Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts

Development and optimization of non-platinum group metal (non-PGM) electrocatalysts for oxygen reduction reaction (ORR), consisting of transition metal-nitrogen-carbon (M-N-C) framework, is hindered by the partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. In this study, we have analyzed more than 45 M-N-C electrocatalysts synthesized from three different families of precursors, such as polymer-based, macrocycles, and small organic molecules. Catalysts were electrochemically tested and analyzed structurally using exactly the same protocol for deriving structure-to-property relationships. We have identified possible active sites participating in different ORR pathways: (1) metal-free electrocatalysts support partial reduction of O₂ to H₂O₂; (2) pyrrolic nitrogen acts as a site for partial O₂ reduction to H₂O₂; (3) pyridinic nitrogen displays catalytic activity in reducing H₂O₂ to H₂O; (4) Fe coordinated to N (Fe-N_x) serves as an active site for four-electron (4e^-) direct reduction of O₂ to H₂O. The ratio of the amount of pyridinic and Fe-N_x to the amount of pyrrolic nitrogen serves as a rational design metric of M-N-C electrocatalytic activity in oxygen reduction reaction occurring through the preferred 4e^- reduction to H₂O.