Electrosynthesis of hydrogen peroxide (H 2 O 2 ) through oxygen reduction reaction (ORR) is an environmentfriendly and sustainable route for obtaining a fundamental product in the chemical industry. Co−N 4 single-atom catalysts (SAC) have sparkled attention for being highly active in both 2e − ORR, leading to H 2 O 2 and 4e − ORR, in which H 2 O is the main product. However, there is still a lack of fundamental insights into the structure−function relationship between CoN 4 and the ORR mechanism over this family of catalysts. Here, by combining theoretical simulation and experiments, we unveil that pyrrole-type CoN 4 (Co−N SAC Dp ) is mainly responsible for the 2e − ORR, while pyridine-type CoN 4 catalyzes the 4e − ORR. Indeed, Co−N SAC Dp exhibits a remarkable H 2 O 2 selectivity of 94% and a superb H 2 O 2 yield of 2032 mg for 90 h in a flow cell, outperforming most reported catalysts in acid media. Theoretical analysis and experimental investigations confirm that Co−N SAC Dp �with weakening O 2 /HOO* interaction�boosts the H 2 O 2 production.

Electrochemical production of hydrogen peroxide (H 2 O 2 )t hrough two-electron (2 e À )o xygen reduction reaction (ORR) is an on-site and clean route.O xygen-doped carbon materials with high ORR activity and H 2 O 2 selectivity have been considered as the promising catalysts,h owever,t here is still alackofdirect experimental evidence to identify true active sites at the complex carbon surface.H erein, we propose ac hemicalt itration strategy to decipher the oxygen-doped carbon nanosheet (OCNS 900 )c atalyst for 2e À ORR. The OCNS 900 exhibits outstanding 2e À ORR performances with onset potential of 0.825 V( vs.R HE), mass activity of 14.5 Ag À1 at 0.75 V( vs.R HE) and H 2 O 2 production rate of 770 mmol g À1 h À1 in flow cell, surpassing most reported carbon catalysts.Through selective chemical titration of C = O, C À OH, and COOH groups,wefound that C = Ospecies contributed to the most electrocatalytic activity and were the most active sites for 2e À ORR, which were corroborated by theoretical calculations.

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