Mixed Ionic-Electronic Conduction in N1 Doped Lanthanum Gallate Perovskites
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Crystal Physics and Electroceramics Laboratory Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, MA 02139, U.S.A.
ABSTRACT Lanthanum gallate is a promising material for "monolithic" fuel cells or oxygen pumps, i.e. one in which the electrolyte and electrodes are formed from a common phase. We have investigated La,.-SrxGa,.yNiyO 3 (LSGN,.y) with x=0.1 and y=0.2 and 0.5 as a potential cathode material for such an electrochemical device. The o(P0 2,T) for LSGN,0 20 points to a p-type electronic conductivity at high PO2 and predominantly ionic conductivity at low P0 2. LSGN10. 50 has an electronic conductivity suitable for SOFC applications of approximately 50 S/cm in air at high temperature. AC impedance spectroscopy on an electron blocking cell of the form M/LSG/LSGN/LSG/M was used to isolate the ionic conductivity in the LSGNIo.20 material. The ionic conductivity was found to have a similar magnitude and activation energy to that of undoped LSG material with a,= 0.12 S/cm at 800°C and EA= 1.0 ± 0.1 eV. Thermal expansion measurements on the LSGN materials were characterized as a function of temperature and dopant level and were found to match that of the electrolyte under opeating conditions.
1. INTRODUCTION Developing electrodes for solid oxide fuel cells or oxygen pumps is a difficult task due to the need to simultaneously satisfy electrical, chemical and thermo-mechanical requirements. A promising approach is to dope the electrolyte material with multivalent cations in order to introduce electronic conductivity[l]. If the structure of the electrolyte is not radically altered, then an electrode may be obtained which retains a similar thermal expansion to the electrolyte, is chemically compatible, and is a mixed conductor. A mixed ionic and electronic conducting (MIEC) electrode is expected to have lower overpotential losses than a purely electronic conductor, due to the expansion of the triple phase boundary to the whole surface of the mixed conductor. At temperatures of 800-1000° C, an electronic conductivity (Oe) of the order of 10 100 S/cm and an ionic conductivity (oi) at least as large as that in the electrolyte, i.e. of the order of 0.01 -0.1 S/cm is desirable.
129 Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
This approach for forming MIECs has been investigated in pyrochlores [1-2] and in electrodes for zirconia fuel cells [3]. The recently discovered perovskite oxygen ion conductor La-.xSrxGa,.yMgyO 3 [4,5] is an ideal candidate for this approach given the flexibility to substitute ions on either the A or B sites to enhance the ionic or electronic conductivity. Other perovskite oxides such as LaI.XSrxMnO 3 and (La,Sr)(Co,Fe)0 3 are already known to exhibit either high electronic or mixed conductivities. Solid solutions with the gallate can be expected to exhibit high MIEC as well. In this paper we study the effects of Ni as a B site dopant in La 1 .•SrxGaO 3. We expect to introduce metallic or polaronic conductivity with increasing
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