▎ 摘　　要

We report great promises of single (transition metal) atom catalysts (SACs) anchored to single- and double- vacancy sites of the defective graphene structures for hydrogen evolution reaction (HER). Among 120 candidates with all 3d, 4d, and 5d-block transition metals considered, the inexpensive Fe-based SAC catalyst shows a theoretical overpotential of 5 mV, the lowest value reported to date theoretically or experimentally. With the help of various electronic structural analysis, we reveal that the key to the observed superior HER activity of Fe-based SAC is the enhanced ionicity of bonding between hydrogen and the corresponding SAC and the lack of ensemble effect that causes the *H binding weaker to be nearly at the top of the HER activity volcano.