Direct Insight into the Role of Cationic Lactam Nitrogen on Oxygen Electroreduction

Nitrogen-doped carbon catalysts are promising metal-free candidates for the oxygen reduction reaction (ORR). However, the identification of genuine active species remains challenging due to the complexity of local N environments, where the role of lactam has long been overlooked. Herein, N-doped nanodiamond (NND) and carbon nanotubes (NCNT) as metal-free model catalysts are fabricated for ORR. Spectroscopic analyses and temperature-programmed desorption (TPD) measurements reveal the dominance of lactam in an sp³-hybridized carbon framework of NND, whereas NCNT is mainly composed of pyridinic and graphitic N, providing clear chemical tools for active site investigation. Through electrochemical Coulombic screening experiments, it was demonstrated that ORR on NND mainly proceeds via lactam-derived cationic N intermediates, while NCNT could expedite the reaction through anionic intermediates associated with pyridinic N. Further postreaction characterizations evidence the formation of cationic N+ in NND, which interacts strongly with neighboring carbon atoms (e.g., C-O-) to participate in ORR actively. Taken together, this work not only highlights the pivotal role of cationic lactam N toward ORR but also gives insights into the future design of metal-free carbon catalysts via fine N engineering.