A study of Fe – substituted (La 0.8 Sr 0.2 ) 0.95 MnO 3-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 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¯ Fuel Oxidant Anode Electrolyte Cathode External Load Direct Current Exhaust gases and Heat Interconnect Anode Electrolyte Cathode Repeating elements Fuel Cell Components Ni Cermet YSZ LSM Doped LaCrO 3 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), Ce 0.8 Gd 0.2 O 1.9 (CGO) etc are being investigated.

Ln 0.4 Sr 0.6 Co 0.8 Fe 0.2 O 3-  ( 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 (La 0.8 Sr 0.2 ) 0.95 Mn 1-x Fe x O 3-  (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 Ce 0.8 Sm 0.2 O 2-  were also studied.

The perovskite oxides, (La 0.8 Sr 0.2 ) 0.95 Mn 1- x Fe x O 3-  (LSMF), where 0  x  1, La 0.95 MnO 3-z, (LM) and (La 0.8 Sr 0.2 ) 0.95 MnO 3-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. Ce 0.8 Sm 0.2 O 2-  (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 Ce 0.8 Sm 0.2 O 2-  (SDC), La 0.95 MnO 3-z (LM), (La 0.8 Sr 0.2 ) 0.95 MnO 3-y (LSM) and (La 0.8 Sr 0.2 ) 0.95 Mn 1- x Fe x O 3-  (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/VAL = length, A = electrode area I = current, V = voltage

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

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 CompoundSystema 0 (Ǻ)c 0 (Ǻ)Bulk density (gm/cc) TEC(  1 x 10 6 /°C) 25 – 800  C Ce 0.8 Sm 0.2 O 2-γ Cubic La 0.95 MnO 3-z,Hexagonal (La 0.8 Sr 0.2 ) 0.95 MnO 3-y Hexagonal (La 0.8 Sr 0.2 ) 0.95 Mn 0.8 Fe 0.2 O 3-δ Hexagonal

Crystallite sizes of LM, LSM and LSMF2 prepared by solid state route and nitrate route SampleSolid State route 1423 K (nm) Nitrate route 1173 K 1473 K (nm) La 0.95 MnO 3-z, (La 0.8 Sr 0.2 ) 0.95 MnO 3-y (La 0.8 Sr 0.2 ) 0.95 Mn 0.8 Fe 0.2 O 3-δ

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

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

Typical XRD patterns of LSMF2, SDC and mixture of LSMF2 + SDC heated to 1673 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 eV for LM (low temperature), eV for LM (high temperature), eV for LSM and eV for LSMF2 seem to be quite reasonable for a small polaron hopping mechanism.

Conclusions Structural, thermal expansion, and electrical properties of the oxides (La 0.8 Sr 0.2 ) 0.95 Mn 1-x Fe x O 3-  along with La 0.95 MnO 3-z and (La 0.8 Sr 0.2 ) 0.95 MnO 3-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 La 0.95 MnO 3-z (LM), (La 0.8 Sr 0.2 ) 0.95 MnO 3-y (LSM) and (La 0.8 Sr 0.2 ) 0.95 Mn 0.8 Fe 0.2 O 3-  (LSMF2) with Ce 0.8 Sm 0.2 O 2-  (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.