Residual metals present in “metal-free” N-doped carbons

To date, "metal-free" catalysts originating from graphene and CNTs have been revealed to contain metal impurities. Many types of such "metal-free" carbons have been synthesized by metal-involving methods, and it is urgent to confirm the origin of the catalytic performance. Herein, we verified that residual metals were inevitably present in these N-doped carbons and the catalytic performance for the hydrogen evolution reaction (HER) was due to the remaining metals, present at only a ppm level.