Fig. 4. (a) ORR polarization curves of Ni–N/C_3_X catalysts measured in O₂-saturated 0.1 M KOH and (b) the corresponding H₂O₂ selectivity. (c) Tafel plots for Ni–N/C_3_X catalysts. The numbers in the parentheses represent the Tafel slopes (in mV dec⁻¹). (d) Mass activity of Ni–N/C_3_X catalysts at 0.70 V.