Geometric and Electronic Structural Engineering of Isolated Ni Single Atoms for a Highly Efficient CO2 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 NiNx 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 NiN3 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 (FECO ) of 97% at a potential of -0.7 V, a high CO partial current density (jCO ) of 49.6 mA cm-2 (-1.0 V), and a remarkable turnover frequency of 24 900 h-1 (-1.0 V) for CO2 reduction reactions (CO2 RR). Density functional theory calculations show that compared to pyridinic-type NiNx , the pyrrolic-type NiN3 moieties display a superior CO2 RR activity over hydrogen evolution reactions, resulting in their superior catalytic activity and selectivity.