A82 Journal of The Electrochemical Society, 148 (1) A82-A86 (2001)
S0013-4651/2001/148(1)/A82/5/$7.00 © The Electrochemical Society, Inc.
The Sr(Ce
1-x
M
x
)O
3-x/2
-based perovskites are the first group of
proton conductors that can tolerate the temperatures and thermal
cycling encountered in solid oxide fuel cell applications.
1
Several
studies of cerate-based systems have been reported, and the gadolini-
um-substituted barium cerates have been found to have some of the
highest conductivities in the 700-1000°C range.
2,3
For example, the
proton conductivity (4 10
-2
S/cm) of Ba(Ce
0.8
Gd
0.2
)O
2.9
at 600°C
under fuel cell conditions
2
is comparable to the oxygen-ion conduc-
tivity of yttria-stabilized zirconia at 800°C. Thus Ba(Ce
1-x
Gd
x
)O
3-x/2
(x = 0.1 - 0.2) electrolytes have received considerable attention for
fuel cell applications.
2,4
One of the best reported perovskite electrolyte
fuel cells
2
[a Ba(Ce
0.8
Gd
0.2
)O
2.9
electrolyte with platinum electrodes]
exhibited a short circuit current density of 800 mA/cm
2
at 800°C.
There is considerable interest in reducing costs and improving
cell performances by developing new mixed-conducting oxides to
replace the platinum electrodes. Several manganese-based per-
ovskites have been studied as cathodes in Ba(Ce
1-x
Nd
x
)O
3-x/2
elec-
trolyte fuel cells, and La
0.6
Ba
0.4
MnO
3
was found to have the lowest
polarization resistance (1.1 cm
2
at 800°C).
5
However, the polar-
ization resistance of this purely electronic conducting cathode is sig-
nificantly higher than that of platinum (0.35 cm
2
at 800°C).
5
Our studies have focused on new mixed-conducting perovskites
that are chemically compatible with the barium cerate-based elec-
trolytes. We have chemically modified barium cerate to produce
oxides with a high electronic and ionic (proton and oxygen-ion) con-
ductivity. We have reported the effect of Pr substitution on the elec-
tronic and ionic conductivity of 10 mol % Gd-doped barium cerate.
6,7
The introduction of Pr in the Ba(Ce
0.9-y
Pr
y
Gd
0.1
)O
2.95
solid solutions
increases the p-type electronic conductivity and decreases the ionic
conductivity.
6
Preliminary studies also indicated that the cathodic
overpotential resistance in Ba(Ce
0.8
Gd
0.2
)O
2.9
electrolyte fuel cells
can be lowered by using Pr-based mixed conducting cathodes.
7
The effect of Pr doping in Ba(Ce
1-x
Gd
x
)O
3-x/2
oxides has also
been reported recently.
8,9
Although an increase in the hole conduc-
tivity under oxidizing conditions was observed, the authors wanted
to enhance the proton conductivity of these perovskites, so most of
their measurements were conducted in H
2
atmospheres. They con-
cluded that the Ba(Pr
x
Gd
1-x
)O
3-x/2
(0.1 < x < 0.4) oxides were use-
ful electrolytes in fuel cells at temperatures <600°C, and the use of
these perovskites as cathodes at elevated temperatures (600-800°C)
was not examined. This paper reports our results on the conductivi-
ty, transference number, and cathode performance of the
Ba(Ce
0.8-y
Pr
y
Gd
0.2
)O
2.9-y/2
perovskites.
Experimental
Single-phase perovskite samples of Ba(Ce
0.8-y
Pr
y
Gd
0.2
)O
2.9
(y =
0.4, 0.6, 0.8) were prepared using standard solid state methods.
6
Dense samples (typically 95% theoretical) were obtained by iso-
statically pressing the powders into pellets at 620 MPa for 5 min, and
sintering at 1650°C for 10 h. Both four-probe dc and two-probe ac
measurements were used to measure the total conductivity. Ionic
transference numbers of the mixed conducting oxides were deter-
mined using an electromotive force (emf) technique. Details of the
sample preparation and description of the experimental setup used
have been reported elsewhere.
6
Cathodic overpotentials of mixed-conducting (Pr doped) electrodes
on a Ba(Ce
0.8
Gd
0.2
)O
2.9
electrolyte were measured using both imped-
ance spectroscopy and three-electrode current interruption technique
(CIT).
10
The impedance measurements were performed using a
Hewlett Packard 4192A LF impedance analyzer (5 Hz to 13 MHz). A
standard three-electrode configuration was used for the CIT;
11
current
was applied using a Solartron 1286 electrochemical interface, and
interrupted using a fast (<1 ms) switch. A Tektronix TDS 320 oscillo-
scope was used to measure the voltage decay. The Ba(Ce
0.8
Gd
0.2
)O
2.9
electrolyte disk was polished with 320 grit SiC paper to 1-2 mm thick-
ness, and the electrode powders were ground with ethylene glycol to
form a slurry. The slurry was brush painted on to the electrolyte surface
and heated between 1000 and 1550°C for 0.5-10 h to form a 20-
100 m thick porous electrode layer. The painted areas of the elec-
trodes used in this study were 0.25 to 0.45 cm
2
. Platinum gauze applied
with platinum paste was used as the current collector, and counter
(anode) and reference electrodes. The measurements were performed
under fuel cell conditions with dry air as the oxidant and H
2
/3% H
2
O
as the fuel. The sintering schedule was varied for each electrode to
study the effect of the morphology [examined using scanning electron
microscopy (SEM)] on the cathodic overpotential resistance.
Results and Discussion
Structure.—The X-ray diffraction (XRD) patterns of all the sam-
ples could be indexed using an orthorhombic perovskite cell.
12
The
decreasing lattice volume of the perovskite unit cell with increasing
Pr content (Fig. 1) indicates that Pr is primarily present in the 4+
state (r
Ce4+
= 0.87 Å, r
Pr4+
= 0.85 Å, and r
Pr3+
= 0.99 Å). The pre-
dominance of the tetravalent state for Pr in Ba-based perovskites has
also been confirmed by electron paramagnetic resonance (EPR)
measurements on BaPrO
3
.
13
However, thermogravimetric analysis
(TGA) indicated that the Pr-doped samples were more easily
Electrochemical Characterization of Mixed Conducting
Ba(Ce
0.8-y
Pr
y
Gd
0.2
)O
2.9
Cathodes
R. Mukundan,*
,a,z
P. K. Davies, and W. L. Worrell**
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272, USA
The high proton conductivities of doped barium and strontium cerates has led to their consideration as electrolytes in fuel cells
operating at intermediate temperatures (600-800°C). The electrochemical properties of Pr-doped barium gadolinium cerate
[Ba(Ce
0.8-y
Pr
y
Gd
0.2
)O
2.9
] have been investigated as part of our assessment of new mixed-conducting electrodes in fuel cells with
barium cerate electrolytes. Substitution of Pr for Ce increases the total and electronic conductivities, and the total conductivity of
Ba(Pr
0.8
Gd
0.2
)O
2.9
exceeds 0.75 S/cm at 800°C in dry air. Electromotive force measurements indicate that the Pr-substituted sam-
ples also retain high proton and oxygen-ion conductivities despite their decreased ionic transport numbers. Cathodes prepared
using these mixed-conducting Pr-doped barium-gadolinium cerates exhibit low cathodic overpotentials in Ba(Ce
0.8
Gd
0.2
)O
2.9
elec-
trolyte fuel cells. For example, the cathodic overpotential resistance of a Ba(Pr
0.8
Gd
0.2
)O
2.9
cathode is 0.47 cm
2
at 800°C at cur-
rent densities <100 mA/cm
2
. This is the lowest cathodic overpotential resistance reported for a perovskite electrode on a proton-
conducting electrolyte fuel cell at this temperature.
© 2000 The Electrochemical Society. S0013-4651(00)08-027-7. All rights reserved.
Manuscript submitted August 7, 2000; revised manuscript received October 11, 2000.
** Electrochemical Society Active Member.
** Electrochemical Society Fellow.
*
a
Present address: Los Alamos National Laboratory, Los Alamos, New Mexico 87545.
*
z
E-mail: mukundan@lanl.gov