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• Materials and Methods
• Fig. S1. Photographs of precursor solution (A), composite gel (B), and the final catalyst (C).
• Fig. S2. Comparison of photographs and UV-visible spectroscopy of different solutions.
• Fig. S3. (A) SEM image of composite gel after sol-gel; (B to E) mapping analysis of the composite gel: (B) C element from gelatin, (C) N and (D) O elements from gelatin and ammonium nitrate, and (E) Fe element from iron ferric hydroxide.
• Fig. S4. Comparison of the products derived from different precursor solution containing different components at the same treatment conditions: (A) only gelatin; (B) gelatin and ammonium nitrate; and (C) gelatin and iron nitrate.
• Fig. S5. Comparison of gels obtained by using cobalt nitrate (A) and nickel nitrate (B) as metal salts with the same treatment conditions of iron nitrate.
• Fig. S6. (A) FT-IR spectra, (B) N 1s spectra, and (C) XRD patterns of gelatin, amorphous Fe(OH)₃, and gelatin-Fe(OH)₃ gel.
• Fig. S7. (A) SEM image of composite gel after sol-gel; (B) SEM image and (C) XRD pattern of the product obtained by calcining the gel at 350°C; (D) TEM image of the above HCl-washed product.
• Fig. S8. TEM images of AG-C (A), IG-C (B), and G-C (C); (D) pore size distributions of four samples prepared with different precursors.
• Fig. S9. Raman spectra (A) and I_D/I_G ratio (B) of four samples prepared with different precursors. (C) Comparison of XRD patterns of IAG-C and G-C.
• Fig. S10. (A) Temperature effect of IAG-C activity for ORR; (B) comparison of the electron transfer number of catalysts at 0.65 V; (C) stability of IAG-C after 3 months in air.
• Fig. S11. LSV curves of IAG-C before and after adding KCN in O₂-saturated 0.1 M KOH at 1600 rpm: (A) 10 mM KCN and (B) 50 mM KCN. The green line is the LSV curve after CN⁻¹ poisoning tests, electrode washing, and immersion in fresh 0.1 M KOH for IAG-C.
• Fig. S12. LSV curves at different rotation rates in O₂-saturated 0.1 M KOH at 5 mV s⁻¹, and the inset shows the corresponding K-L plots: (A) G-C, (B) AG-C, (C) IG-C, and (D) Pt/C; (E) electron transfer number (n) and (F) peroxide yield of GC, AG-C, IG-C, IAG-C, and commercial 20 wt % Pt/C in O₂-saturated 0.1 M KOH.
• Fig. S13. (A) Linear sweep voltammetry (LSV) curves for G-C, AG-C, IG-C, IAG-C, and Pt/C in O₂-saturated 0.1 M HClO₄ at 5 mV s⁻¹ at 1600 rpm; LSV curves for IAG-C (B) and Pt/C (C) at different rotation rates in O₂-saturated 0.1 M HClO₄ at 5 mV s⁻¹, and the inset shows the K-L plots.
• Fig. S14. (A) Electron transfer number and (B) peroxide yield (n) of G-C, AG-C, IG-C, IAG-C, and commercial 20 wt % Pt/C in O₂-saturated 0.1 M HClO₄; (C) chronoamperometric response of IAG-C and Pt/C in O₂-saturated 0.1 M HClO₄ followed by addition of 3 M methanol; (D) chronoamperometric response of IAGC and Pt/C in O₂-saturated 0.1M HClO₄ solution at 0.5 V (versus RHE) at 1600 rpm.
• Fig. S15. (A) High-angle annular dark-field scanning transmission electron microscopy (STEM) image of the IAG-C catalyst; (B) C-, (C) N-, (D) O-, and (E) Fe-elemental mapping of the square region.
• Fig. S16. (A) High-angle annular dark-field scanning transmission electron microscopy (STEM) image of the IAG-C catalyst; (B) C-, (C) N-, (D) O-, and (E) Fe-elemental mapping of the square region after stability test. (F) N 1s XPS spectra of IAG-C after stability test.
• Fig. S17. TEM images of Fe₃O₄@AGC with different content of Fe₃O₄ in the hybrids: (A) Fe₃O₄@AGC-4, (B) Fe₃O₄@AGC, and (C) Fe₃O₄@AGC-6; (D) TG curves of the three hybrids to determine the content of carbon; (E) XRD patterns of three hybrids. The diffraction peaks of the composites are perfectly indexed to pure-phase Fe₃O₄ (JCPDS No.65-3107).
• Fig. S18 Charge-discharge curves of Fe₃O₄@AGC (A), bare Fe₃O₄ (B), and AGC (C) at a current density of 100 mA g⁻¹; (D) Nyquist plots for the Fe₃O₄@AGC and bare Fe₃O₄-based cells with lithium metal as counter electrode.
• Fig. S19. The calibration CV curves of Ag/AgCl electrode (A) in 0.1 M KOH and (B) in 0.1 M HClO₄ with respect to RHE.

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