"Metal-free" catalytic oxygen reduction reaction on heteroatom-doped graphene is caused by trace metal impurities

The oxygen reduction reaction (ORR) is of high industrial importance. There is a large body of literature showing that metal-based catalytic nanoparticles (e.g. Co, Mn, Fe or hybrid Mn/Co-based nanoparticles) supported on graphene act as efficient catalysts for the ORR. A significant research effort is also directed to the so-called "metal-free" oxygen reduction reaction on heteroatom-doped graphene surfaces. While such studies of the ORR on nonmetallic heteroatom-doped graphene are advertised as "metal-free" there is typically no sufficient effort to characterize the doped materials to verify that they are indeed free of any trace metal. Here we argue that the claimed "metal-free" electrocatalysis of the oxygen reduction reaction on heteroatom-doped graphene is caused by metallic impurities present within the graphene materials. Carbon materials: Heteroatom-doped graphene surfaces are used as electrocatalysts for the oxygen reduction reaction. The claimed "metal-free" electrocatalysis of the oxygen reduction reaction is caused by metallic impurities (see picture) present within the graphene materials.