Oxygen Reduction Reaction Mechanism of Nitrogen-Doped Graphene Derived from Ionic Liquid

It is of great significance to develop N-doped carbon materials possessing high catalytic activity, excellent durability and low cost for oxygen reduction reaction (ORR) due to imperative for energy devices with high energy density such as fuel cell and metal-air batteries. Herein N-doped graphene is prepared by annealing a homogeneous mixture of graphene oxide and ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄) in nitrogen atmosphere. By entrapping effect, the ionic liquid serves as both nitrogen source and restacking protectant in formation of high quality N-doped graphene sheets. Electrochemical characterizations reveal that the obtained N-doped graphene possesses excellent electro-catalytic properties for ORR in alkaline condition. The microstructure and ORR catalytic activities are highly sensitive to calcination temperature and the optimal temperature is 900 °C. Density functional theory analysis indicates from the atomic point of view that N atoms with different configurations have different effects on the ORR performance enhancement. Pyridinic N exhibits the highest ORR catalytic activity followed by graphitic N depending on the number of active sites. Based on the experimental and simulation results, the beneficial properties of the as-prepared N-doped graphene for ORR are ascribed to the superior conductivity of graphene, high nitrogen doping content and high proportion of the active graphitic and pyridinic N species.