What is the Real Origin of the Activity of Fe-N-C Electrocatalysts in the O 2 Reduction Reaction? Critical Roles of Coordinating Pyrrolic N and Axially Adsorbing Species.

Fe-N-C electrocatalysts have emerged as promising substitutes for Pt-based catalysts for the oxygen reduction reaction (ORR). However, their real catalytic active site is still under debate. The underlying roles of different types of coordinating N including pyridinic and pyrrolic N in catalytic performance require thorough clarification. In addition, how to understand the pH-dependent activity of Fe-N-C catalysts is another urgent issue. Herein, we comprehensively studied 13 different N-coordinated FeN x C configurations and their corresponding ORR activity through simulations which mimic the realistic electrocatalytic environment on the basis of constant-potential implicit solvent models. We demonstrate that coordinating pyrrolic N contributes to a higher activity than pyridinic N, and pyrrolic FeN 4 C exhibits the highest activity in acidic media. Meanwhile, the in situ active site transformation to *O-FeN 4 C and *OH-FeN 4 C clarifies the origin of the higher activity of Fe-N-C in alkaline media. These findings can provide indispensable guidelines for rational design of better durable Fe-N-C catalysts.