Geometric and Electronic Structural Engineering of Isolated Ni Single Atoms for a Highly Efficient CO 2 Electroreduction.

Tuning the coordination environment and geometric structures of single atom catalysts is an effective approach for regulating the reaction mechanism and maximize the catalytic efficiency of single-atom centers. Here, a template-based synthesis strategy is proposed for the synthesis of high-density NiN x sites anchored on the surface of hierarchically porous nitrogen-doped carbon nanofibers (Ni-HPNCFs) with different coordination environments. First-principles calculations and advanced characterization techniques demonstrate that the single Ni atom is strongly coordinated with both pyrrolic and pyridinic N dopants, and that the predominant sites are stabilized by NiN 3 sites. This dual engineering strategy increases the number of active sites and utilization efficiency of each single atom as well as boosts the intrinsic activity of each active site on a single-atom scale. Notably, the Ni-HPNCF catalyst achieves a high CO Faradaic efficiency (FE CO ) of 97% at a potential of -0.7 V, a high CO partial current density (j CO ) of 49.6 mA cm -2 (-1.0 V), and a remarkable turnover frequency of 24 900 h -1 (-1.0 V) for CO 2 reduction reactions (CO 2 RR). Density functional theory calculations show that compared to pyridinic-type NiN x , the pyrrolic-type NiN 3 moieties display a superior CO 2 RR activity over hydrogen evolution reactions, resulting in their superior catalytic activity and selectivity.