Iron-nitrogen-carbon materials (Fe-N-C) are considered as one of the most promising none precious candidates for oxygen reduction reaction. However, the role of iron carbides, one of the important accompanied species in Fe-N-C catalyst for oxygen reduction reaction, remains controversial. Herein, two different types of 3D graphene-based Fe-N-C catalysts with exposed and encapsulated Fe carbides were synthesized via pyrolysis of self-polymerization compounds of 2,6-Diaminopyridine using Fe(OH) 3 and FeCl3 as both initiators and Fe sources, respectively. Interestingly, Mossbauer spectroscopy and X-ray photoelectron spectroscopy measurements show that both catalysts are composed of same Fe species with (FeN4)-N-II/C, iron carbide, iron nitride, metallic iron where the proportion of each corresponding component is similar. However, there is a big gap between the two catalysts in catalytic performance for oxygen reduction reaction. The kinetic current densities of the former catalyst (3.565 mA cm(-2)) at 0.9 V vs. RHE is 4.68 times and 1.50 times higher than that of the latter (0.7617 mA cm(-2)) and Pt/C catalyst (2.376 mA cm(-2)), respectively. This sharp contrast directly identifies the critical role of exposed Fe carbides nanodots on Fe-N-C in promoting Fe-N-C activity for oxygen reduction reaction. The new discovery brings a new insight into developing advanced none precious metal catalysts. (c) 2018 Elsevier Ltd. All rights reserved.