  Oxygen vacancies in transition metal oxides facilitate catalysis critical for
energy storage and generation. However, it has proven elusive to promote
vacancies at the lower temperatures required for operation in devices such as
metal-air batteries and portable fuel cells. Here, we use thin films of the
perovskite-based strontium cobaltite (SrCoOx) to show that epitaxial strain is
a powerful tool towards manipulating the oxygen content under conditions
consistent with the oxygen evolution reaction, yielding increasingly oxygen
deficient states in an environment where the cobaltite would normally be fully
oxidized. The additional oxygen vacancies created through tensile strain
enhance the cobaltite catalytic activity towards this important reaction by
over an order of magnitude, equaling that of precious metal catalysts,
including IrO2. Our findings demonstrate that strain in these oxides can
dictate oxygen stoichiometry independent of ambient conditions, allowing
unprecedented control over oxygen vacancies essential in catalysis near room
temperature.
