Tomoyuki SHIMIZU
Application of the layered bismuth copper oxide, as the cathode of a solid oxide fuel cell
Kazunori SATO
Lowering the operation temperature of solid oxide fuel cells (SOFCs) to the intermediate range (600-800¡î) can reduce the production cost as well as the material versatility and the prolonged cell-lifetime. However, lowering the operating temperature leads to a decrease in both the electronic conductivity and the oxide ionic conductivity for the cathode materials with the mixed ionic and electronic conduction (MIEC).
The layered bismuth copper oxide, Bi2Sr2-XLaXCuO6+¦Ä (Bi2201), has been examined as a promising cathode material of the solid oxide fuel cell (SOFC). The layered crystal structure of Bi2201, which consists of an alternate stacking of the BiO double layer and the CuO2 layer coordinated with Sr2+ and La3+ ions, can show the MIEC. The electrochemical properties of Bi2201 have been investigated by measuring the electromotive force (EMF) for an oxygen concentration cell. When La content was X=1.0, the EMF showed the highest value.
In this study, the reactivity with Bi2201, whose La content was X=1.0, and the electrolytes, 8 mol% Y2O3 - 92 mol% ZrO2 (YSZ: yttria-stabilized zirconia) and Ce0.8Sm0.2O1.9 (SDC: samaria-doped ceria), was investigated by X-ray diffraction analysis. It was found that YSZ reacts with Bi2201 after the sintering at higher than approximately 800¡î for 12 h in air, but SDC did not react with Bi2201 under the same condition.
A two-electrode cell was fabricated with the SDC electrolyte and NiO/SDC (60:40 mass%) anode to compare the cell performance for the Bi2201 cathode with that for the Sm0.5Sr0.5CoO3 (SSC) cathode. The maximum power density of a cell using the Bi2201 cathode was about 60% less than a cell using the SSC cathode at 700¡î. However, the differences in the maximum power density, ohmic resistance, and polarization resistance decreased with decreasing the operating temperature. The MIEC in SSC occurs in the crystal lattice; however, the MIEC in Bi2201 is most likely to be controlled by the oxide-ionic conduction along the BiO double layer and the electronic conduction along the Cu2O layer. This specific crystal structure of Bi2201 can affect the temperature dependence the cathodic polarization resistance, which results in a decrease in the activation energy for the electrochemical reduction of oxygen at the cathode.
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