Synthesis and Application of Calcium Doped Lanthanum Strontium Titanate as Anode Support for Fuel Cell Applications

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.