  The slow kinetics of the oxygen reduction reaction (ORR) is one of the key
challenges in developing high performance energy devices, such as solid oxide
fuel cells. Straining a film by growing on a lattice-mismatched substrate has
been a conventional approach to enhance the ORR activity. However, due to the
limited choice of electrolyte substrates to alter the degree of strain, a
systematic study in various materials has been a challenge. Here, we explore
the strain modulation of the ORR kinetics by growing epitaxial
La0.6Sr0.4CoO3-{\delta} (LSCO) films on yttria-stabilized zirconia substrates
with the film thickness below and above the critical thickness for strain
relaxation. Two orders of magnitude higher ORR kinetics is achieved in an
ultra-thin film with ~0.8% tensile strain as compared to unstrained films.
Time-of-flight secondary ion mass spectrometry depth profiling confirms that
the Sr surface segregation is not responsible for the enhanced ORR in strained
films. We attribute this enhancement of ORR kinetics to the increase in oxygen
vacancy concentration in the tensile-strained LSCO film owing to the reduced
activation barrier for oxygen surface exchange kinetics. Density functional
theory calculations reveal an upshift of the oxygen 2p-band center relative to
the Fermi level by tensile strain, indicating the origin of the enhanced ORR
kinetics.
