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 CentreTrombay 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.

e¯

e¯

Fuel

Oxidant

Anode

Electrolyte

Cathode

External Load

Direct Current

Exhaust gases

and Heat

InterconnectAnode

ElectrolyteCathode

Repeatingelements Fuel Cell

Components

Ni Cermet

YSZ

LSM

Doped LaCrO3 or Metallic Alloys

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)0.95Mn1-

xFexO3- (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 co-

precipitation 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)0.95Mn1-xFexO3- (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 areaI = current, V = voltage

20 30 40 50 60 70

La0.95

MnO3-z

2

(La0.8

Sr0.2

)0.95

MnO3-y

I/I 0

(La0.8

Sr0.2

)0.95

Mn0.8

Fe0.2

O3-

XRD patterns of LM, LSM and LSMF2 prepared by nitrate route (Heated to 1173 K)

20 30 40 50 60 70

La0.95

MnO3-z

2

(La0.8

Sr0.2

)0.95

MnO3-y

I/I 0

(La0.8

Sr0.2

)0.95

Mn0.8

Fe0.2

O3-

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 route1173 K 1473 K (nm)

La0.95MnO3-z, 43-46 31-34 41-43

(La0.8Sr0.2)0.95MnO3-y 49-51 26 -

(La0.8Sr0.2)0.95Mn0.8Fe0.2O3-δ 46-50 15-16 31-38

200 300 400 500 600 700 800 900 1000 1100 1200

0.0

0.2

0.4

0.6

0.8

1.0

La0.95

MnO3-z

(La0.8

Sr0.2

)0.95

MnO3-y

(La0.8

Sr0.2

)0.95

Mn0.8

Fe0.2

O3-

(La0.8

Sr0.2

)0.95

Mn0.6

Fe0.4

O3-

(La0.8

Sr0.2

)0.95

Mn0.4

Fe0.6

O3-

(La0.8

Sr0.2

)0.95

Mn0.2

Fe0.8

O3-

(La0.8

Sr0.2

)0.95

FeO3-

Lin

ear

therm

al expansio

n (

%)

temperature K

Thermal expansion behavior of LM, LSM and LSMF with varying x from 300 – 1123 K

400 600 800 1000 1200

0.0

0.2

0.4

0.6

0.8

1.0

Lin

ea

r th

erm

al e

xp

an

sio

n (

%)

temperature K

SDC La

0.95MnO

3-z

(La0.8

Sr0.2

)0.95

MnO3-y

(La0.8

Sr0.2

)0.95

Mn0.8

Fe0.2

O3-

Thermal expansion behavior of SDC, LM, LSM and LSMF2 from 300 – 1123 K

20 30 40 50 60 70

LSMF2

I/I 0

2

SDC

LSMF2 +SDC 1673 K

Typical XRD patterns of LSMF2, SDC and mixture of LSMF2 + SDC heated to 1673 K

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

3.0

3.5

4.0

4.5

5.0

La0.95

MnO3-z

(La0.8

Sr0.2

)0.95

MnO3-y

(La0.8

Sr0.2

)0.95

Mn0.8

Fe0.2

O3-

log (

T)

(S c

m-1)

1000/T (K-1)

1200 1100 1000 900 800 700 600 500

Activation Energies (Ea)

LM = 0.271 eV (LT)LM = 0.261 eV (HT)LSM = 0.143 eV LSMF = 0.203 eV

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.

ConclusionsStructural, 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 electrode- Electrolyte–Negative electrode (PEN) assembly are necessary to make use of these materials in actual SOFC devices.