Developing efficient catalysts based on inexpensive and abundant materials for the sluggish oxygen reduction reaction (ORR) constitutes one of the grand challenges in the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Despite recent advances in developing active and durable non-platinum group metal catalysts as promising alternatives to Pt, a less empirical synthesis approach is still missing due to the lack of proper understanding of the mechanistic origin of the ORR and the underlying surface properties under working conditions that govern catalytic activity.

Here, by employing in situ x-ray absorption spectroscopy (XAS) together with complementary characterization techniques such as Mössbauer spectroscopy etc., we propose that the distorted Fe²⁺-N₄-C_x moiety, in which the central Fe²⁺ ion is located out of the N₄-plane and at low spin state without the fifth axial ligand, is the active site responsible for the high ORR activity of pyrolyzed Fe-based catalysts. The local structure of this active site under ex situ conditions is drastically different from that obtained under working conditions, and is thus disconnected from the ORR activity. In situ XAS studies further reveal that the ORR is mediated by the reversible Fe^2+/3+ redox transitions with potential-dependent population of active sites.¹ Accordingly, the high ORR activity of the distorted Fe²⁺-N₄-C_x moiety is attributable to the out-of-plane Fe displacement with low spin state, which leads to high  Fe^2+/3+ redox transition potential that minimizes the site-blocking effect at the targeted potential, as well as appropriate number of e_g-electrons that favors O₂ adsorption.