Fabbri E Nachtegaal M Binninger T Cheng X Kim BJ Durst J Bozza F Graule T Schäublin R Wiles L Pertoso M Danilovic N Ayers KE Schmidt TJ 2017 The growing need to store increasing amounts of renewable energy has recently triggered substantial R&D efforts towards efficient and stable water electrolysis technologies. The oxygen evolution reaction (OER) occurring at the electrolyser anode is central to the development of a clean, reliable and emission-free hydrogen economy. The development of robust and highly active anode materials for OER is therefore a great challenge and has been the main focus of research. Among potential candidates, perovskites have emerged as promising OER electrocatalysts. In this study, by combining a scalable cutting-edge synthesis method with time-resolved X-ray absorption spectroscopy measurements, we were able to capture the dynamic local electronic and geometric structure during realistic operando conditions for highly active OER perovskite nanocatalysts. Ba0.5Sr0.5Co0.8Fe0.2O3-δ as nano-powder displays unique features that allow a dynamic self-reconstruction of the material's surface during OER, that is, the growth of a self-assembled metal oxy(hydroxide) active layer. Therefore, besides showing outstanding performance at both the laboratory and industrial scale, we provide a fundamental understanding of the operando OER mechanism for highly active perovskite catalysts. This understanding significantly differs from design principles based on ex situ characterization techniques. https://sonar.ch/global/documents/277160 eng info:eu-repo/semantics/altIdentifier/doi/10.1038/nmat4938 info:eu-repo/semantics/altIdentifier/pmid/28714982 Nature materials. - 2017 Dynamic surface self-reconstruction is the key of highly active perovskite nano-electrocatalysts for water splitting. http://purl.org/coar/resource_type/c_6501