ECS Transactions, 25 (2) 1679-1686 (2009)
10.1149/1.3205706 © The Electrochemical Society
Modified Sol-gel Method Used for Obtaining SOFC Electrolyte Materials
Crina Suciu1, Egil Severin Erichsen2, Alex C. Hoffmann3
Eugen Dorolti4 and Romulus Tetean4
1
Prototech AS, 5072 Bergen, Norway
Laboratory for Electron Microscopy, University of Bergen, 5007 Bergen, Norway
3
Institute for Physics and Technology, University of Bergen, 5007 Bergen, Norway
4
Physics Faculty, Babes-Bolyai University, Cluj-Napoca, Romania
2
A new modified sol-gel method using sucrose and pectin as
organic precursors was used to obtain 7ScSZ and 11ScSZ
nanoparticles as electrolyte materials for solid oxide fuel cells
(SOFCs). Scandia stabilized zirconia (ScSZ) offers significantly
higher conductivity than most other electrolyte materials (e.g.
yttria stabilized zirconia, YSZ). The high cost of scandia can be
compensated to some degree by the use of thin electrolyte films.
Also, it is important to reduce the costs incurred by all the
intermediate fabrication steps of SOFCs. The new organic
precursors used in the modified sol-gel method have the
advantages of being ubiquitous, cheap, easy to handle and store,
and environmentally friendly. The produced nano-powders were
pressed to pellets and sintered in two different ways to obtain
different grain sizes. The powders and pellets were investigated
using TEM (Transmission Electron Microscopy), XRD (X-ray
Diffraction), physical adsorption (BET), and EIS (Electrochemical
Impedance Spectroscopy) to determine the morphology, particle
size, crystal structure, crystallite size and specific surface area of
the powders and the electrical properties of the sintered pellets.
Introduction
Solid oxide fuel cells (SOFCs) are complex electrochemical devices for power
generation that offer significant advantages over conventional power generation
technologies, decreasing the environmental impact and increasing the efficiency of power
production. There remain a number of challenges that may delay the full
commercialisation of SOFC, mostly related to the materials selection, such as increasing
the current density that can be achieved, and improving the cell reliability, which is
related to the interactions between materials and between the cell components. Thus,
progress depends mainly on materials development.
High temperature solid oxide fuel cells (SOFCs) have, due to their high efficiency
and their environmental friendliness (in that they do not emit SOx, NOx or particles),
continuously stimulated researchers’ attention in the past years. There are many materials
problems remaining to be solved to obtain high-performance fuel cells, many of these
problems arising from the high operating temperature of 10000C. The electrolyte
resistance is often a significant contribution to the total cell resistance of SOFCs and so a
solid electrolyte with high ionic conductivity and good long-term stability should be
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devised for high performance SOFCs (1). Yttria stabilized zirconia (YSZ) with a
conductivity of about 0.1 S/cm at 10000C and 0.3 S/cm at 8000C, is extensively used and
commercialized as SOFC electrolyte (2). Lately, scandia stabilized zirconia (ScSZ) has
been reported to possess the highest conductivity and attracted attention as solid
electrolyte materials in SOFCs. In the Sc2O3-ZrO2 phase diagram seven phases are
identified: monoclinic, tetragonal, cubic, metastable tetragonal-rich t’, and ! (Sc2Zr7O11),
" (Sc2Zr5O13), # (Sc4Zr3O12) rhombohedral phases. The metastable t’ phase has been
reported to possess the higher conductivity, while monoclinic and ! phase possess the
lowest conductivities (3,4).
In the YSZ system the highest conductivity is found at the composition near the lower
solubility limit of Y2O3 in ZrO2 because of the interaction of the net positively charged
oxygen vacancies and net negatively charged Y3+ ion substitutes for Zr4+ ions and the
distortion of the cubic lattice by the ionic radius difference between Zr4+ and Y3+. In the
case of the ScSZ system, the lower concentration limit of Sc2O3 for stability of the cubic
structure is around 8 mol% but, in contrast to the YSZ system, the highest conductivity is
not found at this lower limit; samples with high Sc2O3 content also show high
conductivity. This might be due to the difference of ionic radii of Y3+ and Sc3+; the ionic
radius of Sc3+ is closer to that of Zr4+ and the distortion of the lattice by the substitution
of Sc3+ with Zr4+ does not affect the migration of the oxygen ions as much.
Due to the existence of the many phases, the structure of ScSZ is very sensitive to the
fabrication process and thermal history. Besides calcination, the sintering process plays
an important role for the microstructure. During sintering/densification of a green
compact to a pellet or a thin layer, phenomena such as crystallite coarsening, volume
reduction, decrease in porosity and increase in grain size will take place simultaneously.
The high cost of scandia, insufficient long-term stability and the aging effect caused
by annealing at high temperature are drawbacks in using ScSZ as electrolyte material.
The cost of scandia may be reduced by large-scale use since scandium is quite common
in nature and only a few technological applications of the compound have been
developed as yet.
Sc2O3-ZrO2 bulk materials need high sintering temperature (above 15000C) and a
long annealing time which leads to difficulties in preparing homogeneous materials with
good reproducibility. Therefore, low-temperature synthesis processes are needed to
obtain homogeneous ScSZ nanomaterials at low cost, simple and with good
reproducibility (5).
Nanocrystalline materials have received much interest lately due to the reduction of
the grain size to the nanometer scale and to the large fraction of atoms in the grain
boundary region. These properties influence the mechanical and sintering behavior of the
nanocrystalline materials. For example the sintering temperature can be lowered and thus
energy is saved and also new electrochemical devices can be fabricated (6,7).
For this paper, the objectives are: (a) to study the new modified sol-gel method, using
sucrose and pectin as organic precursors, for obtaining 7 and 11 mol% ScSZ
nanoparticles; ScSZ is proving a promising material for SOFC electrolytes but the
complexity of the solid solution requires a comprehensive knowledge of the relationship
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between the microstructure created by the new synthesis method and the associated
properties; and (b) to study the grain size-dependent electrical properties of the ScSZ
dense ceramic samples which were obtained from the as-prepared nanopowders. To help
achieve this, two different sintering schemes were used; so-called “normal sintering” and
“fast sintering” (see details below) to obtain two different crystal sizes in the pellets.
Experimental
ScCl3.6H2O (Sigma-Aldrich, p=99.999%) and ZrCl4 (Sigma-Aldrich, p=99.9) were
used as precursor salts in the previously calculated ratios. The raw salts were dissolved in
distilled water on a warming plate under stirring. After complete dissolution, a mixture of
sucrose and pectin (commercial usage) in a mass ratio of 10:0.5 was added slowly under
heating and stirring until complete dissolution. The heating was kept constant at a
temperature below 900C to do not destroy the pectin properties. From the clear solution
the sol and the gel started to form slowly. The mixture was continuously dried for about 5
h until the xerogel was obtained taking the character of a black resin (see Figure 1). The
final dried xerogel was subjected to a calcination step at 6500C for 3 h with a heating rate
of 2000C/h. The final oxidic powders, denoted as 7ScSZ and 11ScSZ, were investigated
and characterized by TEM, XRD and BET to establish the morphology, crystal structure,
and particle and crystallite sizes.
The final oxidic powders were uniaxially pressed at 600 bars into pellets followed by
a cold isostatic press step at 1200 bars for 2 minutes. The final pellets were sintered in
two different ways using the “normal” sintering for sample 11ScSZ and “fast” sintering
program for sample 7ScSZ (see Figure 2). The final pellets were investigated by EIS and
by the Archimedes method to establish the electrical properties and the full density.
Figure 1. The procedure for ScSZ nanopowders fabrication.
Results and Discussion
X-ray diffraction (XRD) was carried out on a Bruker D8 Advance diffractometer
using the K! radiation of copper (λ = 0.15406 nm). The intensities were measured from
2θ = 20 to 100°. The structure refinement was performed according to the Rietveld
technique, supported by the FULLPROF computer code (8,9) under the assumption of
Thompson-Cox-Hastings line profile. The auto-coherent domain (or crystallite) size was
deduced from line broadening by means of the Scherrer formula. The best-fit agreement
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Figure 2. The “normal” (in black) and “fast” (in red) sintering of the ScSZ pellets.
is given by the classical agreement factors (10) RB and $2. Figure 3 presents a comparison
of the observed and calculated (100-x) ZrO2 – xSc2O3 (x = 7 and 11%) powder patterns.
The refinement revealed that the two compounds adopt a cubic structure (Fm-3m) with a
small variation in the unit-cell parameters. This evolution is proportional with the amount
of the ZrO2 and Sc2O3 phases observed in the compounds. The crystallographic data are
presented in Table I.
TABLE I. Crystallographic data of 7ScSZ and 11ScSZ nanopowders.
Experimentally
Unit-cell
Theoretical
Sample
composition
determined
parameters
(%)
Composition
(I)
ZrO2
Sc2O3
ZrO2
7ScSZ
94
7
92.28
11ScSZ
90
11
88.76
(%)
Sc2O3
Crystallite size
Rb
(nm)
ZrO2
Sc2O3
ZrO2
Sc2O3
ZrO2
Sc2O3
7.72
5.092(9)
5.046(7)
3.44
4.09
11.62
10.50
11.24
5.090(0)
5.036(7)
1.84
3.88
10.51
9.42
The compositions determined experimentally by XRD presented in Table I are in a
very good agreement with the 7 and 11 mol% ScSZ predicted during the syntheses
process. Although the two materials, Sc2O3 and ZrO2, form a solid solution, their XRD
patterns can be distinguished. The crystallite sizes of the obtained powders can be
calculated from the peaks broadening corresponding to each phases and is around 10 nm
for Sc2O3 and 11 nm for ZrO2 (Table I). This confirms that the results are reliable and is
also evidence that the new modified sol-gel method used during the experiments gives
products of a high quality with good reproducibility.
The green density was measured after the pressing steps according to the formula
!green=m/("r2h) where m is the mass, r is the radius and h the height of the pellet. The
body density was calculated using Archimedes method (see Table II).
The theoretical density was calculated using the XRD data using the formula
%theoretical = "(nM)/Na3 (g/cm3) where n is number of atoms per unity cell, M is the
molecular weight, N is Avogadro’s number and a is lattice parameter. The porosity was
calculated using %green and %theoretical in the relation P = 1-(%green /%theoretical).
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Intensity (arb. units)
E x p e rim a n ta l
C a lc u la te d
D iff
B ra g g p o s itio n
Intensity (arb. units)
E x p e rim a n ta l
C a lc u la te d
D iff
B ra g g p o s itio n
20
30
40
50
60
70
80
90
100
2 θ a n g le (d e g )
Figure 3. X-ray diffraction of 7ScSZ and 11ScSZ compounds. The 'Diff' line correspond
to the difference between the experimental and calculated data.
TABLE II. Properties of the obtained pellets before and after the sintering steps
Sample Name
Green
Body
Theoretical
Porosity
density
Density
Density
(g/cm3)
(g/cm3)
1.9037
5.1764
5.862
0.117
7ScSZ – fast
sintering
1.8173
5.3319
5.688
0.063
11ScSZ –
normal
sintering
Linear
Shrinkage
0.401
0.4571
The BET specific surface area (SBET) and the pore volume (VP) distribution were
measured at 77.5K using the Gemini 2380 nitrogen physisorbtion analyzer
(Micromeritics Co. Ltd.). The pore size distributions were calculated using the adsorption
branch of the isotherms, based on the BJH method (11). The BJH pore diameter (rp) was
calculated as 4VP/SBJH. The obtained powders were previously degassed at 2000C for 2 h.
Table III shows the data obtained directly and indirectly from the BET/BJH analyses
of the powders. The average particle diameters shown were calculated using the
experimentally determined specific surface area and the theoretical particle density
assuming the particles to be spherical. The pore radii, rP, and the total pore volume, VP, of
the powders shown in the Table III are consistent with the other results, in that the pore
radius is similar to the particle radius and the porosity of the powder can be calculated to
be about 0.6.
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TABLE III. BJH pore diameter rp, pore volume VP, and BET surface area SBET of the obtained 7 and 11 mol%
ScSZ.
Sample
7ScSZ
11ScSZ
rP (nm)
10.9496
10.4508
VP (cm3g-1)
0.2819
0.3010
80.85
99.01
SBET (m2g-1)
Particle size (nm)
12.66
10.66
The morphology of the nanopowders was observed by transmission electron
microscopy (TEM, JEOL-JEM-1011). The powders were dispersed in distilled water and
stirred for 1 min. One drop of the suspension was deposited on a copper grid (Agar
G2400C, 400 Square Mesh, copper 3.05mm), which had previously been covered by an
Formvar powder (Agar Scientific LTD, Essex) dissolved in chloroform followed by a
carbon deposition process under vacuum (208C Agar Turbo carbon coater) and a glow
discharge. The obtained powders are constituted of an assembly of homogeneous and
weakly agglomerated nanoparticles. The mean particle size calculated from the TEM
images is about 10nm (see Figure 4) in good agreement with the other methods of
powder characterization. Further sintering experiments indicated that the as-prepared
nanocrystalline powders display good sinterability.
Figure 4. TEM images of the obtained powders calcined at 6500C.
The electrical properties of the materials were determined by using impedance
spectroscopy. The obtained ScSZ nanopowders were pressed into pellets by using the
cold isostatic press (CIP) at 1200 bars for 2 minutes followed by a sintering process at
15000C for 10h at a heating rate of 2000C/h. The densities of the pellets were determined
by the Archimedes method to be 90 and 92%. Platinum paste was then painted on both
sides of the pellets and dried slowly at elevated temperatures up to 7000C to eliminate all
the organic substance present in the Pt paste. The as-prepared pellets were placed in a
furnace, open to air, with a temperature control system. The two electrodes were
connected via Pt wires to a Solartron analytical 1260 Impedance/Gain-Phase Analyzer.
The electrochemical impedance spectra were recorded after thermal equilibration for 0.5
h in the temperature range of 200 to 10000C at 500C intervals. The AC amplitude was to
500mV at the lower temperatures and the frequencies swept between 1Hz and 1MHz.
The EIS results were analyzed using the Z-plot software based on complex non-linear
least squares fitting. Two parallel RQ (R-resistance, Q-constant phase element) in series
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was adopted as equivalent circuits to distinguish the bulk and the grain boundary
conductivities, respectively.
Figure 5 shows the temperature dependence of the electrical conductivity of ScSZ
sintered pellets. Both materials exhibit similar conductivity between 400 and 10000C. At
temperatures below 4000C the materials behave differently, the log-log plot showing that
the conductivity of the material with lower doping level, 7ScSZ, is significantly the
higher of the two.
10
7ScSZ
11ScSZ
5
ln(sigma*T)
0
0
0.5
1
1.5
2
2.5
-5
-10
-15
1000/T
Figure 5. Temperature dependence of the electrical conductivity of ScSZ.
Figure 6 shows the complex impedance for the two materials at the temperature of
4000C. The left semicircular shape corresponds to the response of the bulk material,
while the right one (visible at least for the 7ScSZ material) corresponds to the response of
the grain boundaries, the two being distinguishable because the grain boundary response
time is much longer than that of the grain interior (the crystalline bulk material) (12). It
is clear from the plot that there is a significant difference between the resistances within
the crystalline materials itself, i.e. within the bulk of the grains. Such a difference might
be expected, since the level of doping is different between the two, and demonstrates that
the conductivity of the less-doped material is the higher at this temperature. A difference
in the grain boundary response might also be expected due to the different sintering
schemes used, and indicated by the difference in the porosity of the two materials, but
this is not visible due to the fact that the right semicircle for the 11ScSZ material is
obscured by the electrode response, which has the longest response time of them all.
Conclusions
Using the new modified sol-gel method with sucrose and pectin as organic precursors
highly crystallized, homogeneous and weakly agglomerated nanocrystalline ScSZ
powders can be obtained. The crystals of the obtained nanopowders are in the fluorite
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Figure 6. Complex impedance measurements of the materials at 4000C.
cubic structure with a crystallite size of ~10nm. Two types of powder, 7ScSZ and
11ScSZ, were successfully pressed and sintered into pellets with acceptable relative
densities. The complex conductivities of the two materials were compared by EIS, and
shown to be the same over a wide temperature range relevant for the operation of SOFCs.
At lower temperature, below 4000C, the conductivity of the lower doped material, 7ScSZ,
was found to be significantly lower than that of the other one, and, at least part of this
difference was shown to be due to differences in the conductivities of the actual
crystalline material in the interior of the grains. It is concluded that using homogeneous
nanocrystalline powders, obtained through the new modified sol-gel method, can be a
feasible way to fabricate solid oxide fuel cells ceramics.
Acknowledgments
The authors gratefully acknowledge the Norwegian Research Council for funding this
work.
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