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In an electrocatalytic process, the cognition of the active phase in a catalyst has been regarded as one of the most vital issues, which not only boosts the fundamental understanding of the reaction procedure but also guides the engineering and design for further promising catalysts. Here, based on the oxygen evolution reaction (OER), the stepwise evolution of the dominant active phase is demonstrated in the LaNiO3 (LNO) catalyst once the single‐crystal thin film is decorated by LNO nanoparticles. It is found that the OER performance can be dramatically improved by this decoration, and the catalytic current density at 1.65 V can be enhanced by ≈1000% via ≈109 cm−2 nanoparticle adhesion after extracting the contribution of surface enlargement. Most importantly, a transition of the active phase from LNO to NiOOH via surface reconstruction with the density of LNO nanoparticles is demonstrated. Several mechanisms in terms of this active phase transition are discussed involving lattice orientation‐induced change of the surface energy profile, the lattice oxygen participation, and the A/B‐site ions leaching during OER cycles. This study suggests that the active phases in transition metal‐based OER catalysts can transform with morphology, which should be corresponding to distinct engineering strategies.

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The ferroelectric properties in single-crystalline perovskite oxide can be engineered by strain, which is conventionally achieved via interfacial lattice mismatch between a substrate and an epitaxial ferroelectric overlayer. To realize the continuous tuning of strain in ferroelectrics and explore its functionality in the morphology of freestanding membranes, in this study, we have performed a comprehensive investigation on flexible BaTiO3 (BTO) membranes with varied wrinkle patterns. The freestanding BTO membranes can be obtained from the rigid epitaxial heterostructures by water-dissolving the Sr3Al2O6 (SAO) sacrificial interlayer, and the different varied wrinkle patterns are gained by tailoring the mixing ratios of polydimethylsiloxane (PDMS) supporters following the thermal strain and elastic theory. Then, we have found that the spontaneous polarization switching behavior of freestanding wrinkled BTO membranes is curvature-dependent, and the coercive field of BTO can be reduced by 15.03% once it suffers ∼1.34% tensile and compressive strain. Our study provides a convenient and robust approach to morphological changes of ferroelectrics and holds great promising potential for applications in flexible oxide electronics.