Oxygen Reduction Reaction on N-Doped Graphene under Acidic Conditions: Integrating N-Dopant Site Structure Changes with Competing Elementary Reaction Steps

Abstract

Nitrogen-doped carbon (CNx) 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 CNx. Experimental half-cell ORR activity studies of CNx 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 CNx catalysts.