Electrocatalytic reduction of carbon dioxide in confined microspace utilizing single nickel atom decorated nitrogen-doped carbon nanospheres

Carbon dioxide electroreduction reaction (CO2RR), as a rational regulation of CO2 resource utilization, demands effectively selective catalysts for converting CO2 into high-value-added chemicals. Carbon-based nanoreactors featuring rationally designed porous framework structures might provide a unique chemical environment for confining and stabilizing the active metal species, consequently improving the CO2RR activity. Herein, nitrogen -doped porous carbon nanospheres decorated by single Ni atom (Ni-NCN) featuring a Ni-N4 structure were synthesized using the modified sol-gel method for the reduction of CO2 to CO. The synergistic effect of the Ni-N4 active sites homogenously distributed in the interconnected pore structure and the favorable chemical confined microspace of carbon nanospheres endows it with excellent CO2RR activity. In the H-type cell, Ni-NCN displays a CO Faradaic efficiency up to 96.6 % and a CO current density of 9.8 mA cm-2 at -0.83 V (vs. RHE), as well as a high turnover frequency (TOF) of 10658 h-1 at -1.33 V (vs. RHE). In the flow cell, the mass transfer can be further facilitated by the formation of three-phase interface. The Faradaic efficiency and current density of CO2RR catalyzed by Ni-NCN is enhanced to 97.9 % and 102.4 mA cm-2 at -1.13 V (vs. RHE), and the wide potential window ranges from -0.53 V to -1.33 V (vs. RHE) with the Faradaic efficiency more than 95 %. Density functional theory (DFT) calculations reveal that the high selectivity of Ni-N4 sites is mainly ascribed to the high energy barrier that restrains the hydrogen evolution reaction (HER). Meanwhile, the lower CO binding energy on Ni-N4 site helps the escape of CO to increase the TOF of active sites. The in-situ Fourier transform infrared (FTIR) spectroscopy verifies that the intermediate *COOH can be more stable in the confined envi-ronment of Ni-NCN to promote the selectivity of CO2RR. The strategy of constructing confined microspace paves a new path for the rational design of high-efficient single atom catalysts for CO2 reduction.