▎ 摘　　要

Nonprecious metal catalysts are known of significance for electrochemical N(2)reduction reaction (NRR) of which the mechanism has been illustrated by ongoing investigations of single atom catalysis. However, it remains challenging to fully understand the size-dependent synergistic effect of active sites inherited in substantial nanocatalysts. In this work, four types of small iron clusters Fe-n(n= 1-4) supported on nitrogen-doped graphene sheets are constructed to figure out the size dependence and synergistic effect of active sites for NRR catalytic activities. It is revealed that Fe(3)and Fe(4)clusters on N(4)G supports exhibit higher NRR activity than single-iron atom and iron dimer clusters, showing lowered limiting potential and restricted hydrogen evolution reaction (HER) which is a competitive reaction channel. In particular, the Fe-4-N(4)G displays outstanding NRR performance for "side-on" adsorption of N(2)with a small limiting potential (-0.45 V). Besides the specific structure and strong interface interaction within the Fe-4-N(4)G itself, the high NRR activity is associated with the unique bonding/antibonding orbital interactions of N-N and N-Fe for the adsorptive N(2)and NNH intermediates, as well as relatively large charge transfer between N(2)and the cluster Fe-4-N(4)G.