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Electrical Performance of Calcium doped Lanthanum Ferrite for use in Single-Step Co-fired Solid Oxide Fuel Cells (SOFCs)

Published online by Cambridge University Press:  01 February 2011

Peter A. Zink ,
Kyung Joong Yoon ,
Uday B. Pal  and
Srikanth Gopalan
Peter A. Zink
Affiliation:
[email protected], Boston University, Division of Materials Science and Engineering, Brookline, Massachusetts, United States
Kyung Joong Yoon
Affiliation:
[email protected], Boston University, Division of Materials Science and Engineering, Brookline, Massachusetts, United States
Uday B. Pal
Affiliation:
[email protected], Boston University, Division of Materials Science and Engineering, Brookline, Massachusetts, United States
Srikanth Gopalan
Affiliation:
[email protected], Boston University, Division of Materials Science and Engineering, Brookline, Massachusetts, United States
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Abstract

In the single-step SOFC co-firing process YSZ electrolytes with sintering aid densify at a temperature of ˜1300°C. Electrodes employed in the single-step co-fired SOFC must therefore sinter with the right microstructure at ˜1300°C. Calcium-doped lanthanum ferrite, La0.8Ca0.2FeO3±δ (LCF-20) was identified in earlier studies as a possible stable cathode material for the single-step co-fired SOFC. LCF-20 is also expected to be a more stable cathode material than LSCF (strontium and cobalt doped lanthanum ferrite). Four-probe conductivity tests yielded ˜93 S/cm at 800°C and showed an increase in conductivity as pO2 increases, characteristic of p-type conduction. LCF has a higher electrical conductivity compared to LSCF and LCM+YSZ cathode materials. Oxygen ion conductivity of LCF-20 obtained from permeability measurements is higher than that of YSZ and LSF-20. Therefore LCF has excellent mixed conducting properties to serve as a catalytically active cathode material for co-fired solid oxide fuel cells operating at intermediate temperatures. Electrochemical Impedance Spectroscopy (EIS) measurements were made on symmetrical cells fabricated with YSZ electrolyte and the electrode materials. A gadolium doped ceria (GDC) barrier layer was employed to prevent LCF/YSZ reaction. Comparison of LCF/GDC/YSZ/GDC/LCF EIS data to LCM+YSZ/YSZ/LCM+YSZ EIS data gathered using identical test conditions and electrode microstructures shows that LCF has a measured polarization resistance (Rp) of approximately half that seen in LCM+YSZ. Variations in cathode thickness and porosity show the best performance with a cathode of a critical thickness and finer porosity. Slight stoichiometric deviations in LCF result in the formation of a Ca-Fe-O liquid phase during electrode sintering at around 1220 C. The liquid phase migrates into and through the GDC layer. EDX line scans show the second phase to be rich in Ca and Fe. Thicker GDC layers seem to prevent the liquid phase from reaching the electrolyte/GDC interface. Structural analysis with TEM will be performed. The properties and the effect of the Ca-Fe-O phase on the cathodic performance of the cell are being investigated; however, preliminary results indicate that minor amounts of the Ca-Fe-O phase will not interfere with the electrochemical performance of the LCF cathode. The high temperature instability in LCF has been observed in other studies, but has not been studied specifically in the literature.

Keywords

ionic conductorcatalyticthermogravimetric analysis (TGA)
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1126: Symposium S – Solid-State Ionics–2008 , 2008 , 1126-S11-02
Copyright
Copyright © Materials Research Society 2009

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References

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