Study on the Perovskite-Type Oxide Cathodes in Proton-Conducting SOFC
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0972-AA01-06
Study on the Perovskite-Type Oxide Cathodes in Proton-Conducting SOFC Hidenori Yahiro, Hiroyuki Yamaura, and Makiko Asamoto Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, 790-8577, Japan
ABSTRACT The cathode performances of perovskite-type oxide electrodes were investigated in the H2-O2 fuel cell with proton-conducting electrolyte. Among the perovskite-type oxides tested in the present study, La1-xSrxFeO3 (LSFO) showed the best cathode performance. The cathode performances of LSFO depended on the Sr content and the heat-treatment temperature prior to electrochemical measurements. The cathode reactions of both LSFO and Pt electrodes are discussed briefly based on the cathode overpotentials measured as a function of oxygen partial pressure. Finally, the structure and the electrical conductivity of the proton-conducting ceramic film fabricated on LSFO electrode substrate are presented. INTRODUCTION Several doped perovskite-type oxides such as Yb-doped SrCeO3[1] and Nd- or Gd-doped BaCeO3 [2, 3] belong to the group of solid state protonic conductors at high temperature and are of interest for their applications in hydrogen sensors [4, 5], hydrogen pumps [6, 7], membrane reactors [8, 9], and fuel cells [10-13]. The use of proton-conducting electrolyte in solid oxide fuel cells (SOFCs) has some advantages compared with that of the oxide-ion conductor. For instance, the protonic ceramic fuel cells form water at the cathode compartment. It means that the fuel unreacted keeps pure at the anode compartment, requiring no recirculation. Iwahara and coworkers [10] reported operation of a H2-O2 fuel cell using proton-conducting electrolyte, SrCe0.95Yb0.05O3 (SCYO). Bonanos et al. [13] demonstrated that a H2-O2 fuel cell using BaCe0.8Gd0.2O3 (BCGO) electrolyte yielded a high current density. Recently, Coors [14] pointed out that the protonic ceramic fuel cell is ideal for use with hydrocarbon fuels such as natural gas. The electrical conductivity of protonic ceramics was studied as a function of oxygen partial pressure [15, 16]. In a hydrogen atmosphere corresponding to very reducing oxygen partial pressure, it is reported that the electrical properties of both SCYO and BCGO are dominated by the ionic conductivity at low temperature while the electron conductivity becomes dominant at high temperature. For example, a transition from ionic to electronic conductivity in a hydrogen atmosphere was observed at about 1023 K for SCYO [15]. This suggests that SOFC using SCYO electrolyte should be operated below 1023 K. However, the decrease in the operating temperature results in an increase of the overpotential, mainly at the cathode. In fact, Uchida et al. [17] have pointed out that the cathode overpotential of Pt in the proton-conducting SOFC could not be neglected below 1173 K, whereas the anode overpotential was negligibly small. Therefore, the development of a superior cathode has become an important subject. Mixed conducting ABO
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