Nitrogen-doped graphene derived from ionic liquid as metal-free catalyst for oxygen reduction reaction and its mechanisms

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 cells and metal-air batteries. Herein, N-doped graphene is prepared by annealing a homogeneous mixture of graphene oxide (GO) and ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) in N2 atmosphere. By entrapping effect, the ionic liquid serves as both N source and restacking protectant in formation of high-quality N-doped graphene sheets. Electrochemical characterizations reveal that the obtained N-doped graphene possesses excellent electrocatalytic properties for ORR in alkaline condition with onset potential of −39 mV (vs. Hg/HgO) and current density of 5.83 mA cm−2 at −0.9 V (vs. Hg/HgO) at 2500 rpm. The microstructure of the prepared catalysts and their ORR catalytic activities are highly sensitive to calcination temperature and the optimal temperature is 900 °C. Density functional theory (DFT) analysis indicates from the atomic point of view that N atoms with different configurations contribute unequally to the ORR performance enhancement. Pyridinic N at the edge of graphene plays the most significant role in improving ORR performance owing to the largest number of active sites and lower band gap. 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, the high N doping content and the high proportion of active pyridinic N species.