Understanding the influence of strain-modified oxygen vacancies and surface chemistry on the oxygen reduction reaction of epitaxial La0.8Sr0.2CoO3-δ thin films

One of the key challenges in developing high-performance electrochemical energy devices is the slow kinetics of the oxygen reduction reaction (ORR). Implementing epitaxial strain induced by a lattice mismatch between a film and a substrate has shown great potential for enhancing the ORR by increasing the concentration of oxygen vacancies. However, strain also influences the surface chemistry which also impacts the ORR. Here, we show the role of strain in concurrent changes of oxygen vacancies and surface chemistry, which in turn significantly influence the ORR activity by employing epitaxial La0.8Sr0.2Co3-δ (LSC) thin films with three different strain states. Tensile strained LSC films dramatically enhanced ORR activity up to two orders of magnitude compared to compressively strained LSC films. Among the tensile strained films, moderately tensile strained LSC films show the highest ORR activity, which is attributed to both increased concentration of oxygen vacancies and suppressed Sr segregation. Our study provides a new design strategy to enhance the ORR activity by controlling the strain state to tune the Sr segregation, oxygen vacancies, and oxygen surface exchange kinetics. •The optimal strain to enhance the oxygen reduction reaction of La0.8Sr0.2Co3-δ was revealed.•Tensile strain increased the concentration of oxygen vacancies compared to compressive strain.•Tensile strain also increased surface Sr segregation.•Moderate tensile strain was beneficial for enhancing the ORR activity of LSC.