Abstract:
La0.2Sr0.25Ca0.45TiO3 is a carefully selected composition to provide optimal
processing and electrical characteristics for use as an anode support in solid oxide fuel
cells (SOFCs). In the present study, the optimization of the preparation process of A-site
deficient perovskite, La0.2Sr0.25Ca0.45TiO3 (LSCTA-) powders and their characterization
for integration into the SOFC anode supports have been focussed. LSCTA- powder was
investigated in different yet connected important aspects using high-tech methods like
tape casting, microstructure optimization and testing in symmetrical and button cell set
ups.
The major part of the present research deals with the process optimization of
LSCTA-. A modified Pechini method was successfully applied to produce single phase
perovskite at 900 oC. The effect of calcination temperature on the phase, morphology and
sintering characteristics was studied using XRD, SEM and dilatometry techniques. The
optimal calcination temperature of 1000 oC was selected for further studies as the powder
calcined at this temperature displayed a similar sintering profile to commercial 8 mol%
yttria-stabilized zirconia (YSZ), the typical choice for electrolyte. LSCTA- showed an n-
type conduction nature where conductivity of a dense LSCTA- specimen sintered in air
increased by three orders of magnitude after in-situ reduction in 5% H2/Ar. These
encouraging characterization results supported the SOFC anode candidateship of LSCTA-.
In the second part of study, the synthesized powder was processed in aqueous tape
casting which is a quick and rapid technique to fabricate thin SOFC anodes. Slurry
formulation was optimized for both the dense and porous green tapes. The rectangular
bars fabricated from green tapes by lamination were sintered and tested for conductivity
measurements using van der Pauw set up. The effect of ceria impregnation on the
conductivity of porous LSCTA- bars was studied. The conductivity behaviour of porous
bars under redox cycling showed a two-stage process that exhibited strong reversibility.
For the reduction process, addition of impregnated ceria reduced the onset delay period
and increased the apparent rate constant, k values by 30-50% for both stages. The co-
impregnation of Ni further resulted in an increase of conductivity of porous bars.
Another aspect of the study was the microstructure optimization of LSCTA- tapes.
To introduce the porosity in LSCTA- tapes, commercial pore formers like graphite,
polymethylmethacrylate (PMMA) and glassy carbon (GC) were used. It was observed
that pre-sintering the powder helps to get a good microstructure with commercial pore
formers. An interesting feature for inducing porosity in LSCTA- tapes was the synthesis
of homogeneous and well dispersed carbon micro spheres (CMS) from an optimized
hydrothermal method and their further application as pore formers.
As a part of the research, the anode performance of LSCTA- was tested in YSZ
electrolyte supported symmetrical cells. The effect of impregnates like ceria (CeO2),
gadolinium doped ceria (CGO), with and without Ni, on the performance of symmetrical
cells was investigated. It was found that co-impregnation of CeO2 and CGO with Ni have
pronounced effect in decreasing the impedance of bare LSCTA- in symmetrical cells.
Further, the anode performance was tested in button cells using a three electrode set up.
A significant improvement in cell performance could be achieved by optimizing the
anode support with various impregnates both qualitatively and quantitatively.
Finally, LSCTA- was doped at B site with Ni (LSCTN) and Fe (LSCTF). The
doped compositions offered higher conductivity values than the parent LSCTA-.
Compared to pre-reduced LSCTA- having conductivity of 38 S cm-1, the pre reduced 5%
Ni doped LSCTA- (LSCTN-5) and 5% Fe doped LSCTA- (LSCTF-5) offered conductivity
values of 47 S cm-1 and 66 S cm-1 at 880 oC, respectively.
In conclusion, structurally stable LSCTA- could be a good alternative to state of
the art SOFC anode exhibiting good mechanical, morphological and electrical properties.
Catalyst introduction via impregnation or doping could enhance the electrical and
catalytic properties of these perovskites making them viable alternatives for
electrochemical applications.