Shape Control of Mn 3 O 4 Nanoparticles on Nitrogen- Doped Graphene for Enhanced Oxygen Reduction Activity

Three kinds of Mn3O4 nanoparticles with different shapes (spheres, cubes, and ellipsoids) are selectively grown on nitrogen-doped graphene sheets through a two-step liquid-phase procedure. These non-precious hybrid materials display an excellent ORR activity and good durability. The mesoporous microstructure, nitrogen doping, and strong bonding between metal species and doped graphene are found to facilitate the ORR catalytic process. Among these three kinds of Mn3O4 particles, the ellipsoidal particles on nitrogen-doped graphene exhibit the highest ORR activity with a more positive onset-potential of -0.13 V (close to that of Pt/C, -0.09 V) and a higher kinetic limiting current density (J(K)) of 11.69 mA cm(-2) at -0.60 V. It is found that the ORR performance of hybrid materials can be correlated to the shape of Mn3O4 nanocrystals, and specifically to the exposed crystalline facets associated with a given shape. The shape dependence of Mn3O4 nanoparticles integrated with nitrogen-doped graphene on the ORR performance, reported here for the first time, may advance the development of fuel cells and metal-air batteries.