Functions in cooperation for enhanced oxygen reduction reaction: the independent roles of oxygen and nitrogen sites in metal-free nanocarbon and their functional synergy

Heteroatom-doped nanocarbons are known to behave differently in the oxygen reduction reaction (ORR) in aqueous systems. In the present study, we reveal the dissimilar roles of oxygen and nitrogen sites in nanocarbons and show that the different functions can jointly promote the ORR in cooperation when appropriately combined. The oxygen sites in general could efficiently facilitate both the HO2- generation and the reduction at high overpotential, while the active nitrogen sites could activate the ORR with a lower energy requirement and simultaneously reduce both O2 and HO2- at low overpotential. A further study to integrate the functionalities in a single nanocarbon reveals the oxidation-resistant nature of the active nitrogen functional group, a chemical basis for the reactivity.