Journal of Power Sources 160 (2006) 1211–1216 Sm 0.5 Sr 0.5 CoO 3 + Sm 0.2 Ce 0.8 O 1.9 composite cathode for cermet supported thin Sm 0.2 Ce 0.8 O 1.9 electrolyte SOFC operating below 600 ◦ C Xinge Zhang ∗ , Mark Robertson, Sing Yick, Cyrille Deˆ ces-Petit, Edward Styles, Wei Qu, Yongsong Xie, Rob Hui, Justin Roller, Olivera Kesler, Radenka Maric, Dave Ghosh Institute for Fuel Cell Innovation, National Research Council Canada, 3250 East Mall, Vancouver, BC, Canada V6T 1W5 Received 4 February 2006; received in revised form 9 March 2006; accepted 10 March 2006 Available online 5 June 2006 Abstract The cathode is a key component in low temperature solid oxide fuel cells. In this study, composite cathode, 75wt.% Sm 0.5 Sr 0.5 CoO 3 (SSC) + 25 wt.% Sm 0.2 Ce 0.8 O 1.9 (SDC), was applied on the cermet supported thin SDC electrolyte cell which was fabricated by tape casting, screen-printing, and co-firing. Single cells with the composite cathodes sintered at different temperatures were tested from 400 to 650 ◦ C. The best cell performance, 0.75 W cm -2 peak power operating at 600 ◦ C, was obtained from the 1050 ◦ C sintered cathode. The measured thin SDC electrolyte resistance R s was 0.128  cm 2 and total electrode polarization R p (a + c) was only 0.102  cm 2 at 600 ◦ C. © 2006 Elsevier B.V. All rights reserved. Keywords: SOFC; Composite cathode; SDC electrolyte; Anode supported; Low operating temperature 1. Introduction Various processes are associated with cathode resistance in a solid oxide fuel cell (SOFC): the transport of oxygen gas through the porous cathode, the adsorption of oxygen onto the cathode surface, the reduction and dissociation of the oxygen molecule (O 2 ) into the oxygen ion (O 2- ), and the incorporation of the oxygen ion into the electrolyte, to be transported across to the anode. As the operating temperature of an SOFC is lowered to reduce system and material cost and degradation, the cathode begins to limit the cell performance. The cathode resistance was 70–85% of the total cell resistance from 550 to 800 ◦ C for anode supported SOFCs with the standard (La, Sr)MnO 3 (LSM)–YSZ composite cathode on YSZ electrolyte [1]. There is extensive interest in mixed ionic/electronic conductor (MIEC) cathode materials for SOFCs that operate below 600 ◦ C, such as Sm 0.5 Sr 0.5 CoO 3 (SSC) [2,3], (La, Sr)CoO 3 (LSC) [4] (La, Sr)(Co, Fe)O 3 (LSCF) [5], and (Ba, Sr)(Co, Fe)O 3 [6], as well as their combination with Sm 0.2 Ce 0.8 O 1.9 (SDC) or Gd 0.2 Ce 0.8 O 1.9 (GDC) to form composite cathodes [2,5,6]. Those new cath- ∗ Corresponding author. Tel.: +1 604 2213077; fax: +1 604 2213088. E-mail address: xinge.zhang@nrc.gc.ca (X. Zhang). odes have provided a superior performance compared to (La, Sr)MnO 3 (LSM) cathodes due to electrochemically active area extension from just the triple phase boundary (TPB) to the entire MIEC surface. For example, low-current polarization resistance at 600 ◦ C measured for LSCF–GDC cathodes on YSZ elec- trolyte is about 0.3  cm 2 [7], on GDC electrolyte is 0.19  cm 2 [5] and for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 on SDC electrolyte, it is in the range of only 0.055–0.071  cm 2 [6]. Sm 0.5 Sr 0.5 CoO 3 (SSC) has been studied as a cathode mate- rial for SOFCs with YSZ, LSGM and SDC or GDC electrolytes [2,3,8–14]. Dense SSC showed overpotential about 50% lower than LSC under similar conditions. The rate-determining step of dense SSC cathodes was shown to be adsorption–desorption at the surface of the electrode, the same as for LSC. But the adsorp- tion and desorption rate constants of SSC were approximately one order of magnitude larger than the corresponding values for LSC. Furthermore, the electrochemical performance of SSC porous cathodes can be improved by adding doped ceria, which results in the suppression of growth of SSC particles, thereby maintaining the porosity and increasing the TPB area. The per- formance can also be improved by optimizing the conductivity and size distribution of particles of each solid phase to yield a larger TPB contact area that is accessible for oxygen reduction. Adding SDC to the SSC cathode is also believed to be of benefit 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.03.035