Novel High-Performance Electrodes for Medium-Temperature Solid Oxide Fuel Cells

在各类燃料电池中，固体氧化物燃料电池有望提供最高的能量转换效率。目前，由于最先进的电解液和电极在低温下的性能不足，固体氧化物燃料电池的工作温度被限制在非常高的值约1000℃。然而，在中温(-800℃)下运行SOFC是可取的，因为高温运行会导致许多严重的问题：SOFC组件退化、材料选择有限等。第一个是减少固体电解质中的欧姆损耗。除了降低欧姆损耗之外，开发高性能的电极也是必不可少的，因为在这样的温度下，电极的反应速度必须减慢。我们开发了一种用于中温SOFC的多孔催化反应层。混合导电氧化物颗粒，掺撒玛利亚CeO_2[(CeO_2)_t；0.8；(Smo_&lt；1.5…More&gt；)_&lt；0.2&gt；以SDC为阳极，La(SR)MnO_3(LSM)为阴极，在其表面结合高度分散的(纳米)金属电催化剂。本研究工作的目的是通过制造高性能和高可靠性的新型电极来促进高性能中温SOFC的发展。1999财年，我们成功地开发出了性能优异的新型阴极。La(Sr)CoO_3(LSC)具有比LSM更高的阴极性能，但在高温下，LSC更容易与YSZ电解液反应。研究发现，薄而致密的SDC中间层可以有效地避免LSC和YSZ之间发生不利的固相反应。控制LSC阴极的微结构可以在不增加欧姆电阻的情况下略微降低无IR过电位。负载量为3wt%的高分散铂催化剂可以进一步提高正极性能，特别是在较低的工作温度下。在空气中，当过电位(η)为-0.05V时，阴极上的电流密度为1A/cm2。在2000财年，我们成功地用掺Y氧化铈(YDC)来代替SDC来提高阳极的性能。结果表明，在800℃~1000℃的工作温度下，YDC的多孔阳极表现出比SDC更好的性能。混合导体YDC具有较高的电子导电率，对提高阳极反应速度有显著的作用。高分散的Ru催化剂(3wt%)有效地提高了YDC阳极的性能。在η=0.1V，800℃的加湿氢气中，电流密度达到0.4A/cm2，是一种很有前途的中温SOFC电极。较少

Solid oxide fuel cells (SOFCs) are expected to provide the highest energy conversion efficiency among various types of fuel cells. At present, the SOFC operating temperatures are restricted to very high values of about 1000℃ because of insufficient performances of the state-of-the-art electrolyte and electrodes at low temperatures. It is desirable, however, to operate SOFCs at a medium temperature (-800℃) because high-temperature operation causes many serious problems : degradation of SOFC components, limited choice of materials, etc. Two major obstacles must be solved to operate mediunm-temperature SOFCs. The first is to reduce an ohmic loss in the solid electrolyte. Besides reducing the ohmic loss, it is essential to develop high performance electrodes because the electrode reaction rates must be slow down at such temperatures.We have developed a porous catalyzed-reaction layer for medium-temperature SOFCs. Mixed conducting oxide particles, samaria-doped ceria [(CeO_2)_<0.8>(SmO_<1.5 … More >)_<0.2>, denoted as SDC] for the anode and La(Sr)MnO_3 (LSM) for the cathode, were employed in combination with highly dispersed (nanometer-sized) metal electrocatalysts on their surfaces.The aim of this research work is to contribute to development of high-performance medium-temperature SOFC by fabricating novel electrodes with high performance and high reliability.In FY1999, we succeeded to develop new cathodes with exceptionally high-performance. La(Sr)CoO_3 (LSC) has been known to exhibit a higher cathodic performance than LSM.However, LSC tends to react easier with yttria-stabilized zirconia (YSZ) electrolyte than LSM at high temperatures. We found that a thin and dense SDC interlayer was effective to avoid unfavorable solid-state reactions between LSC and YSZ.Control of the microstructure of LSC cathode appreciably lowered the IR-free overpotential without an increase of the ohmic resistance. The cathode performance was enhanced further with highly dispersed Pt catalysts of 3 wt% loading, especially at low operating temperature. The current density on the Pt-LSC cathode at an overpotential (η) of -0.05 V was 1 A/cm^2 at 800℃ in air.In FY2000, we succeeded to enhance the anode performance by employing yttria-doped ceria (YDC) in place of SDC.We found that the porous YDC anode exhibited higher performance than that of SDC at the operating temperature of 800℃ to 1000℃. High electronic conductivity in the mixed conducting YDC appreciably contributes to enhancing the anode reaction rate. Highly dispersed Ru catalysts (3 wt%) effectively enhanced the performance of the YDC anode. The current density of 0.4 A/cm^2 at η= 0.1 V was achieved at 800℃ in humidified H_2. In conclusion, they are promising electrodes for medium-temperature SOFCs. Less