Subnano Amorphous Fe-Based Clusters with High Mass Activity for Efficient Electrocatalytic Oxygen Reduction Reaction.

The development of cost-effective and efficient oxygen-relative electrocatalysts with high mass activity is extremely critical for modern sustainable fuel cells. Here, we present a new type of subnano amorphous transition-metal clusters supported on a hierarchical carbon framework as a promising oxygen reduction reaction (ORR) electrocatalyst, synthesized by a novel "amino-induced spatial confinement" strategy. This developed Fe subnano-cluster/3D-C could deliver outstanding ORR performance with a large mass activity of ∼8600 A gFe-1 at a half-wave potential of 0.92 V, ∼10 times that of the benchmarking Pt/C electrocatalyst. The atomic characterizations and theoretical calculations jointly reveal the robust surface-covalent Fe-N bonds, and the synergistic effect of hetero Fe2+/0 species is essentially beneficial for the adsorption of *O2 and the formation of key *O intermediate during the ORR process, contributing to high oxygen-relative electrocatalytic activity for subnano amorphous Fe clusters.