Journal of New Materials for Electrochemical Systems 10, 243-248 (2007)
© J. New Mat. Electrochem. Systems
Flash Combustion Synthesis and Characterisation of
Nanosized Proton Conducting Yttria-doped Barium Cerate
*
M. Jacquin, Y. Jing, A. Essoumhi, G. Taillades, D. J. Jones and J. Rozière
Laboratoire des Agrégats Moléculaires et Matériaux Inorganiques, UMR 5072 Université Montpellier II,
Place E .Bataillon, 34095 Montpellier Cedex 5. France
Received: October 13, 2006, Accepted: April 5, 2007
Abstract: BaCe0.9Y0.1O2.95 (BCY10) was synthesised by a quick and simple combustion method using glycine as fuel and nitrate as oxidant. The mixture was ignited at 600 °C, resulting in a fine powder which was characterised by transmission and scanning electron microscopies, and X-ray diffraction. The nano-sized crystallites obtained allow the preparation of fully densified materials with densities up to
98 %. Water uptake was investigated in compressed and sintered samples of BCY10, bulk and total conductivities were determined with
impedance spectroscopy in the range 300 – 600 °C. Densified yttria doped barium cerate materials show a bulk conductivity of 2.3 × 10-2 S
cm-1 and a total conductivity of 1.2 × 10-2 S cm-1 at 500 °C.
Keywords: Ceramic oxide, yttria-barium cerate, flash combustion, nanoparticles, water uptake, proton conductivity
proton hopping through the oxygen ion sublattice [1].
The conventional method of preparing this type of ceramic is to
perform a solid state reaction from a stoichiometric mixture of
barium carbonate, cerium and yttrium oxides at temperatures
higher than 1000 °C. This method is relatively simple but timeconsuming, energy intensive and leads directly to materials of
large grain size in which grain boundary effects are significant.
However, nanoparticles are essential for good sinterability and for
preparation of fully densified materials.
In addition to the solid state route, several wet chemical techniques like oxalate co-precipitation, modified Pichini process [4,
5] or combustion synthesis have been employed to obtain ultra fine
and more homogeneous powders. We have developed a flash combustion route allowing the preparation of yttrium-doped barium
cerate nanopowders. Combustion synthesis methods have been
used in the past for the preparation of various oxides including
alumina powder [6], alumina-ceria composites [7], ferrites [8-10],
zinc oxide [11] and recently zirconate [12]. This quick, safe and
low cost route takes advantage of the exothermic and selfsustaining redox reaction between high oxygen content metal salts
(e.g. nitrates) and a suitable fuel (e.g. urea, glycine, citric acid) that
acts as a reducing agent. The parameters that influence the reaction product are the type of fuel, the fuel to oxidizer ratio, and the
ignition temperature. Use of suitable fuel in combustion syntheses
ensures stability of the chemical composition and high quality of
1. INTRODUCTION
Proton ceramic fuel cells (PCFC) represent a promising technology for electric power conversion with potentially high conversion
efficiency and low environmental impact. In contrast to solid oxide
fuel cells (SOFC) operating between 800 and 1000 °C, and to
polymer electrolyte based devices working below 150 °C, PCFCs
function in an intermediate temperature range (400 – 600 °C). At
this temperature, the problem of thermal ageing of SOFC components and the use of expensive catalysts in PEMFC are avoided.
A key factor in developing PCFC is to identify appropriate electrolyte materials with proton conduction properties at these intermediate temperatures. Among potential materials, rare earth doped
barium cerate compositions exhibit high proton conductivity and
low activation energy in a water vapour atmosphere [1, 2]. The
mechanism of proton conductivity in these materials is related to
the substitution of cerium by trivalent rare earth cations (M = Y3+,
Yb3+, Gd3+…) which creates oxide ion vacancies giving BaCe1xMxO3-x/2 compositions. At moderate temperature, water from the
gas phase dissociates [3] according to the defect reaction where
two positively charged protonic defects ( OH O• ) are formed:
H 2 O + VO•• + OOx → 2OH O•
(1)
Thus, proton conduction is a consequence of thermally actived
*To whom correspondence should be addressed: Email: [email protected]
Tel: +33-467-14-32-11, fax: +33-467-14-33-04
243
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M. Jacquin et al. / J. New Mat. Electrochem. Systems
products, and produces non-toxic gases. The research of Hwang et
al. has selected glycine among different fuels, due to its low price
and its most negative heat of combustion [10].
The basis of the combustion synthesis technique comes from the
thermochemical concepts used in the field of propellants and explosives. Jain et al. [13] introduced a simple method of calculating
the oxidizing to reducing character of the mixture, which consists
of establishing a simple valency balance to calculate the
stoichiometric composition of the redox mixture. According to
Chick [14] the usual products of the combustion reaction are CO2,
H2O and N2. Therefore, carbon, hydrogen, barium, cerium and
yttrium are considered as reducing element with valencies +4, +1,
+2, +3 and +3 respectively. Oxygen is considered as an oxidizing
element with the valence of -2, nitrogen is considered to be 0. In
the case of synthesis of BCY10, barium, cerium and yttrium nitrates are used as metal salts and the total calculated valence of
metal nitrates by arithmetic summation is –25. The calculated
valence of glycine is +9.
Previous studies [11, 16] have shown that non-stoichiometric
conditions and an ignition temperature of 600 °C are needed for a
proper combustion synthesis. Only at fuel-to-oxidiser ratio larger
than 2 a crystalline powder is formed, while at a ratio greater than
3, a carbonaceous residue remains after combustion. Following
the principle of propellant chemistry [15], for stoichiometric redox
reaction between a fuel and an oxidizer, the glycine needed to
balance the total oxidizing and reducing valencies leads to (-25) +
n(+9) = 0, or n = 2.77 mol of glycine per mole of BCY10.
In this paper we report the synthesis of BaCe0.9Y0.1O2.95 in fuelrich conditions with glycine-to-nitrates ratio fixed at 2.5, or n =
6.95 mol of glycine per mole of BCY10. The powder obtained has
been characterised by transmission and scanning electron microscopies and X-ray diffraction. In order to obtain fully densified
pellets, the sintering process (heating rate, intermediate and final
temperature) after compaction of the powder has been optimised.
Those pellets have been characterised by impedance measurements and water uptake has been investigated thermogravimetrically.
2. EXPERIMENTAL
Powders of BaCe0.9Y0.1O2.95 were synthesised from a combustion reaction. Starting materials were high purity Ba(NO3)2,
Y(NO3)3.6H2O, Ce(NO3)3.6H2O and glycine (Aldrich). The appropriate molar ratios of nitrates and glycine were mixed in a minimum amount of deionized water to obtain a limpid solution. A
glycine to nitrate ratio of 1:2.5 was used. The aqueous solution
was concentrated on a hot plate at 200 ° C, generating a viscous
liquid. Upon complete evaporation of the water, the dried mixture
was placed into an oven at 600 °C to start the combustion reaction
which occurred within 2-3 mins. The powder obtained was annealed at 600 °C for 10 h, and then calcined at 900 °C for 10 h.
The morphology and the grain size after annealing at 600 °C,
and after calcining at 900 °C, were observed by transmission electron microscopy (TEM, JEOL 1200 EX) and scanning electronic
microscope (SEM, HITACHI 52600N). Phase purity, identity and
homogeneity of the calcined powder were confirmed by X-ray
diffraction (XRD) analyses performed at room temperature using
a Seifert θ-θ diffractometer with CuKα radiation. XRD patterns
were recorded between θ of 5 ° and 50 ° with a step size of 0.02 °
(a)
(b)
Figure 1. Scanning electron micrographs of BCY10 powder: as
synthesised (a) and calcined (10 h, 900 °C) (b).
and a measuring time of 3 s at each step.
BCY10 powder was compacted under vacuum and pressed
under 220 MPa into disks of diameter 13 mm and thickness 0.8
mm. In order to obtain fully densified pellets, the sintering process
(heating rate, intermediate and final temperature) after compaction
of the powder has been optimised. First, four final sintering temperatures (1300, 1350, 1420 or 1500 °C, in air for 10 h) were
tested, with a heating rate of 3 °C min-1. All pellets were characterised by XRD analyses. Then the sintering process has been
modified with an intermediate isotherm at 900 °C for 2 h reached
with a heating rate of 1.5 °C min-1.
Water uptake and conductivity measurements were made on
two different pellets, with compacities of 98 % and 80 %.
To determine water uptake at 600 ° C, each pellet sintered was
first dried for several hours at 900 °C under dry nitrogen, then
cooled down to 600 °C. The sample was held at this temperature,
in a dry nitrogen atmosphere until its weight stabilised. Wet nitrogen (3% H2O, 97% N2) was then passed over the sample by flowing N2 (50 dm3 min-1) through a water bubbler at room temperature. The water uptake at 600 °C was measured by recording the
weight change with a thermobalance (Netzch TG 439).
Platinum electrodes were sputtered on each face of a BCY10
disk and the sample placed in a closed cell. Wet nitrogen gas
(bubbler at room temperature) was allowed to flow through the
cell. Impedance spectra were recorded in the 300-600 °C temperature range using an HP4192A impedance analyser. Each measurement was taken after 1 h stabilisation at each temperature. Imped-
Flash Combustion Synthesis and Characterisation of Nanosized Proton Conducting Yttria-doped Barium Cerate
/ J. New Mat. Electrochem. Systems
245
(a)
(b)
Figure 3. Powder X-ray diffraction pattern of BCY10 powder calcined at 900 °C.
(c)
Figure 2. Transmission electron micrographs of BCY10 powder:
as-synthesised (a), annealed at 600 °C (b) and calcined at 900 °C
(c).
ance spectra were obtained in the 10 Hz – 10 MHz frequency range
with 10 logarithmically spaced frequencies and using a 100 mV
oscillating voltage.
3. RESULTS AND DISCUSSION
Assuming complete combustion, the overall reaction between
barium, cerium and yttrium nitrate to form BaCe0.9Y0.1O2.95 can be
written as:
Ba(NO3)2 + 0.9 Ce(NO3)3 + 0.1 Y(NO3)3 + 8/3 H2NCH2COOH
→ BaCe0.9Y0.1O2.95 + 16/3 CO2 + 23/6 N2 + 20/3 H2O
(2)
Fig. 1 shows typical scanning electron micrographs of the as
synthesised (a) and calcined (10 h, 900 °C) (b) products after man-
ual grinding. For the as synthesised BCY10 a continuous network
is formed. The morphology shows flakes with holes and large voids
as already observed in different reported studies of flash combustion syntheses of transition metal oxides with a fuel-rich molar
ratio. This morphology is due to the liberation of a large amount of
CO2, N2 and H2O gases during the reaction. After heat treatment at
900° C and with high magnification, non-aggregated nanoparticles
can be observed having a coral-like appearance (Fig. 1b).
Transmission electron micrographs of BCY10, as-synthesized
(a), annealed (600 °C) (b) and calcined (900 °C) (c) (Fig. 2) were
taken after grinding and sonication of an ethanol suspension. The
crystallite sizes are nanometrics, but a slight evolution of the particle size with the temperature of heat treatment is observed. The
estimation of the crystallite size is respectively 3 nm, 15 nm and 48
nm, and in each case the particle size distribution is homogeneous.
Fig. 2c shows that the particles of the calcined powder have hexagonal morphology.
X-ray diffraction was performed on BCY10 after sintering at 900
°C for 10 h in order to check the purity of the crystalline product
and to allow estimation of the crystallite size of the powder. A
calcination at 900 °C results in a dominant perovskite phase structure and the powder pattern (Fig. 3) is in agreement with those
previously reported [17]. The space group is Pmcn with the lattice
parameters a = 8.772 Å, b = 6.249 Å, c = 6,221 Å, values close
those reported by Takeuchi et al. and Knight [18-20]. The grain
size of the calcined product was estimated using the Scherrer formula [21]. It was found that the particle size is ~47 nm, very close
to that observed in TEM.
The compacities of BCY10 disks obtained after compaction of
the powder under 220 MPa under vacuum and sintered at various
temperatures (1300, 1350, 1420 and 1500 °C) are given in Table 1.
Compacity increases with the temperature, with 70 % at 1300 °C,
75 % at 1350 °C and 80 % at 1420 and 1500 °C. These pellets were
also characterised by X-ray diffraction, and results are presented in
Figure 4. As of 1300 °C, cerium oxide traces are detected, and the
intensity of the characteristic peaks of CeO2 strongly increases with
the temperature of sintering. The increasing presence of cerium
oxide is related to the decomposition with temperature, of the
perovskite structure at the surface of the disk. At high temperature,
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M. Jacquin et al. / J. New Mat. Electrochem. Systems
Table 1. Evolution of the pellet compacity with the temperature of
sintering
Temperature of sintering (°C)
Compacity (%)
1300
70
1350
75
1420
80
1500
80
Figure 6. Water uptake by pelletized P80 at 600 °C under wet
nitrogen (3% H2O, 97% N2). See text for further explanations.
0.5
(a)
Figure 4. Powder X-ray diffraction pattern of BCY10 disk compacted under 220 MPa under vacuum and sintered at different
temperature.
Δ m / m ( wt %)
0.4
0.3
(b)
0.2
0.1
0.0
0
25
50
75
100
Time (min)
Figure 7. Comparison of P80 and P98 water uptake at 600 °C under wet nitrogen (3% H2O, 97% N2).
Figure 5. Powder X-ray diffraction pattern of BCY10 disk compacted under 220 MPa under vacuum and sintered at 1350 °C,
before (a) and after (b) surface polishing.
barium is easily lost from BCY10 leading to the formation of barium deficient phases and BaO [17, 22] according to :
BaCe0.9Y0.1O2.95 → Ba1-xCe0.9Y0.1O2.95-x + x BaO
Then, higher Ba losses cause decomposition of this perovskite in
BaO, CeO2 and Y2O3.
The diffraction pattern of a pellet sintered at 1350 °C and polished (Figure 5(b)), indicates that perovskite decomposition occurs
only at the surface (Figure 5(a)). Moreover, this surface appears to
be more porous than the heart of the disk, since after polishing the
compacity of the pellet was increased to 80 % (75 % before polish-
ing). This sample is denoted P80 and the sintering temperature is
fixed at 1350 °C.
Then, in order to increase this 80 % density, an intermediate
isotherm at 900 °C for 2 h reached with a heating rate of 1.5 °C
min-1, has been added to the sintering process, leading to a very
dense pellet with a density of 98 %. This sample is denoted P98.
The amount of water that can be incorporated was investigated
using isothermal gravimetry at 600 °C on P80 and P98, with constant nitrogen pressure. Water uptake measurement of P80 is given
in figure 6. The pellet was first dried at 900 °C under dry nitrogen
and then cooled to 600 °C until the sample mass remained constant
(region I). Wet nitrogen was then flowed through the thermobalance (point A), causing a rapid mass increase which corresponds to
water uptake by the sample. A plateau is reached at 0.11 wt %
(region II). At point B, dry nitrogen was again introduced, leading
to a weight loss approximately equal to the water uptake (region
III). On repeating this procedure (region IV), the second cycle from
point C (injection of wet nitrogen) to point D (dry nitrogen) shows
that water uptake by P80 is reproducible in intensity and shape. The
hysteresis observed in the first cycle (rapid water uptake, slower
loss) appears to be reduced in cycle 2. Water uptake by P98, com-
247
Flash Combustion Synthesis and Characterisation of Nanosized Proton Conducting Yttria-doped Barium Cerate
/ J. New Mat. Electrochem. Systems
-1.0
100 Ω
-1.5
-2.0
-1
log σ (S.cm )
-Z imaginary (Ω)
(a)
(b)
360 Ω
(c)
-2.5
-3.0
-3.5
1.86 kΩ
-4.0
1.0
1.2
1.4
Z real (Ω)
- Z imaginary (Ω)
1.8
2.0
2.2
2.4
1000/T (K)
Figure 8. Impedance spectra of P98 (arbitrary units), 5 Hz to 13
MHz: (a) 510 °C, (b) 350 °C, (c) 250 °C.
(a)
1.6
1 kΩ
(b)
360 Ω
Figure 10. Temperature dependence of P98 and P80 conductivities. P98: bulk (▲) and total (■), P80: bulk (Δ) and total (□).
Three representative Nyquist plots (Zreal vs. Zimag), obtained for
P98, at 245, 350 and 510 °C, over the 10 Hz – 13 MHz frequency
range in moist nitrogen are shown in Fig. 8. Throughout the temperature range investigated, the analysis of the impedance spectra
revealed three contributions. Up to 400°C, the spectrum comprises
two semicircles, with the high frequency arc assigned to the bulk
response, and the low frequency one attributed to the grain boundary conductivity [12, 26, 28]. In this temperature range, at least half
of the arc is visible and the frequency at the apex of each arc can be
determined, and the capacitance corresponding to each arc can be
calculated using the equation (3):
C=
Z real (Ω)
Figure 9. Impedance spectra of P80 (a) and P98 (b) (arbitrary
units), 5 Hz to 13 MHz, 350 °C.
pared to P80, and measured on same conditions, is shown in figure
7. A plateau is reached at a higher value than P80 (0.14 wt %) and
the time to reach this maximum is 30 mins compared to 4 mins for
the P80 sample. The water uptake is kinetically slower, due to the
higher compacity of the pellet.
In rare earth-doped barium cerate, the degree of water uptake is
directly related to the oxygen vacancy concentration, each yttrium
dopant being compensated by 1/2 oxygen vacancy. The “water”
content of BCY10 depends on the number of these vacancies (1/2
H2O fill 1/2 vacancy), and the theoretical maximum water uptake
of BaCe0.9Y0.1O2.95 is 0.28 wt % which correspond to 100 % of the
nominal oxygen vacancies. In this experiment, the maximum
weight increase of 0.14 % corresponds to filling of 50 % of vacancies. It was already observed that the maximum theoretical water
uptake in rare earth doped cerate is never reached [23-26] and our
result is in good agreement with the water incorporation study at
different temperatures of Kruth et al. on BCY10 [27], with a
weight increase of 0.11 % at 600 °C. A maximum of 80 % of oxygen vacancies can be filled, but at a temperature of 250 °C.
1
R ( 2πf max )
(3)
The bulk element has capacitance of about 10-11 F and the grain
boundary element about 8.10-10 F. When increasing temperature,
these two semicircles shift to higher frequency and around 350 °C,
a third component appears, which corresponds to the electrode
response [12, 28]. At highest temperatures, the first semicircle can
no longer be resolved because of the frequency limitation of the
impedance analyser. However, both bulk and grain boundary resistances can be determined by extrapolation. At temperatures higher
than 600 °C, the sample resistance becomes smaller than the measurement accuracy. Comparing the impedance spectra at the same
temperatures for the two materials, P98 and P80 (Figure 9), it is
evident that the increase of the compacity produces a very important decrease for the grain boundary contribution to the total resistance of the sample. Consequently the P98 sample exhibits a lower
total resistance (3 times lower) than the P80 sample.
The temperature dependence of the bulk and total conductivities
of P80 and P98 are given in Fig. 10. The bulk and the grain boundary conductivities were calculated from the respective resistance
value of the semicircles, using pellet dimensions. The total conductivity was calculated using
Rtotal = Rbulk + Rgrain boundary
(4)
The data obtained for the two BCY10 bulk conductivities, between
180 and 500 °C are comparable. There are in full agreement with
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M. Jacquin et al. / J. New Mat. Electrochem. Systems
those reported by Coors et al. [28] and slightly higher by about half
log unit than the values described by Kreuer [3]. At 500 °C, both
P80 and P98 show a bulk conductivity of 2.3 × 10-2 S.cm-1.
The bulk activation energies Ea determined with the Arrhenius
equation
piles à combustible (PAN-H) de l’Agence Nationale de la Recherche under the Tectonic project (ANR-05-PANH-015-03) is acknowledged with thanks.
(5)
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σ = A exp (-Ea/kbT)
where kb is Boltzman constant, is 0.47 eV, value close to the
reported values [3, 28] and is typical of protonic conductors.
The total conductivities of P80 and P98 are also included in Fig.
10 for comparison with the bulk conductivities. As it is recognized
for a longer time, the conductivity is strongly dependant on the
microstructure and therefore on the details of the synthesis procedure. The presence of grain boundaries leads to a decrease in proton mobility and an increase in the sample resistance, thus the conductivity of polycrystalline samples is generally significantly lower
than the corresponding bulk conductivity [3]. The total conductivity of P98 is higher than the P80’s one, with for example, at 500
°C, 1.2 × 10-2 S.cm-1 and 5.0 × 10-3 S.cm-1 respectively. The total
conductivity of P80 is about an order of magnitude lower than the
bulk conductivity, whereas the total conductivity of P98 is very
close to the bulk: the more densified material shows an improvement of the grain boundary conductivity.
In terms of activation energies, for P80, the obtained values are
the same for both bulk and total conductivities. These results suggest that in spite of an extensive grain boundary region due to the
small size of the BCY10 particles, the contact between nanograins
is good with low impedance in this region. For P98, a detailed examination of the temperature dependence of the total conductivity
reveals a proton conductivity loss for the highest temperatures
which can be related to a partial dehydration. But until 500 °C, the
total activation energy is 0.46 eV, value very close to the bulk’s
one.
4. CONCLUSION
The objective of this research was to prepare a proton conducting
BaCe0.9Y0.1O2.95 – type ceramic oxide from nano-powders. The
flash combustion synthesis method has been used successfully and
the association of X-ray diffraction and transmission and scanning
electron microscopies provides converging results indicating the
existence of nano-sized crystallites. These nanoparticles allow the
preparation of fully densified materials (98%) at relatively low
temperature (1350 °C). Sintered samples are easily hydrated in a
water-containing atmosphere. Up to 0.14 wt % water uptake at 600
°C by a BCY10 pellet corresponds to 50 % filled oxygen vacancies,
a result in good agreement with the literature.
Proton conductivity of these materials, determined by impedance
spectroscopy, is comparable to the highest value reported to date.
Bulk conductivity, is 2.3 × 10-2 S cm-1 at 500 °C. The total conductivity at this same temperature is 1.2 × 10-2 S cm-1, and the temperature dependence is close to that of the bulk.
Such characteristics show that flash combustion is an interesting
alternative approach to the preparation of proton conducting oxides
for intermediate temperature fuel cells.
ACKNOWLEDGMENTS
Funding by the Plan d’Action National pour l’Hydrogène et les
REFERENCES