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The electrical conductivity of SmBaCo 2 O 5+δ (SBCO) showed a Metal-Insulator Transition (MIT) at about 200°C with maximum conductivity of 570 S·cm -1 at 200°C and minimum conductivity 170 S·cm -1 at 900 o C. The area specific resistance (ASR) of composite cathode (50wt% of SBCO and 50wt% of Ce 0.9 Gd 0.1 O 1.95 , SBCO:50) on a Ce 0.9 Gd 0.1 O 1.95 (CGO91) electrolyte was observed to be 0.05 Ω·cm 2 at 700°C. SmBa 0.5 Sr 0.5 Co 2 O 5+δ (SBSCO) electrode was sintered onto CGO91 at 1000 o C to form symmetrical cells and exhibited an ASR value of 0.092 Ω·cm 2 at 700 o C. The lowest ASR value was observed when the composite cathode of 50wt% of SBSCO and 50wt% of CGO91 (SBSCO:50) was used and these were 0.10 Ω·cm 2 at 600 o C and 0.013 Ω·cm 2 at 700 o C, respectively. The coefficient of thermal expansion (CTE) was 21.9 x 10 -6 K -1 at 700 o C. However, the CTE of SBSCO:50 was shown to be 13.6 x 10 -6 K -1 at 700 o C which are similar to the value of CGO91 electrolyte for intermediate temperature-operating solid oxide fuel cell (IT-SOFC) applications. Background and IntroductionLowering the operating temperature (500-750 o C) of solid oxide fuel cell (SOFC) can improve compatibility and ease of material selection and reduce the fabrication cost as well as enhance the reliability and the life of SOFC system (1-8). In spite of the advantages in intermediate temperature ranges, the lower temperature of SOFC increases the resistance of SOFC cell. Significantly, the cathode for intermediate temperatureoperating SOFC (IT-SOFC) is a major resistance contributor (9,10). As cathode materials for IT-SOFC, mixed ionic and electronic conductors (MIECs) with perovskite structure have increased attention for 12,13). As cathode materials, complex perovskite oxides for IT-SOFC, (La,Sr)(Co,Fe)O 3-δ (LSCF) were investigated because of the rapid surface exchange kinetics, the mixed conductivity and high oxygen vacancy concentration (14). Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ has also been investigated as a cathode material for IT-SOFC application by Shao et al. (15).Recently, layered perovskite oxides showing the chemical form of LnBaMn 2 O 5+ δ were investigated in the field of colossal magnetoresistance (CMR). Layered perovskite used for SOFC cathode materials shows the chemical form of LnBaCo 2 O 5+ δ and lanthanide materials of Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Y can be substituted (16,17). Most usefully, the oxygen ion diffusivity and surface exchange coefficient give the ECS Transactions, 25 (2) chance to apply these layered perovskite oxides to be used for cathode material for IT-SOFC because of their unique structural, physical and electrochemical characteristics. However, little published information are available concerning with SmBaCo 2 O 5+δ (SBCO) and LnBa 0.5 Sr 0.5 Co 2 O 5+δ (Ln = Pr, Nd, Sm, and Gd) as cathode materials for IT-SOFC. In this study, the electrochemical properties of SmBaCo 2 O 5+ δ and LnBa 0.5 Sr 0.5 Co 2 O 5+δ (Ln = Pr, Nd, Sm, and Gd) were measured and their electrical properties discussed. Most usefully, ...