Fig. 1. (a) Flow chart of NGY-Fe catalyst preparation; (b) SEM image and (c) TEM image of NGY-Fe sample prepared after γ irradiation; (d) elemental distribution (C, N and Fe) plots of NGY-Fe samples obtained by SEM's built-in EDS

Fig. 2. (a) High-resolution core-grade XPS full spectra of NGY-Fe samples and GY samples after γ irradiation; (b) Raman spectra of NGY-Fe samples; high-resolution core-level XPS peak splitting spectra of (c) C1s, (d) Fe2p and (e) N1s

Fig. 3. (a) Cyclic voltammetry curves of NGY-Fe catalysts and commercial Pt/C catalysts in 0.1 mol/L KOH electrolyte; (b) polarisation curves of NGY-Fe catalyst and commercial Pt/C catalyst at 1 600 r/min (color online)

Fig. 4. (a) LSV curves of the optimal sample NGY-Fe_1/2 at different rotational speeds; (b) K-L curves corresponding to the optimal sample NGY-Fe_1/2 at different potentials; (c) nyquist plots of samples NGY-Fe_1/2 and commercial Pt/C catalyst (color online)

Fig. 5. Number of (a) transferred electrons (n) of samples NGY-Fe_1/2 and commercial Pt/C catalysts obtained according to RRDE experimental tests; (b) H₂O₂ yield during oxygen reduction reaction

Fig. 6. Tafel slope calculated based on the polarization data of oxygen reduction reaction of NGY-Fe_1/2 catalyst and Pt/C catalyst

Fig. 7. (a) I-t curves of NGY-Fe_1/2 catalyst and Pt/C catalyst; (b) Methanol tolerance curves for NGY-Fe_1/2 catalyst and Pt/C catalyst