Modified LSM-YSZ Cathodes for Reduced Temperature Solid Oxide Fuel Cells (SOFC) Chun Lu, Tal Sholklapper, Craig P. Jacobson, Steven J. Visco, and Lutgard C. DeJonghe Lawrence Berkeley National Laboratory Materials Science Division 1 Cyclotron Road, Building 62R0203 Berkeley, CA 94720 Introduction Most state-of-art solid oxide fuel cells (SOFC) use 8mol% yttria stabilized zirconia (YSZ) as the electrolyte, Ni-YSZ as the anode, and La 1-x Sr x MnO 3- (LSM)-YSZ as the cathode (1). Due to the insufficient performance of the electrolyte and electrodes at low temperatures (<700 o C), cells have been operated at high temperatures (800 o C- 1000 o C). Lowering cell-operation temperatures will expand the materials selection, suppress the degradation of SOFC components, and consequently extend cell lifetime. Therefore, a thin-film electrolytes as well as alternative electrolytes with higher oxide-ion conductivity than that of YSZ have been extensively explored to reduce cell ohmic loss at reduced temperatures (2-6). However, at low temperatures cathode polarizations lead to a significant cell-performance loss because the electrochemical reaction rates decrease dramatically (7). To enhance the reactions, alternative cathode materials including Sm 0.6 Sr 0.4 CoO 3- (SSC) have been developed (8). Unfortunately, due to the thermal expansion mismatch with YSZ and deleterious reaction with YSZ at high-temperature processing steps, SSC can not be directly used in the YSZ-based SOFCs. To take advantage of SSC in reduced-temperature cathodes in YSZ-based cells, in this study SSC is infiltrated into the porous LSM-YSZ cathodes at 800 o C using a precipitation method. The effect of SSC addition on the cathode polarizations and cell performance at low temperatures (600 o C-700 o C) is reported. Experimental 20g YSZ powder (Tosoh-Zirconia; TZ-8Y) was ball milled with 2 wt% Menhaden fish oil, in isopropanol alcohol (IPA), for 24 hours. 2 wt% polyvinylbutyral (PVB) and dibutylphthalate (DBT) binders were added to the solution that was additionally milled for 1 hr. The solution was dried under a heat lamp, and the dried powder was then ground and sieved down to 150 microns. 3g of the sieved powder was pressed in a metal die with a diameter of 1.5 inch, at 10kpsi. The resultant discs were sintered at 1400 o C for 4 hrs to obtain dense YSZ discs with a diameter of ~1.19 inch. Symmetric cells were prepared using an aerosol spray method (6). 1g YSZ and 1g La 0.7 Sr 0.3 MnO 3- (Praxair Specialty Ceramics) were attritor-milled in IPA, for 1 hr, with the addition of 0.1g fish oil and 1 drop of DBT. The solution was then sonicated for 5 mins, and sprayed on both sides of YSZ discs to form symmetric cells having 1cm 2 electrodes. The electrodes were then sintered at 1150 o C for 4 hrs, and absorbed aqueous solution containing urea and Sm, Sr, Co nitrates in the ratio of 0.6:0.4:1. The electrodes were heated at 90 o C for 2 hrs. Pt current collectors were placed on the electrodes and fired at 800 o C for 2hrs. Single cells in this work were fabricated using the techniques detailed elsewhere (6). The SSC was incorporated into the LSM-YSZ cathodes using the precipitation method as mentioned above. The cell I-V (current density-voltage) curves were collected with lab-coded software, and its performance was also characterized using a Solartron 1260 frequency response analyzer connected with a Solartron 1286 electrochemical interface. The cell microstructures were observed with a scanning electron microscopy. The formation of SSC perovskite phase was analyzed using XRD examination. Results Fig. 1 shows the XRD pattern of the prepared powder sintered at 800 o C for 2hrs. The majority of the pattern corresponds to Sm 0.6 Sr 0.4 CoO 3- with the perovskite phase, although there are some peaks from impurities. This allows us to incorporate SSC into LSM-YSZ cathodes and avoid the adverse reactions between SSC and YSZ. Acknowledgements This work was supported by the U. S. Department of Energy, through the National Energy Technology Laboratory. References 1. N. Q. Minh and T. Takahashi, Science and Technology of Ceramic Fuel Cells, Elsevier 1995. 2. K. Huang, R. Tichy, J. B. Goodenough and C. Milliken, J. Am. Ceram. Soc., 81 (10) 2581 (1998). 3. R. Doshi, V. Richards, J. D. Carter, X. Wang, and M. Krumpelt, J. Electrochem. Soc., 146 (4), 1273 (1999). 4. C. Xia, F. Chen and M. Liu, Electrochemical and Solid-State Letters, 4 (5) A52 (2001). 5. C. Wang, W. L. Worrell, S. Park, J. M. Vohs, R. J. Gorte, J. Electrochem. Soc., 148 (8), A864 (2001). 6. S. De Souza, S. J. Visco, and L. C. DeJonghe, J. Electrochem. Soc., 144, L35 (2002). 7. H. Uchida, S. Arisaka, and M. Watanabe, J. Electrochem. Soc., 149 (1), A13 (2002). 8. J. M. Ralph, C. Rossignol, and R. Kumar, J. Electrochem. Soc., 150 (11), A1518 (2003). 0 1000 2000 3000 4000 5000 6000 20 30 40 50 60 70 80 2 Theta Lin (Counts) * * * * * * * Fig. 1 XRD pattern of the prepared SSC powder sintered at 800 o C for 2hrs. (*) perovskite phase