Temperature Dependence of Effective Thickness of Ni-GDC Anode for SOFC

Abstract

ABSTRACTS Solid oxide fuel cell (SOFC) is highly efficient due to its high operating temperature (700 ˚C ~ 1000 ˚C) and therefore is expected as one of the major power generation systems in the near future. However, the high temperature restricts the material that can be used, and causes degradation of performance due to heat cycle. Therefore, it is required to lower the operating temperature, which leads to high overvoltage. To lower the overpotential, not only the material but also the microstructure of the electrode is important. The anode of SOFC is generally made from a so-called cermet, which is a composite of ceramic and metal. Ni-YSZ (yttria stabilized zirconia) has been widely used as cermet, but GDC (Gadolinium Doped Ceria), which shows higher ionic conductivity than YSZ at low temperatures, is recently been used[1]. We have reported the effects of anode thickness and particle diameter of Ni particles catalyst on electrode performance using Ni-GDC cermet anode. When the diameter of Ni particles was large, anode thickness dependence of overvoltage was larger than when its diameter was small, indicating an increase of active reaction zone [2]. However, the structure of the examined electrode was not uniform, because the number of times the electrode was sintered was different for the anode with different thickness. Therefore, in this study, the anodes were made by using uniaxial pressing to construct a uniform anode structure. EXPERIMENTAL Powders of NiO, Gd0.1Ce0.9O1.9 and pore forming agent were used to fabricate the anode. The powders were mixed and molded by hand press, and then the obtained pellet was sintered. GDC was used as the electrolyte, and Sm0.5Sr0.5CoO3 (SSC) was used as the cathode. The operating temperature of SOFC was 900 ˚C, 800 ˚C and 700 ˚C. The anode and the cathode were supplied with H2 and O2, respectively. Current density vs. voltage was measured using a galvanostat. RESULTS and DISCUSSIONS The performance of the cell with thick anode and thin anode were comparable at high temperature. In comparison, at low temperature, maximum current density of the cell with thicker anode was higher. The results indicate that the effective reaction zone was comparable at high temperature, irrespective of the anode thickness, and that the effective reaction zone of the thick anode was wider than the thin anode at low temperature. At high temperature, the resistance due to the reaction is expected to be relatively small, therefore, the reaction occured only in the vicinity of the electrolyte, and effective reaction zone was suppressed irrespective of the anode thickness. On the other hand, at low temperature, the resistance due to the reaction is relatively large, therefore, the entire electrode is effectively used. Thus, the thick anode showed lower overpotential at low temperature. REFERENCES [1] B. Zhu, Albimsson, C. Andersson, K. Borsand, M. Mellander, J. Electrochem. Soc., 97, (40), 391-399 (1997) [2] H. Fukunaga, T. Ohno, C. Arai, T. Takatsuka, K. Yamada, J. Electrochem.Soc.,7,(1),1527-1531 (2007)