Preparation and characterization of lead ruthenate based composite cathodes for SOFC applications
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Preparation and characterization of lead ruthenate based composite cathodes for SOFC applications Vincenzo Esposito1,2, Enrico Traversa1, and Eric D. Wachsman2 1 Department of Chemical Science and Technology, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133, Rome, Italy 2 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, U.S.A. ABSTRACT Nanometric pyrochlore lead ruthenate (Pb2Ru2O6.5) powders were prepared using a new chemical route as a possible candidate for cathode materials in solid oxide fuel cells (SOFCs). Pb2Ru2O6.5 was mixed with yttria-stabilized zirconia (YSZ) and erbia-stabilized bismuth oxide (ESB) to fabricate porous composite electrodes. Pure pyrochlore or composite electrodes were deposited as thick films onto YSZ and ESB electrolytes. Powders and films were analyzed using X-ray diffraction (XRD) analysis and field emission scanning electron microscopy (FE-SEM). Electrochemical features of the electrodes were investigated using electrochemical impedance spectroscopy (EIS) measurements at different temperatures in air for symmetric cells. The composite electrodes were compared to single Pb2Ru2O6.5 phase electrodes to evaluate the effect on polarization of the addition of the pure ionic conductor phase in the electrodes. INTRODUCTION Catalytic activity of RuO2 and other transition metal oxides toward oxygen in red-ox reactions is well known [1] and Ru is largely used as a catalyst in low temperature fuel cells [2]. Ruthenium based pyrochlores have been investigated due to their high metallic or semiconducting electrical conductivity [3, 4] that allows their use in catalysis [5] and for electrochemical sensors [6]. In particular, Pb2Ru2O6.5 possess electric and catalytic properties similar to those of RuO2 [4, 7-10], and has been evaluated as possible candidate for cathode materials in SOFCs. Pb2Ru2O6.5 is a metallic conductor because of the interaction between Ru2O6 and Pb2O' through the Ru-O-Pb bonds [4], described with a Mott–Hubbard mechanism for metallic conductivity [11]. The use of a pure metallic conductor instead of a mixed ionic-electronic conductor (MIEC) as a cathode in SOFCs can limit the oxygen ionic charge transfer because of the absence of the bulk path exchange process between cathode and electrolyte [12]. Bulk path exchange is known to improve the cathode performance [13]. To overcome this problem, composite electrodes are used to increase the electrode triple phase boundary (TPB) and to decrease the polarization ohmic drops [14]. This paper reports a study of composite ESB-Pb2Ru2O6.5 and YSZ-Pb2Ru2O6.5 electrodes, fabricated to combine the electronic conductivity of lead ruthenate with the ionic conductivity of ESB and YSZ. The prepared composites were used to fabricate electrodes that were deposited onto ESB pellet or YSZ tape-cast electrolytes. Electrochemical impedance spectroscopy (EIS) measurements were used to evaluate the electrochemical performance of symmetric cells made of either single pyrochlore
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