Coordinatively unsaturated nickel–nitrogen sites towards selective and high-rate CO2 electroreduction

Abstract

High Faradaic efficiency and appreciable current density are essential for future applications of the electrochemical CO₂ reduction reaction (CO₂RR). However, these goals are difficult to achieve simultaneously due to the severe side reaction – the hydrogen evolution reaction (HER). Herein, we successfully synthesized coordinatively unsaturated nickel–nitrogen (Ni–N) sites doped within porous carbon with a nickel loading as high as 5.44 wt% by pyrolysis of Zn/Ni bimetallic zeolitic imidazolate framework-8. Over the Ni–N composite catalysts, the CO current density increases with the overpotential and reaches 71.5 ± 2.9 mA cm⁻² at −1.03 V (vs. a reversible hydrogen electrode, RHE), while maintaining a high CO Faradaic efficiency of 92.0–98.0% over a wide potential range of −0.53 to −1.03 V (vs. the RHE). Density functional theory calculations suggest that the CO₂RR occurs more easily than the HER over the coordinatively unsaturated Ni–N site. Therefore, highly doped and coordinatively unsaturated Ni–N sites achieve high current density and Faradaic efficiency of the CO₂RR simultaneously, breaking current limits in metal–nitrogen composite catalysts.