Hierarchically open-porous carbon networks enriched with exclusive Fe–Nx active sites as efficient oxygen reduction catalysts towards acidic H2–O2 PEM fuel cell and alkaline Zn–air battery

•Well-defined open-porous carbon networks are prepared with g-C3N4 and ZIF-8 as precursors.•High fraction of Fe–Nx and hierarchical porosity boost oxygen reduction reaction.•The material exhibited an excellent performance in both acidic H2–O2 PEM fuel cell and alkaline Zn–air battery.