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 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 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 ) of 97% at a potential of -0.7 V, a high CO partial current density (j ) of 49.6 mA cm (-1.0 V), and a remarkable turnover frequency of 24 900 h (-1.0 V) for CO reduction reactions (CO RR). Density functional theory calculations show that compared to pyridinic-type NiN , the pyrrolic-type NiN moieties display a superior CO RR activity over hydrogen evolution reactions, resulting in their superior catalytic activity and selectivity.