Stretching Epitaxial La0.6Sr0.4CoO3-{\delta} for Fast Oxygen Reduction

TL;DR

This study demonstrates that tensile strain in epitaxial La0.6Sr0.4CoO3-{eta} films significantly enhances oxygen reduction reaction kinetics, with potential implications for high-performance energy devices like fuel cells.

Contribution

It systematically investigates strain effects on ORR kinetics in epitaxial LSCO films, revealing a two-order magnitude increase in activity due to tensile strain, supported by experimental and theoretical analysis.

Findings

Ultra-thin tensile-strained films show 100x higher ORR kinetics.

Sr segregation is not responsible for activity enhancement.

Strain increases oxygen vacancy concentration and shifts oxygen 2p-band center.

Abstract

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.