MOF-derived 1D/3D N-doped porous carbon for spatially confined electrochemical CO2 reduction to adjustable syngas

Abstract

Electrochemical reduction of CO₂ to syngas (CO and H₂) offers an efficient way to mitigate carbon emissions and store intermittent renewable energy in chemicals. Herein, the hierarchical one-dimensional/three-dimensional nitrogen-doped porous carbon (1D/3D NPC) is prepared by carbonizing the composite of Zn-MOF-74 crystals in situ grown on a commercial melamine sponge (MS), for electrochemical CO₂ reduction reaction (CO₂RR). The 1D/3D NPC exhibits a high CO/H₂ ratio (5.06) and CO yield (31 mmol g⁻¹ h⁻¹) at −0.55 V, which are 13.7 times and 21.4 times those of 1D porous carbon (derived from Zn-MOF-74) and N-doped carbon (carbonized by MS), respectively. This is attributed to the unique spatial environment of 1D/3D NPC, which increases the adsorption capacity of CO₂ and promotes electron transfer from the 3D N-doped carbon framework to 1D carbon, improving the reaction kinetics of CO₂RR. Experimental results and charge density difference plots indicate that the active site of CO₂RR is the positively charged carbon atom adjacent to graphitic N on 1D carbon and the active site of HER is the pyridinic N on 1D carbon. The presence of pyridinic N and pyrrolic N reduces the number of electron transfer, decreasing the reaction kinetics and the activity of CO₂RR. The CO/H₂ ratio is related to the distribution of N species and the specific surface area, which are determined by the degree of spatial confinement effect. The CO/H₂ ratios can be regulated by adjusting the carbonization temperature to adjust the degree of spatial confinement effect. Given the low cost of feedstock and easy strategy, 1D/3D NPC catalysts have great potential for industrial application.