The behavior of SOEC was investigated as compared with that of solid oxide fuel cell (SOFC) using small cells prepared on yttria stabilized zirconia (YSZ) planar discs. Ni-YSZ cermet negative electrode showed asymmetric behavior indicating the existence of diffusion limited process in the electrolysis direction, although the behavior was strongly dependent on the electrode preparation. The behavior of positive elec trodes made by using perovskite type oxides was also investigated.When polarized anodically, the positive electrode showed degradation behavior which ended up with electrode delamination from electrolyte. The degradation rate was remarkably decreased by improving the initial polarization performance of the elec trode or by using a mixed ceria intermediate layer between YSZ and electrode. Polarization measurements of SOEC were conducted at 1173K, 1223K and 1273K with various water content in hydrogen simulating the atmosphere of various water electrolysis rate. The cell was shown to work at a high current density and at high water electrolysis rate without suffering from diffusion limiting current.
Anode characteristics of a Ce 0.8 Sm 0.2 O 1.9 /Ni cermet on the Ce 0.8 Sm 0.2 O 1.9 electrolyte were studied by a three-probe ac impedance method combined with dc polarization, instead of the conventional current-voltage curve measurement. The experimentally obtained Cole-Cole plots of ac impedance showed a shift with added dc voltage. Both bulk resistance and electrode reaction resistance increased with increasing anodic dc current. The shift of bulk resistance is explained by the electronic conduction of the electrolyte. The increase of electrode reaction resistance is thought to be a result of discrepancy between the ionic and the external current, differing from Ni/yttria-stabilized zirconia anode. An interface resistance lower than 0.1 ⍀/cm 2 was obtained below 700°C by using a coprecipitation method to prepare the anode precursor and cofiring it with the electrolyte to improve the contact.Solid oxide fuel cells ͑SOFCs͒ technology is a high efficiency and clean power generating technology in the new century. To commercialize it, the reduction of its operation temperature, 1000°C at present time, is the primary focus. The operation at a lower temperature than 800°C permits utilization of metallic interconnectors. It will also help to extend the life of a cell stack. The reduced temperature, however, requires new electrolytes of higher conductivity or thinner films of conventional electrolyte and also high performance electrodes ͑anode and cathode͒.As a high performance anode material of SOFCs, ceria had been confirmed 1-3 to be active for direct oxidation of CH 4 . Such works are very meaningful for direct CH 4 feeding SOFCs. A recent paper 4 gave a good summary of the literature on this respect. For SOFCs using yttria-stabilized zirconia ͑YSZ͒ electrolyte and H 2 fuel, the Ni/YSZ cermet has been widely studied 5-8 and utilized as the anode. However, with the development of reduced-temperature SOFCs, the study of Ni/ceria cermet anode is increasing. 9-11 The Ni/ceria cermet was shown to be effective both as a catalyst of methane steam reforming and as anode for H 2 /H 2 O fuel. The high performance of a Ni/ceria cermet originates from the high ionic conductivity and characteristic electronic conductivity of doped ceria.On the other hand, since the electrical conductivity of the traditional YSZ electrolyte is relatively low, there is a limit to reduction of the operating temperature of YSZ-based SOFCs. Substitution electrolyte materials such as lanthanum gallate, 12 scandia-stabilized zirconia, 13 and Gd-or Sm-doped ceria 14,15 are now actively studied. Among them, doped ceria is the most compatible electrolyte with Ni/ceria cermet. This compatibility is important, because it can avoid the risk of interface reactions during the fabrication process of cells by cosintering at a temperature as high as 1400°C. Cosintering technology is believed to be the most effective process to reduce the manufacturing cost of the cells.The problems of doped ceria as an electrolyte, are the electronic conductivity and the expan...
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