▎ 摘　　要

Electrochemical oxygen reduction reaction (ORR) in acids via a selective 2e^- pathway offers great opportunities for electrosynthesis of H2O2, allowing on-site environmental treatment in industry. Unfortunately, despite some progress, the apparent activity of most electrocatalysts (especially in a flow cell reactor) still requires further improvement to meet the industrial demands, where high H2O2 productivity with low energy input is desired. Herein, we report a free-standing ORR electrode comprising cobalt single atoms on a vertically aligned graphene nanosheet assembly (CoN4/VG), which is demonstrated to exhibit a hierarchical porous structure maximizing the utilization of catalytic active atoms without sacrificing the mass/charge transport efficiencies. Within a H-cell setup, the as-prepared ORR electrode gives a H2O2 selectivity close to 100% from 0.3 to 0.5 V versus reversible hydrogen electrode (RHE) in 0.1 M HClO4, sustaining a record-breaking H2O2 productivity of 706 mmolH2O2 gcatalyst^-1 h^-1 at 0.3 V vs. RHE for 36 hours. Further employing this electrode in a gas-diffusion flow reactor yields a peroxide concentration of 1100 mg L^-1 (4000 mmolH2O2 gcatalyst^-1 h^-1) continuously at -1.8 V of cell voltage, corresponding to an energy consumption of 3.81 W h gH2O2^-1, which represents the most energy-efficient flow system for rapid H2O2 generation in acidic media.