fig1

Figure 1. (A) Schematics of ORR 2e^- pathway and 4e^- pathway with the corresponding intermediates^[22]; (B) Trends in oxygen reduction activity plotted as a function of the oxygen binding energy^[23]; (C) Schematic model of Fe-N-C. Fe (orange), N (blue), O (red), and C (gray)^[25]; (D) Free energy diagrams of 2e^- ORR on the SACs at U = 0.7 V versus RHE^[28]; (E) Schematic illustration for the formation of Co SAs/N-C through pyrolysis of Zn/Co MOFs^[29]; (F) Thermodynamic-activity relation trends for the 2e^- pathway for H₂O₂ product of different M-N-C^[30]; (G-H) In situ TEM images in synthetic process of Fe SAs/S-NC (Fe₃O₄@SiO₂/CdS@PDA)^[32]; (I) Aberration-corrected HAADF-STEM images corresponding to the selected area in (H) of Fe SAs/S-NC^[32]. ORR: Oxygen reduction reaction; M-N-C: metal-nitrogen-carbon; SACs: single-atom catalysts; Fe SAs/S-NC: Fe single atoms arranged on S, N co-doped carbon skeleton; RHE: reversible hydrogen electrode; MOFs: metal-organic frameworks; HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy.