Fig. 4. Flow cell performance of catalysts.

a Schematic illustration of the air-breathing flow cell configuration with natural air diffusion electrode. b Comparison of polarization curves of Co HSACs in the flow cell and H-type cell with natural air. Electrolyte: 0.5 M KOH. c Stability test of the Co HSACs under 300 mA cm⁻² in 0.5 M KOH for practically producing 80 mM H₂O₂ solution. The volume of 0.5 M KOH solution is 500 mL. The electrolyte feeding rate was fixed at 2 mL min⁻¹, and the oxygen is fed by the natural air diffusion. EDTA was added to the catholyte for the flow cell. The electrolyte was sampled every 2 h to determine the Faradaic efficiency and the H₂O₂ concentration, and the electrolyte was refreshed at the same time interval. Source data are provided as a Source Data file.