Boosting Defective Carbon by Anchoring Well-Defined Atomically Dispersed Ni–N₄ Sites for Electrocatalytic CO₂ Reduction

To enhance the faradic efficiency of the electrocatalytic CO₂ reduction reaction (CO₂RR) with stable catalysts, atomically dispersed Ni–N₅ active sites composed of planar Ni–N₄ (in nickel phthalocyanine) coordinated with the N atom in the carbon matrix (denoted as NiPc/NC) were proposed to reduce CO₂ into CO products. Extended X-ray absorption fine structure (EXAFS) spectroscopy and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements confirmed that the Ni–N₅ structure composed of single Ni atoms in NiPc and N doped in the carbon matrix. Density functional theory (DFT) calculations reveal that an energy barrier of only +0.89 eV is required for the process to take place on the surface of NiPc@pyridinic N. This barrier is significantly lower than in the case of NiPc@graphitic N (+2.12 eV), NiPc@pyrrolic N (+1.60 eV), and NiPc@C (+2.64 eV). This result suggests that the high CO₂RR activity originates from the synergistic effect between the coordinatively unsaturated Ni–N₄ sites and the surface pyridinic N species. The faradic efficiency of CO₂ reduction into the CO product was ≥93% over the NiPc/NC catalyst in a wide potential range of −0.5 to −0.8 V (vs a reversible hydrogen electrode, RHE). The peak CO faradic efficiency was 98% at a potential of −0.5 V due to the synergistic effect of Ni–N₄ sites in NiPc and pyridinic N atom doped in NC.