Perovskite LaCoO3 is being increasingly explored as an effective low-cost electrocatalyst for the oxygen evolution reaction (OER). Sr doping in LaCoO3 (La1xSrxCoO3) has been found to substantially increase its catalytic activity. In this work, we report a detailed study on the evolution of the electronic structure of La1xSrxCoO3 with 0 x 1 and its correlation with electrocatalytic activity for the OER. A combination of X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) was used to unravel the electronic density of states (DOS) near the Fermi level (EF), which provide insights into the key electronic structure features for the enhanced OER catalytic activity. Detailed analysis on the Co L-edge XAS suggest that LaCoO3 has a low spin state with t2g 6 eg 0 configuration at room temperature. This implies that the high OER catalytic activity of LaCoO3 should not be rationalized by the occupancy of eg = 1 descriptor. Substituting Sr2+ for La3+ in LaCoO3 induces Co4+ oxidation states and effectively dopes hole states into the top of valence band. A semiconductor-to-metal transition is observed for 0.2, due to the holeinduced electronic DOS at the EF and increased hybridization between Co 3d and O 2p. Such an electronic modulation enhances the surface adsorption of the *OH intermediate and reduces the energy barrier for interfacial charge transfer, thus improving the OER catalytic activity in La1xSrxCoO3. In addition, we found that the La1xSrxCoO3 surface undergoes amorphization after certain period of OER measurement, leading to a partial deactivation of the electrocatalyst. High Sr doping levels accelerated the amorphization process.

K.H.L. Zhang is grateful for funding support by the National Natural Science Foundation of China (Grant No. 21872116). J.C. gratefully acknowledges the financial support by the National Natural Science Foundation of China (Grant No. 21621091 and 21373166). Freddy E. Oropeza and Victor A. de la Peña O'Shea are grateful for the funding supported by the Spanish AEI (NyMPhA PID2019-106315RB-I00). Victor A. de la Peña O'Shea also wishes to thank ''Comunidad de Madrid" and European Structural Funds for their financial support to FotoArt-CM project (S2018/NMT-4367) and the Fundación Ramón Areces. C. M. Tian gratefully acknowledges financial support by the China Scholarship Council (CSC). K.H.L. Zhang and Jan P. Hofmann also ackonwledge the Sino-German Mobility Program (Grant No. M-0377). Preliminar spectroscopic measurements were performed at the CLAESS beamline at the ALBA synchrotron.