Fe-N-x-C materials are extensively investigated due to their outstanding performance for oxygen reduction reaction (ORR). However, even though Fe3C, pyridinic N, and Fe-N coordinations have been widely recognized as active sites for catalyzing the ORR, there is almost no report about the dominating functional groups responsible for the enhancement of ORR performance, especially in acidic media. Here we developed a robust and universal non precious metal catalyst, Fe3O4 nanoparticles (particle size: similar to 50 nm) embedded in 3D cross-linking hierarchically porous N-doped carbon (noted as Fe3O4@HPNC), as a superb ORR performance catalyst in both basic and acidic media. More importantly, in this hybrid there exists no Fe3C phase, which makes it easier to identify the prominent group contributing to the enhanced ORR performance. Taking into account that the enhancing factor of the ORR half-wave potential at Fe3O4-300@HPNC with respect to that at HPNC in acidic media (36.6%) is much higher than that in basic media (7.6%), it could be deduced that the role of the Fe-N coordinations for ORR in acidic media is more dominating than that in basic media. Optimized Fe3O4-300@ HPNC exhibits comparable ORR activity in acidic, or even better in basic media, superior stability and MeOH tolerance compared with the commercial Pt/C. (c) 2017 Elsevier Ltd. All rights reserved.