) are both smaller than
the ionic radii of the M cations. Reduction from 180° of the
M-O-Mo bond angles is caused by a cooperative rotation
The Essay on Determination of the amount of dissolved oxygen
Topic : Determination of the amount of dissolved oxygen in a water sample by iodometry-the winkler’s method. Objective: To determine the amount of dissolved oxygen in a water sample by iodometry- the winkler’s method. Apparatus: volumetric pipette, 3 conical flask, burette, burette clamp, Pasteur pipette, reagent bottle, conical flask stopper, retord stand, white tile Materials: 2 ml manganese ...
of the MO6/2 and MoO6/2 octahedra; these rotations increase
as the geometric tolerance factor t ) (rA + rO)/ 2(rB + rO)
decreases. The rA, rB, and rO are respectively the roomtemperature ionic radii of the A-site cation (ninefold
coordination), the mean B-site ionic radius (sixfold coordination), and the oxide-ion radius (twofold coordination) with
the Shannon13 ionic radii. The t factors are 0.932 and 0.945
for M ) Co and M ) Ni, respectively. This assignment for
the valence states places the Mo6+/Mo5+ reduction potential
above the M3+/M2+ reduction potentials of M ) Co and Ni.
The double-perovskite B-site cations generally exhibit
some antisite disorder and antiphase boundaries. The value
of the order parameter ξ can be calculated as ξ ) 2(gM (12) Huang, K. Q.; Goodenough, J. B. J. Alloys Compd. 2000, 303-304,
454.
(13) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
Sr2MMoO6 (M ) Co, Ni) for Solid Oxide Fuel Cells
Chem. Mater., Vol. 21, No. 11, 2009 2321
0.5) from the refined occupancy of M ions at its correct site
(gM).
Sr2MMoO6 with M ) Co and Ni have a highly ordered
structure, which is ascribed to the big difference in radius
and valence between Co2+ and Mo6+, Ni2+, and Mo6+.
The iodometric titration method can be used to determine
directly the valence of some cations in the double perovskites.14 According to their standard oxidation-reduction
potential, Co3+ and Ni3+ can efficiently oxidize I- to I2.
Therefore, we used this method to check the contents of Co3+
and Ni3+ ions. A weighed amount of double-perovskite
powder was dissolved in diluted HCl aqueous solution and
then excess KI was added. The purple iodine produced in
situ was immediately titrated with a standard volumetric
solution of sodium thiosulfate; starch was used as an indicator
of the end point. The whole reaction can be expressed as
follows:
2M3+ + 2I- ) I2 + 2M2+
(M ) Co, Ni)
2S2O32- + I2 ) 2I- + S4O62-
(1)
(2)
The percentages of Co3+/Co and Ni3+/Ni in the as-prepared
Sr2CoMoO6 and Sr2NiMoO6 samples sintered in air were
6.7% and 4.2%, respectively.
We have shown in our previous work that some oxygen
vacancies exist in the reduced Sr2MgMoO6.5,6 Bernuy-Lopez
and Marrero-Lopez et al.15,16 separately noted that reduction
´
of Sr2MgMoO6 can give rise to a limited number of oxygen
vacancies and hence to some lower Mo oxidation states than
Mo6+. Here, we focus on the temperature dependence of the
oxygen vacancies in the as-prepared Sr2CoMoO6 and
Sr2NiMoO6 samples in air and a 5% H2/Ar atmosphere.
The perovskite structure does not accept interstitial oxygen.
Therefore, the presence of M3+ species in the air-sintered
samples requires the presence of some cation vacancies.
Partial evaporation of some of the constituent metals during
sintering at 1250 °C in air led to a cationic nonstoichiometry
in the as-prepared samples.17,18 We investigate how oxygen
is lost and reincorporated on raising and lowering of the
temperature with TGA. Since the Mo6+/Mo5+ redox couple
is at a higher energy than the M3+/M2+ couples, reduction
of the samples will, first, reduce the M3+ to M2+. (Note:
throughout we indicate formal valences on the cations, not
the actual charges they carry.)
On heating in air, Figure 1A, both samples show the onset
of weight loss at 300 °C and a step loss at 650 °C,
corresponding to ca. 0.05 O atoms/f.u., that is shown to be
irreversible in the case of M ) Co. The M ) Co sample
also shows a second reversible weight-loss step at 900 °C;
the M ) Ni sample does not. This difference and the smaller
oxidation of Ni2+ versus Co2+ reflects, we believe, a greater
resistance to oxygen and Mo loss by the stronger preference
(14) Vazquez-Vazquez, C.; Blanco, M. C.; Lopez-Quintela, M. A.; Sanchez,
´
´
´
´
R. D.; Rivas, J.; Oseroff, S. B. J. Mater. Chem. 1998, 8, 991.
(15) Bernuy-Lopez, C.; Allix, M.; Bridges, C. A.; Claridge, J. B.;
Rosseinsky, M. J. Chem. Mater. 2007, 19, 1035.
(16) Marrero-Lopez, D.; Martınez, J. P.; Ruiz-Morales, J. C.; Perez-Coll,
´
´
´
D.; Aranda, M. A. G.; Nunez, P. Mater. Res. Bull. 2008, 43, 2441.
´
(17) Huang, Y. H.; Linden, J.; Yamauchi, H.; Karppinen, M. Chem. Mater.
´
2004, 16, 4337.
(18) Ivanov, S. A.; Eriksson, S. G.; Tellgren, R.; Rundlof, H.; Tseggai, M.
¨
Mater. Res. Bull. 2005, 40, 840.
Figure 1. TG curves for Sr2CoMoO6 (solid line) and Sr2NiMoO6 (dotted
line): (A) in air with a heating rate of 2 °C/min; (B) in 5% H2/Ar with
heating rates of 2 and 10 °C/min.
of Ni2+ for octahedral sixfold oxygen coordination. The step
weight losses signal oxygen loss in a finite volume fraction,
i.e., at defects. Oxygen vacancies trapped by formation of
molybdyl (ModO) species at Mo-rich antiphase boundaries,
for example, would provide an irreversible, finite oxygen
loss. We postulate antisite Mo or Mo-rich antiphase boundaries are the operative defect since these would be common
to both samples. The second step in the M ) Co sample at
900 °C reflects, according to this reasoning, reversible
trapping of an oxygen vacancy at some other defect associated with the M2+ ion since it does not occur in the M ) Ni
sample. Given the presence of Mo vacancies, we tentatively
assign this step to trapping of oxygen vacancies at Co
neighboring a Mo vacancy. A Co2+ ion at a Mo vacancy is
stable in a lower oxygen coordination whereas the Ni2+ ion
is much less stable in lower coordination.
Next, the TGA curves of Figure 1B were recorded on
heating the as-prepared M ) Co and M ) Ni samples under
a 5% H2/Ar atmosphere, which resembles that at the anode
of an operating SOFC. In this atmosphere, both samples
begin to decompose above 800 °C where we have confirmed
initiation of the reduction of MoO3 with TGA measurements
on MoO3 in the same atmosphere (not shown), which
contains ModO units. In 5% H2/Ar, the step found at 650
°C in air does not appear with either sample. On the other
hand, a significant reversible weight loss occurs in a step
above 300 °C in the M ) Co sample, but not in the M ) Ni
sample. Figure 2 shows that this weight loss in the M ) Co
sample is due to a reversible loss of oxygen. On heating in
2322
Chem. Mater., Vol. 21, No. 11, 2009
Huang et al.
Figure 2. TG curves recorded for Sr2CoMoO6 with a heating rate of 2 °C/
min first in 5% H2/Ar up to 800 °C, and then for the same sample in air
after cooling it rapidly down in 5% H2/Ar (broken line).
air, a nearly complete regain of the weight lost occurs below
650 °C, and the weight-loss steps at 650 and 950 °C reappear.
We therefore conclude that the small, irreversible weight loss
in air above 350 °C in both samples, Figure 1, reflects
reduction of M3+ to M2+; it occurs more abruptly in the M
) Ni sample because of the stronger octahedral-site preference of the Ni2+ ion. In the 5% H2/Ar atmosphere, the M3+
ions would be reduced to M2+ already at room temperature
and the vacancies introduced by this reduction are apparently
trapped at the defects postulated to be Mo-rich antiphase
boundaries. In the M ) Co sample, the weight loss above
300 °C in 5% H2/Ar would then represent a reduction of
Mo6+ to Mo5+ by a reversible loss of oxygen that introduces
mobile bulk oxygen vacancies. The stronger octahedral-site
preference of Ni2+ inhibits further loss of oxygen in the 5%
H2/Ar atmosphere once the Ni3+ have been reduced to Ni2+
and the oxygen vacancies so introduced have been trapped.
Heating the M ) Co sample in air after reduction in 5%
H2/Ar reintroduces oxide ions that move, below 650 °C, to
annihilate the trapped vacancies, vacancies that are again
created on further heating above 650 °C.
In ref 19, a TGA curve was presented for H2-reduced
Sr2CoMoO6 to demonstrate its reoxygenation in air up to
800 °C. Even though the authors do not pay attention to the
slight lowering of the curve after it reaches its maximum
around 650 °C, it is clear that their TGA curve is highly
consistent with the present data and conclusions.
The Mo L3-edge, Ni K-edge, and Co K-edge XAS
measurements were performed to confirm the cation valences.
Since a simultaneous standard was not possible at the low
energies of the Mo-L3 edges, possible standards were run
periodically in the sequence of samples and the relative
energy was (0.1 eV or better. In view of the low energy
and strong “white line” p-to-d transition at the Mo-L3 edge
self-absorption, degradation/rounding of the absorption edge
peaks in the fluorescence mode was unavoidable. Here the
“white line” (WL) terminology is conventional and refers
to an atomic-like dipole transition into empty states that
typically manifests a sharply peaked near-edge structure; in
(19) Okamoto, H.; Fjellvag, H.; Yamauchi, H.; Karppinen, M. Solid State
˚
Commun. 2006, 137, 522.
Figure 3. (A) Mo L3-edge spectra for Sr2MMoO6 samples (M ) Co, Ni)
and standard MoO3; (B) Co K-edge spectra for Sr2CoMoO6 and standards
LaCoO3, La2CoO4, and CoO; (C) Ni K-edge spectra for Sr2NiMoO6 and
standards LaNiO3, La3NiO6.4, and NiO.
prior years it left a white-line streak on photographic film.20,21
As is routinely done, the spectra presented in this study had
a linear background subtracted (determined over a ca. 80
eV interval below the edge) and were normalized to unity
absorption step height across the edge. Here an average of
the data in the 50-200 eV range above the edge was used
to set the normalization value.
The L3 near-edge WL features of 4d transition-metal
compounds are due to transitions from the 2p to 4d states of
the transition metal (such as Mo).
In an octahedral ligand
field, the d states of Mo6+ are split into a sixfold-degenerate
t2g ground state below a fourfold-degenerate eg state. In
Figure 3a the L3-edge WL features of the octahedrally
coordinated compounds Sr2CoMoO6 and Sr2NiMoO6 are
shown in comparison with the spectra of another octahedrally
coordinated compound, MoO3. The A and B features are
associated, respectively, with the t2g and eg final states of
the octehedrally coordinated compounds. It should also be
noted that the relative intensity of the A feature decreases
as the number of empty t2g states decreases (i.e., as the
(20) Wei, P. S. P.; Lytle, F. W. Phys. ReV. B 1979, 19, 679.
(21) Jeon, Y.; Jisrawi, N.; Liang, G.; Lu, F.; Croft, M.; Mclean, W. L.;
Hart, D. L.; Stoffel, N. G.; Sun, J. Z.; Geballe, T. H. Phys. ReV. B
1989, 39, 5748.
Sr2MMoO6 (M ) Co, Ni) for Solid Oxide Fuel Cells
Figure 4. Magnetic susceptibility χ and reciprocal susceptibility χ-1 for
the as-prepared Sr2MMoO6 (M ) Co, Ni) samples.
electron count increases or valence decreases).
Thus, the
relative intensity ratio of the A to B features, IA/IB, can be
used to track the d-electron/hole count in such Mo compounds; i.e., a higher IA/IB usually corresponds to a higher
Mo valence.
The double-peak features A and B in Figure 3A are seen
in all spectra and can be attributed to the 2p f 4d(t2g) and
2p f 4d(eg) transitions, respectively. Since such doublepeak features could have different widths for different
compounds, it is more reliable to use the centrum or mean
energy Em (or characteristic energy22,23) instead of the edge
energy (defined as the inflection point of the rising part of
the edge) to estimate the Mo valence. The values of Em
estimated for these three spectra are almost identical (2525.5
eV), supporting the belief that the values of the Mo valence
are nearly the same for these three compounds. The very
close Mo valence in these three compounds indicates that
Ni and Co are predominantly in the divalent state like that
for Mg. To confirm this deduction, we show the Co and Ni
K-edges of the Sr2MMoO6 compounds with M ) Co and
Ni, respectively, in Figure 3B,C, along with selected spectra
of known formal valence standards: LaM3+O3, M2+O,
La3Ni2O6.4, and La2Co2+O4. The features A* and B* labeled
on the spectra of the Sr2MMoO6 compounds are similar to
what has been observed in the Ni K-edges of divalent nickel
perovskite compounds La2-xSrxNiO4 (0 e x e 0.2)24 and
can be assigned to the transitions from the M 1s state to the
final 4p states with different 3d electron-ligand hole configurations and orbital orientations. The Sr2MMoO6 compounds have chemical shifts for all the cations (see circles)
consistent with a valence close to 2+ and shifted well down
in energy from the 3+ (see rectangle) standard.
The magnetic susceptibility of the as-prepared Sr2MMoO6
samples was measured to check their electron configuration.
Figure 4 displays the temperature dependence of the magnetic
susceptibility χ and reciprocal susceptibility χ-1 for
(22) Alp, E. E.; Goodman, G. L.; Soderholm, L.; Min, S. M.; Ramanathan,
R.; Shenoy, G. K.; Bommannavar, A. S. J. Phys.: Condens. Matter
1989, 1, 6463.
(23) Liang, G.; Yao, Q.; Zhou, S.; Katz, D. Physica C 2005, 424, 107.
(24) Sahiner, A.; Croft, M.; Guha, S.; Perez, I.; Zhang, Z.; Greenblatt, M.;
Metcalf, P. A.; Jahns, H.; Liang, G. Phys. ReV. B 1995, 51, 5879.
Chem. Mater., Vol. 21, No. 11, 2009 2323
Sr2MMoO6 with M ) Co and Ni. They both show a longrange-ordered antiferromagnetic state at low temperature. The
Neel temperature TN values are 37 and 80 K for M ) Co
´
and Ni, respectively. In the double-perovskite structure, the
magnetically active M2+ ions are separated by magnetically
neutral Mo6+ ions. The exchange interactions propagate
through the intervening MoO6 octahedra; therefore, a relatively high level of covalent bonding is expected within the
M-O-Mo-O-M pathways. The variation of the TN values
depends primarily on the inverse of the energy separating
the M3+/M2+ and M2+/M+ redox energies; the correlation
energy U for Co2+ is larger than the charge-transfer gap ∆
for Ni. Interestingly, with reduction of the air-sintered
Sr2CoMoO6 sample in 5% H2/Ar or H2 at 800 °C for 10 h,
the reduced sample exhibits a spin-glass ferromagnetism in
the measured temperature range from 5 to 380 K. A similar
phenomenon was also observed by Viola et al.25 in oxygendeficient Sr2CoMoO6-δ with a Curie temperature TC )
350-370 K. The observed spin-glass ferromagnetism in
Sr2CoMoO6-δ comes from a ferromagnetic superexchange
between Co2+ (3d7) and Mo5+ (4d1) moments in the cationordered regions with antiferromagnetic M2+-O-M2+ interactions across antiphase boundaries and at antisite M2+.
A good fitting with the Curie-Weiss law is observed
above TN in the χ-1-T curves for M ) Co and Ni. The linear
fitting above 200 K gives an effective paramagnetic moment
(µeff) of 4.34(2) and 3.51(1) µB/f.u. for M ) Co and M )
Ni, respectively. For Sr2CoMoO6, the measured µeff is higher
than the spin-only 3.87 µB for Co2+ ions. The measured µeff
for Sr2NiMoO6 is also high relative to the predicted spinonly 2.83 µB for Ni2+ ions. Interpretation of the discrepancies
between measured and spin-only µeff values is made complicated by the existence of antiphase boundaries where shortrange magnetic order may occur above the TN for the ordered
regions.
Conductivity data taken above 350 °C while heating in
different atmospheres are shown in Figures 5 and 6 for
nominal Sr2CoMoO6-δ and Sr2NiMoO6-δ, respectively. The
conductivity above 350 °C is dominated by electrons on
the Mo6+/Mo5+ couple given the much lower mobility of
the oxygen vacancies. The as-prepared M ) Co sample gave
positive thermoelectric power at room temperature showing
that the conduction was by holes in the Co3+/Co2+ couple
(see Figure S4 in the Supporting Information), but the Co3+
are all reduced to Co2+ above 350 °C. In air, the log(σT) vs
T-1 curves for M ) Co, Figure 5B, are linear below 650
°C, but the curve deviates to higher conductivity above 650
°C where the TGA curves of Figure 1A show a step.
Electrons would be on Mo5+ coordinated by an oxygen
vacancy, but trapping of oxygen vacancies at ModO species
would release electrons to the matrix. The M ) Ni sample
in air had a larger resistivity with a nonlinear Arrhenius
behavior below 650 °C, consistent with a small step in the
TGA curve near 500 K and a saturation of the number of
charge carriers.
(25) Viola, M. C.; Martınez-Lope, M. J.; Alonso, J. A.; Velasco, P.;
´
Martınez, J. L.; Pedregosa, J. C.; Carbonio, R. E.; Fernandez-Dıaz,
´
´
´
M. T. Chem. Mater. 2002, 14, 812.
2324
Chem. Mater., Vol. 21, No. 11, 2009
Huang et al.
Figure 7. XRD patterns for air-sintered Sr2CoMoO6 after being reduced in
5% H2/Ar, H2, and CH4 at 800 °C for 20 h.
Figure 5. (A) Temperature dependence of resistivity (F) and (B) Arrhenius
plots of log(σT) vs T-1 for Sr2CoMoO6 in different atmospheres. The oxygen
pressure pO2 is around 10-18, 10-19, and 10-20 atm in CH4 (dry or wet),
5% H2/Ar, and H2, respectively.
Figure 8. SEM images for the cell with the Sr2CoMoO6 anode: (A) cross
section of the cell configuration, (B) section between LSGM and LDC buffer
layer, (C) the surface of the Sr2CoMoO6 anode, and (D) the cathode SCF.
Figure 6. (A) Temperature dependence of resistivity (F) and (B) Arrhenius
plots of log(σT) vs T-1 for Sr2NiMoO6 in different atmospheres. The oxygen
pressure pO2 is around 10-18, 10-19, and 10-20 atm in CH4 (dry or wet),
5% H2/Ar, and H2, respectively.
After reduction in 5% H2/Ar for 20 h at 800 °C, the
resistivity of the M ) Co sample in that atmosphere shows
a broad maximum near 650 °C where the TGA curve in air
shows a step that we have postulated to be associated with
formation of ModO species that trap oxygen vacancies.
Since the system was carefully sealed so as to prevent
leakage from air, the partial pressure of oxygen in the gas
was extremely low (pO2 ) 10-19 atm).
Mo5+ ions would be
trapped at oxygen vacancies unless formation of ModO
species released the electrons. Nevertheless, at least half of
the electrons introduced by the oxygen vacancies would be
free to reduce the Co3+ to Co2+ or to move on the Mo in
sixfold oxygen coordination. Although a small fraction of
metallic cobalt or nickel was found in the XRD data of the
samples after reduction in 5% H2/Ar at 800 °C for 20 h (see
Figure 7), this fraction was too small to percolate through
the sample. Therefore, the grain conductivity can be assumed
to be dominated by transfer of electrons from Mo5+ to Mo6+
octahedral sites. This assumption is supported by the increase
in conductivity above 600 °C, which could be due to
excitation of electrons from Co2+ to Mo6+ or, more likely,
by release above 600 °C of electrons trapped at fivefoldcoordinated ModO. The increase in resistivity with temperature below 600 °C is more problematic; but the motional
enthalpy of polaronic conduction may increase with temperature or the grain-boundary contribution to the conductivity may be dominant at lower temperatures. An analogous
increase in the conductivity of the M ) Ni sample above
600 °C is also apparent.
The double-perovskite samples were tested as anode
materials in SOFCs having a 300 µm thick LSGM electrolyte
and SrFe0.2Co0.8O6-δ cathode. Figure 8 shows SEM images
for the single cell with Sr2CoMoO6 (SCMO) as the anode
after operating in H2 and CH4. A clear cross section of the
cell configuration can be seen in Figure 8A. Estimated from
the SEM images, the thicknesses of the LSGM, SCMO,
LDC, and SCF layers are 310, 26, 8, and 20 µm, respectively.
The LDC buffer layer obviously exists between LSGM and
Sr2MMoO6 (M ) Co, Ni) for Solid Oxide Fuel Cells
Chem. Mater., Vol. 21, No. 11, 2009 2325
Figure 10. Maximum power density (Pmax) as a function of power cycle
for various anodes Sr2MMoO6 (M ) Co and Ni) at 800 °C in H2, dry CH4,
and wet CH4.
Figure 9. Power density and cell voltage as functions of current density at
800 °C in H2, dry CH4, and wet CH4 for (A) Sr2CoMoO6 and (B)
Sr2NiMoO6.
SCMO, which efficiently prevents a diffusion reaction
between the anode and the electrolyte. Both anode and
cathode layers are porous, which favors transfer of oxygen,
fuel gas, and exhaust products.
Power density and cell voltage as functions of current
density at 800 °C in H2, dry CH4, and wet CH4 for cells
with Sr2MMoO6 (M ) Co and Ni) anodes are presented
in Figure 9. In H2, Sr2CoMoO6 exhibits a maximum power
density (Pmax) of 735 mW/cm2 at a current density of 1380
mA/cm2, almost comparable with Sr2MgMoO6.5 In wet
CH4 (containing 3% H2O), Pmax reaches as high as 527
mW/cm2 at a current density of 920 mA/cm2, which is
higher than that of Sr2MgMoO6. The Sr2CoMoO6 anode
showed a remarkable electrochemical performance in H2
and wet CH4, but its power density in dry CH4 was only
186 mW/cm2, which is much lower than that in wet CH4.
Sr2NiMoO6 showed only a notable power output in dry
CH4.
We used power cycles to test the stability of the anodes
in different fuels. Figure 10 shows the maximum power
density as a function of cycle number for the Sr2MMoO6
(M ) Co and Ni) anodes at 800 °C in H2, dry CH4, and wet
CH4. Each cycle was run from OCV (open circuit voltage)
to 0.2 V and back to OCV, which took 20 min. A total of
50 cycles, i.e., 1000 min, was carried out for each anode.
We define Pmax,1 as the maximum power density for the first
cycle and Pmax,50 for the 50th cycle. power loss was calculated
by (Pmax,1 – Pmax,50)/(Pmax,1) × 100%. Power loss over 50
cycles in H2 is 6.3% and 6.8% for M ) Co and M ) Ni,
respectively. In dry CH4, Sr2CoMoO6 shows a Pmax,1 of 186
mW/cm2 and a power loss over 50 cycles of 24.2%;
Sr2NiMoO6 shows a higher Pmax,1 of 273 mW/cm2 but a more
rapid power loss over 50 cycles of 43.2%. In wet CH4,
Sr2CoMoO6 exhibits a large Pmax,1 of 527 mW/cm2, but the
Pmax value drops to 368 mW/cm2 after 50 cycles, a 30.2%
loss.
The higher power output with H2 fuel for the M ) Co
sample is consistent with its higher concentration of oxygen
vacancies and higher electronic conductivity, which we
believe to be the result of the stronger octahedral-site
preference of the Ni2+ ions. From Figure 10, we can see
that both Sr2CoMoO6 and Sr2NiMoO6 anodes run stably over
power cycling in H2, which can be ascribed to their stable
phases in H2. The XRD pattern in Figure 7 has typically
shown that Sr2CoMoO6 is almost pure after being reduced
at 800 °C in H2 for 20 h.
The performances are quite different for Sr2CoMoO6 and
Sr2NiMoO6 in dry and wet methane. Sr2NiMoO6 shows a
higher power in dry CH4, demonstrating that M ) Ni favors
the main process for direct oxidation of methane. The
reaction can be expressed as
CH4 + 4O2- ) CO2 + 2H2O + 8e-
(3)
The O2- ions are created in the cathode and transported
through the electrolyte from the cathode. Sr2CoMoO6 exhibits
a much higher power in wet CH4, indicating a preferential
reaction pathway on the anode through steam reforming of
methane:
CH4 + H2O ) CO + 3H2
(4)
We should take into account the catalytic effect of
Sr2CoMoO6 on the above reformer reaction. Metallic Co was
observed in Sr2CoMoO6 exposed to CH4 (see the XRD
patterns in Figure 7).
Moreover, Co has been found to have
high catalytic activity for the reforming of methane.26,27
Therefore, it is reasonable to propose that the high performance for Sr2CoMoO6 in wet CH4 fuel is due to the catalytic
effect of surface Co0 on the reformer reaction. Metallic Ni
can also act as a catalyst for steam reforming of CH4,28 but
the power density of Sr2NiMoO6 is lower than that of
Sr2CoMoO6 in wet CH4. We note that Ni easily induces
formation of graphitic carbon on the catalyst, which causes
(26) Lucredio, A. F.; Assaf, E. M. J. Power Sources 2006, 159, 667.
´
(27) Profeti, L. P. R.; Ticianelli, E. A.; Assaf, E. M. Fuel 2008, 87, 2076.
(28) Laosiripojana, N.; Assabumrungrat, S. J. Power Sources 2007, 163,
943.
2326
Chem. Mater., Vol. 21, No. 11, 2009
Huang et al.
catalytic deactivation and blockage of the reactor.27 In
addition, after exposure of the sample to CH4, a tiny amount
of SrCO3 and SrMoO4 also became visible in the XRD data
(Figure 7).
SrCO3 and SrMoO4, which would be on the
surface of the anodes, may block the catalytic pathways for
reforming of the fuel. These impurities would increase with
operation time and may be the reason why the power density
in CH4 drops rapidly over cycling in Figure 10.
transfer of O2- ions from the oxide-ion electrolyte to the
anode surface to replenish the surface oxygen that is lost. In
methane, the rate-limiting processes are the chemical reactions occurring at the surface of the anode, but the dominant
processes are obscured by the exolution onto the anode
surface of Co or Ni and SrCO3 with SrMoO4 in the reducing
atmosphere at the anode.
Conclusions
Acknowledgment. We thank the Robert A. Welch Foundation, Houston, TX, for support of this work. The work at Sam
Houston State University (SHSU) was supported by the National
Science Foundation under Grant No. CHE-0718482 and an
award from Research Corporation. M.K. acknowledges financial
support from the Academy of Finland (Decision Nos. 110433
and 116254).
Y.H.H. acknowledges support from the National
Science Fund for Distinguished Young Scholars of China (No.
50825203).
Magnetic, structural, and XAS data show the existence of
Co2+/Mo6+ and Ni2+/Mo6+ pairs in the double-perovskites
Sr2MMoO6. The Mo6+/Mo5+ reduction potential is low
enough to accept electrons from H2 and CH4, thus allowing
dissociative chemisorption of the fuel. Moreover, displacement of a Mo6+ or a Mo5+ ion within an octahedral site to
form a stable ModO species would trap oxygen vacancies
in Mo-rich antiphase boundaries and release electrons trapped
at oxygen vacancies; also, ModO species may favor reaction
of the fuel with a surface oxide ion by lowering its acidity
and energy for desorption of the oxidized reaction product.
Polaronic conduction on the mixed-valent Mo6+/Mo5+ array
allows transfer of the electrons from the surface reaction to
the current collector, and bulk oxygen vacancies allow
Supporting Information Available: Rietveld refinement of
XRD patterns, magnetic susceptibility, thermoelectric power, and
the average activation energy of electronic conduction for the anode
materials.This material is available free of charge via the Internet
at http://pubs.acs.org.
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