The oxygen reduction reaction (ORR) activity of a series of composite cathodes consisting of a porous Gd 0.20 Ce 0.80 O 2-δ (GDC) scaffold infiltrated with Sr-, Co-, and Y-nitrate solutions has been systematically investigated in this study. The results show that such infiltrated cathodes if calcined at low temperatures such as 350 • C exhibit low polarization resistance (R P ) in the temperature range of 450-700 • C, even though XRD analysis reveals that the calcined product is virtually a mixture of Co 3 O 4 and SrCO 3 . A further study by design-of-experiment suggests that the true ORR-active species is Co 3 O 4 , whereas SrCO 3 serves as a sintering inhibitor to preserve the high surface area of Reduced-temperature solid oxide fuel cells (RT-SOFCs) have the potential to be a commercially viable product due to improved durability and reduced cost, thus attracting a long lasting interest in the SOFC community. In the pursuit of RT-SOFCs, enhancing the kinetics of oxygen reduction reaction (ORR) is leading the development. There are two general approaches currently being practiced to achieve the enhanced ORR activity: 1) searching for new materials with a high intrinsic activity; 2) optimizing microstructure with maximal reactive sites.2,3 For the first approach, the research has been mostly focused on oxides with perovskite, double perovskite and RuddlesdenPopper intergrowth structures. [4][5][6] It has been reported that the lattice parameters of these structures can be tuned to yield high mixed electronic/ionic conductivity and high concentration of oxygen vacancies on the surface, two of which are the important descriptors of high ORR activity.7-9 For the second approach, nanoparticles (NPs) of an active material are introduced into a pre-existing microstructure (either a cathode or electrolyte porous scaffold) as a means of maximizing ORR-active reaction sites. The method of incorporation is solution infiltration/impregnation, 10-12 which has the advantage of allowing creation of NPs at much lower temperatures than scaffold sintering temperature, thus preserving nanostructures desirable for fast ORR.However, most of ORR-active materials found so far are complex oxides, the formation of which generally requires higher temperatures. For example, infiltrated Sr-and Co-doped LaFeO 3 (LSCF) NPs need ∼800• C to form the perovskite structure. 13,14 Under such a circumstance, the preferred nanostructures may be sintered into microstructures, resulting in a loss of the ORR-activity. Nevertheless, there are ample examples in the literature that show significant performance enhancement by high-temperature calcined NPs even though the original NPs have been coarsened to some degree. 11,12 On the other hand, low-temperature calcined NPs have also been shown with high performance even though the desirable phase is not formed at the first place. For example, researchers from Riso recently observed a very low R p for infiltrated La 0.6 Sr 0.4 CoO 3-δ NPs calcined at 350• C, 15,16 where perovskite phase cannot be formed. U...