Transition-metal oxides are commonly used as catalysts for the oxygen reduction reaction (ORR) in fuel cells for efficient power generation. The main barrier to achieving acceptable chemical-to-electrical conversion efficiency in fuel cells is the sluggish ORR kinetics at the cathode. [1] A lack of fundamental understanding of the ORR mechanism limits the development of highly active catalysts to enhance fuel cell efficiency. As single-crystal oxide thin films and superlattices can have physical properties dramatically different from those of bulk materials, such as enhanced ferroelectricity [2] and oxygen-ion conductivity, [3,4] one may expect that thin-film surfaces can have intrinsic ORR activity different from that of the bulk. However, studies to date have shown that polycrystalline [5, 1b] and single-crystal epitaxial oxide films [6,7] have reduced ORR activity (specifically surface oxygen exchange kinetics) relative to the bulk (see Figure S1 in the Supporting Information). Herein, we report the epitaxial growth of a strontiumsubstituted lanthanum cobalt perovskite, La 0.8 Sr 0.2 CoO 3Àd (LSC), on (001)-oriented single-crystal yttria-stabilized zirconia. The epitaxial LSC surface exhibits markedly increased ORR activity by up to two orders of magnitude relative to the bulk, which may be attributed to increased oxygen vacancy concentrations in the films.Pulsed laser deposition (PLD) was utilized to first deposit a gadolinium-doped ceria (GDC, 20 mol % Gd) film having a thickness of approximately 5 nm on a single crystal of yttriastabilized zirconia (YSZ) with the (001) cubic orientation. LSC thin films with thicknesses of 20, 45, and 130 nm were subsequently deposited on the GDC/YSZ (001) cubic substrate.GDC was used as an interlayer to prevent chemical reactions between LSC and YSZ. [8] Atomic force microscopy (AFM) showed that LSC films had a surface root-mean-square (RMS) roughness of 3-6 nm (Figure 1 a).Normal X-ray diffraction (XRD) patterns (Figure 1 b) of LSC thin films only reveal (001) cubic or (00l) pc peaks, which indicates (001) pc LSC//(001) cubic GDC//(001) cubic YSZ. The subscript "pc" denotes the pseudocubic notation, in which the rhombohedral structure of LSC bulk [9] is approximated with a pc % 3.837 (see Figure S2 in the Supporting Information). Four-circle XRD data analysis (see Figure S3 in the Supporting Information) showed that LSC films grown epitaxially on (001) cubic YSZ were single-phase. In addition, off-normal XRD data (Figure 2 a) showed that the [100] pc LSC was rotated by 458 with respect to the [100] cubic GDC, which is expected from a(GDC)% ffiffi ffi 2 p a pc (LSC) (Figure 2 b; that is, [100] pc LSC//[110] cubic GDC//[110] cubic YSZ).Of significance is the fact that the films have much larger relaxed unit cell volumes than the bulk [9] at room temperature. Having a different relaxed unit volume from that of bulk materials is commonly noted for PLD films, and may Figure 1. a) AFM image of an LSC 45nm film with surface RMS roughness of ca. 3.4 nm. b) Normal XRD data of LS...