B, N- and P, N-doped graphene as highly active catalysts for oxygen reduction reactions in acidic media

Abstract

Graphene has been highlighted recently as a promising material for energy conversion due to its unique properties deriving from a two-dimensional layered structure of sp²-hybridized carbon. Herein, N-doped graphene (NGr) is developed for its application in oxygen reduction reactions (ORRs) in acidic media, and additional doping of B or P into the NGr is attempted to enhance the ORR performance. The NGr exhibits an onset potential of 0.84 V and a mass activity of 0.45 mA mg⁻¹ at 0.75 V. However, the B, N- (BNGr) and P, N-doped graphene (PNGr) show onset potentials of 0.86 and 0.87 V, and mass activities of 0.53 and 0.80 mA mg⁻¹, respectively, which are correspondingly 1.2 and 1.8 times higher than those of the NGr. Moreover, an additional doping of B or P effectively reduces the production of H₂O₂ in the ORRs, and shows much higher stability than that of Pt/C in acidic media. It is proposed that the improvement in the ORR activity results from the enhanced asymmetry of the spin density or electron transfer on the basal plane of the graphene, and the decrease in the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the graphene through additional doping of B or P.