. (a) AFM image and (b) corresponding thickness curves of graphene, (c) SEM image and (d) XRD pattern of Fe, N-doped porous carbon catalyst

. XPS spectra of Fe, N-doped porous carbon catalysts (a) Survey spectrum; (b) C1s; (c) N1s; (d) Fe2p

. (a) CV curves of the Fe, N-doped porous carbon catalyst in a nitrogen- and oxygen-saturated 0.1 mol/L KOH aqueous solution; (b) ORR polarization curves of catalysts recorded at different rotating speed; (c) The K-L plots at different potentials including electron transfer number (n) per oxygen of catalyst; (d) OER polarization curves of catalysts at 1600 r/min in an oxygen-saturated 0.1 mol/L aqueous KOH solution

. (a) Discharge polarization and corresponding power density curves of zinc-air battery using Fe, N-doped porous carbon catalyst; (b) discharge-charge polarization curves of zinc-air battery; and (c) cycling measurements for rechargeability at a current density of 10 mA/cm²