Oxygen Reduction Reaction on N‑Doped Graphene under Acidic Conditions: Integrating N‑Dopant Site Structure Changes...

Nitrogen-doped carbon (CN _x ) catalysts are promising alternatives to replace Pt for the electrochemical oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells. To date, there remains an ongoing debate regarding the nature of active sites on CN _x . Experimental half-cell ORR activity studies of CN _x catalysts correlate the fraction of pyridinic sites (characterized ex situ by X-ray photoelectron spectroscopy) to ORR activity. In contrast, density functional theory (DFT) studies using the chemical hydrogen electrode (CHE) approach predominantly show quaternary/oxide sites to have higher activity than that of pyridinic sites. In this work, we integrate the initial thermodynamic stability analysis of the N-dopant structure at 1.23 V-RHE with subsequent competing ORR elementary steps as a function of applied potential. In contrast to earlier studies, our integrated approach predicts high activity for pyridinic sites, with a majority of quaternary sites, except basal quaternary, which show low activity. This enhanced activity occurs through an initial oxidation of the pyridinic sites via H₂O that converts the pyridinic sites to pyridinic oxide structures at 1.23 V-RHE (before the start of ORR). Subsequent ORR steps on this structure result in a stable 4 proton-electron cycle of elementary steps that yield high ORR DFT thermodynamic limiting potentials. Therefore, the DFT-CHE results suggest that pyridinic sites are indeed active for ORR but first must convert to an oxidized structure that mediates the ORR mechanism. Our findings suggest a potential mechanism to reconcile the ongoing debate on the nature of active sites and provide fundamental insights into the ORR activity of CN _x catalysts.