This PDF file includes:

• Fig. S1. Characterization of VA-NCNTs.
• Fig. S2. Electrocatalytic activities of the VA-NCNT catalyst in alkaline electrolyte (O₂-saturated 0.1 M KOH) by half-cell tests.
• Fig. S3. Electrocatalytic activities of the VA-NCNT catalyst in acidic electrolyte (O₂-saturated 0.1 M HClO₄) by half-cell tests.
• Fig. S4. Typical cross-section SEM images of the GDL with the MEA of VANCNTs as the cathode catalyst layer, Nafion membrane (N211) as the separator, and Pt/C as the anode.
• Fig. S5. SEM (A) and TEM (B) images of N-CNT bundles.
• Fig. S6. Typical cross-section SEMimages of the GDLswith the MEAs of (A to C)N-G-CNT (2mg cm−²) and (D to F) N-G-CNT + KB (0.5 + 2 mg cm−²) as the cathode catalyst layers, respectively.
• Fig. S7. Tafel plot (A) and electron transfer number (B) for the N-G-CNT and Pt/C (20%) as the function of electrode potential by RRDE in oxygen-saturated 0.1 M KOH solution at a scan speed of 5 mV s−¹ and a rotation speed of 1600 rpm.
• Fig. S8. Long-time stability and tolerance to methanol/carbon monoxide of metalfree catalyst N-G-CNT.
• Fig. S9. SEM images of catalyst layer cross sections used in RDE measurements.
• Fig. S10. Electrocatalytic activities of the carbon-based metal-free N-G-CNT catalysts in acidic electrolyte (O₂-saturated 0.1 M HClO₄) by half-cell tests.
• Fig. S11. Optimization of cathode catalyst layer composition.
• Fig. S12. Single-cell performance comparison between N-G-CNT and Fe/N/C catalysts at the same catalyst layer composition: catalyst (0.5 mg cm−²)/KB (2 mg cm−²)/Nafion (2.5 mg cm−²).
• Fig. S13. Polarization curves of the N-G-CNT and individual components of N-G or N-CNT.
• Fig. S14. Durability of the catalyst layer composed of metal-free N-G-CNT (2 mg cm−²) + KB (2 mg cm−²) in a PEM fuel cell measured at 0.5 V.
• Fig. S15. The metal-free character of N-G-CNT catalyst.

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