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-N-5 active sites composed of planar Ni-N-4 (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-N-5 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@pyrrolic 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-N-4 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-N-4 sites in NiPc and pyridinic N atom doped in NC.