  Fast, reversible redox reactions in solids at low temperatures without
thermomechanical degradation are a promising strategy for enhancing the overall
performance and lifetime of many energy materials and devices. However, the
robust nature of the cation's oxidation state and the high thermodynamic
barrier have hindered the realization of fast catalysis and bulk diffusion at
low temperatures. Here, we report a significant lowering of the redox
temperature by epitaxial stabilization of strontium cobaltites (SrCoOx) grown
directly as one of two distinct crystalline phases, either the perovskite
SrCoO3-{\delta} or the brownmillerite SrCoO2.5. Importantly, these two phases
can be reversibly switched at a remarkably reduced temperature (200~300
{\deg}C) in a considerably short time (< 1 min) without destroying the parent
framework. The fast, low temperature redox activity in SrCoO3-{\delta} is
attributed to a small Gibbs free energy difference between two topotatic
phases. Our findings thus provide useful information for developing highly
sensitive electrochemical sensors and low temperature cathode materials.
