Transcript Slide 1

A study of Fe – substituted
(La0.8Sr0.2)0.95MnO3-y
as cathode material
for solid oxide fuel cells
B. N. Wani, Mrinal Pai, S.J. Patwe, S. Varma,
S. R. Bhardwaj and N.M Gupta
Applied Chemistry Division,
Bhabha Atomic Research Centre
Trombay Mumbai 400 085. India
Solid oxide fuel cells (SOFCs) are drawing great
interest as a power generation system on account of
high energy efficiency and environmental advantages.
However, there are many material problems remaining
to be solved to obtain a high performance SOFC.
Typical SOFCs with yttria stabilized zirconia (YSZ)
electrolytes operating at about 1273 K have been
extensively studied
High temperature operation can cause degradation
during a long – term service life because of chemical
interaction of cell components or due to thermal
expansion mismatch between the various components.
Ni Cermet
Fuel
e¯
Anode
YSZ
Electrolyte
Cathode
Oxidant
Repeating
elements
External
Load
e¯
LSM
Interconnect
Anode
Electrolyte
Cathode
Direct Current
Exhaust gases
Doped LaCrO3 or
Metallic Alloys
Fuel Cell
Components
and Heat
One possible way to overcome this problem is to reduce
the SOFC operating temperature to 1000 to 1100 K.
Development of suitable Electrodes and Electrolytes for
Intermediate Temperature SOFC (ITSOFCs)
YSZ is the best candidate as an electrolyte material at
high temperature
For ITSOFCs, electrolytes such as samaria doped ceria
(SDC), Ce0.8Gd0.2O1.9 (CGO) etc are being investigated.
Ln0.4Sr0.6Co0.8Fe0.2O3- ( LSCF) where Ln = La, Pr, Nd,
Sm, Gd ) have been investigated as electrodes for the
temperature range of 873 – 1073 K.
But thermal expansion behaviour of these perovskites
did not match the electrolyte CGO.
In this study, we synthesized Fe doped LSM materials
like (La0.8Sr0.2)0.95Mn1-xFexO3- (LSMF) where 0.0  x 
1.0 and investigated their electrical conductivity and
thermal expansion behaviors.
The chemical as well as the mechanical compatibility
of the LSMF materials with Ce0.8Sm0.2O2- were also
studied.
The perovskite oxides, (La0.8Sr0.2)0.95Mn1xFexO3- (LSMF), where 0 x  1, La0.95MnO3-z,
(LM) and (La0.8Sr0.2)0.95MnO3-y (LSM) were
synthesized by standard ceramic route.
These perovskites were also prepared from their
nitrate solutions. 1M nitrate solutions of, Sr, Fe and
Mn were prepared and these solutions were used
to prepare different compositions by the nitrate
method. The mixed nitrate solutions were dried and
calcined at different temperatures namely, 873,
1173, 1373 and 1673 K.
Ce0.8Sm0.2O2- (SDC), was synthesized by coprecipitation route. Nitrate solutions of cerium and
samarium were mixed in stoichiometric ratio and its
hydroxides were precipitated The dried precipitate
was decomposed in air at 823 K to obtain single
phase compositions.
Powder XRD patterns of all the samples were
recorded on a Philips X-ray Diffractometer (PW 1710)
with Ni filtered Cu K radiation and using silicon as an
external standard.
The linear thermal expansion measurements of
the Ce0.8Sm0.2O2- (SDC), La0.95MnO3-z (LM),
(La0.8Sr0.2)0.95MnO3-y (LSM) and (La0.8Sr0.2)0.95Mn1xFexO3- (LSMF) oxides were carried out during heating
from room temperature to 1073 K in air at 8 K /min
using an LKB 3185 fused quartz dilatometer.
The electrical conductivity measurements were
carried out with sintered bars of 5 mm x 5 mm 20 mm
dimensions. They were sintered at 1673 K for 10h. The
electrical conductivity  was calculated by the equation
 = LI/VA
L = length, A = electrode area
I = current, V = voltage
(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-
I/I0
(La0.8Sr0.2)0.95MnO3-y
La0.95MnO3-z
20
30
40
2q
50
60
70
XRD patterns of LM, LSM and LSMF2 prepared by
nitrate route (Heated to 1173 K)
(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-
I/I0
(La0.8Sr0.2)0.95MnO3-y
La0.95MnO3-z
20
30
40
50
60
70
2q
XRD patterns of LM, LSM and LSMF2 prepared
by solid state route (Heated to 1673 K)
Lattice parameters, bulk density and bulk thermal
expansion data for SDC, LM, LSM and LSMF2
Compound
System
a0 (Ǻ) c0 (Ǻ)
Bulk
density
(gm/cc)
TEC(1 x
106/°C)
25 – 800 C
Ce0.8Sm0.2O2-γ
Cubic
5.4369 --
4.53
11.68
La0.95MnO3-z,
Hexagonal
5.5298 13.347
4.35
7.89
(La0.8Sr0.2)0.95MnO3-y
Hexagonal
5.5163 13.329
4.40
10.08
(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-δ
Hexagonal
5.5294 13.372
4.96
10.36
Crystallite sizes of LM, LSM and LSMF2 prepared
by solid state route and nitrate route
Sample
Solid State route
1423 K
(nm)
Nitrate route
1173 K
(nm)
La0.95MnO3-z,
43-46
31-34
41-43
(La0.8Sr0.2)0.95MnO3-y
49-51
26
-
15-16
31-38
(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-δ 46-50
1473 K
Linear thermal expansion (%)
1.0
La0.95MnO3-z
(La0.8Sr0.2)0.95MnO3-y
(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-
(La0.8Sr0.2)0.95Mn0.6Fe0.4O3-
(La0.8Sr0.2)0.95Mn0.4Fe0.6O3-
(La0.8Sr0.2)0.95Mn0.2Fe0.8O3-
(La0.8Sr0.2)0.95FeO3-
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
900
1000 1100 1200
temperature K
Thermal expansion behavior of LM, LSM and LSMF
with varying x from 300 – 1123 K
Linear thermal expansion (%)
1.0
0.8
SDC
La0.95MnO3-z
(La0.8Sr0.2)0.95MnO3-y
(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-
0.6
0.4
0.2
0.0
400
600
800
1000
1200
temperature K
Thermal expansion behavior of SDC, LM, LSM and
LSMF2 from 300 – 1123 K
LSMF2 +SDC 1673 K
I/I0
SDC
LSMF2
20
30
40
50
60
70
2q
Typical XRD patterns of LSMF2, SDC and
mixture of LSMF2 + SDC heated to 1673 K
T (K)
1200
1100
1000
900
800
700
600
500
5.0
Activation Energies (E a)
LM = 0.271 eV (LT)
LM = 0.261 eV (HT)
LSM = 0.143 eV
LSMF = 0.203 eV
-1
log (T) (S cm )
4.5
4.0
3.5
La0.95MnO3-z
(La0.8Sr0.2)0.95MnO3-y
(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-
3.0
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
-1
1000/T (K )
log (T) (S cm-1) versus reciprocal of temperature
for LM, LSM and LSMF2
If the carrier concentration is constant, the plots
of log((T) versus 1/T should be linear, as the
small polaron conduction mechanism follows the
relation
 = (C/T) exp(-Ea/kT)
where Ea is the activation energy and k is the
Boltzmann constant.
The pre exponential factor C includes the carrier
concentration as well as other material
dependent parameters.
The calculated activation energies 0.271 eV for
LM (low temperature), 0.261 eV for LM (high
temperature), 0.143 eV for LSM and 0.203 eV
for LSMF2 seem to be quite reasonable for a
small polaron hopping mechanism.
Conclusions
Structural, thermal expansion, and electrical
properties of the oxides (La0.8Sr0.2)0.95Mn1-xFexO3- along
with La0.95MnO3-z and (La0.8Sr0.2)0.95MnO3-y were studied
in detail. All the oxides in this series were found to be
single phase right up to x=1.
The chemical compatibility between the perovskite
type oxides La0.95MnO3-z (LM), (La0.8Sr0.2)0.95MnO3-y
(LSM) and (La0.8Sr0.2)0.95Mn0.8Fe0.2O3- (LSMF2) with
Ce0.8Sm0.2O2- (SDC) has been established.
Further studies with suitable anode material and the
current – voltage characteristics of a Positive electrodeElectrolyte–Negative electrode (PEN) assembly are
necessary to make use of these materials in actual
SOFC devices.